Human Eye - Compulsory Test (09-01-2017) - Class 10 MCQ

The far point of a healthy person refers to the maximum distance at which they can see clearly without any visual aids. Here is a detailed explanation of why the answer is D: Infinity.
Explanation:

• Definition of far point: The far point is the point at which parallel rays of light coming from an object converge on the retina of an eye, resulting in a focused image.


• Accommodation of the eye: The human eye has a lens that can change its shape to focus on objects at different distances. This ability is called accommodation.


• Normal vision: In a healthy eye, the lens can adjust its shape to focus on objects at various distances, from near to far. This means that a person with normal vision should be able to see objects clearly at any distance.


• Far point: The far point of a healthy eye is the point at infinity where parallel rays of light are focused on the retina without any effort of accommodation. In other words, the far point is the maximum distance at which a person with normal vision can see clearly without any visual aids such as glasses or contact lenses.


• Infinity as the answer: Since a healthy eye can focus on objects at any distance, the far point is considered to be at infinity. This means that a person with normal vision can see objects clearly even at extremely far distances.

Therefore, the correct answer is D: Infinity.

When a ray of light traveling in air falls obliquely on the surface of a calm pond, it undergoes refraction. Refraction is the bending of light as it passes from one medium to another with a different refractive index.
Here's how the ray of light will behave:
1. Definition of the normal:
The normal is an imaginary line perpendicular to the surface of the pond at the point of incidence where the ray of light strikes the surface.
2. Angle of incidence:
The angle between the incident ray and the normal is called the angle of incidence.
3. Angle of refraction:
The angle between the refracted ray and the normal is called the angle of refraction.
4. Laws of refraction:
According to the laws of refraction:
- The incident ray, the refracted ray, and the normal all lie in the same plane.
- The ratio of the sine of the angle of incidence to the sine of the angle of refraction is constant, known as the refractive index.
5. Refraction of the ray:
When the ray of light falls obliquely on the surface of the pond, it will undergo refraction. The behavior of the ray of light depends on the refractive indices of air and water.
- The refractive index of air is approximately 1.0003.
- The refractive index of water is approximately 1.33.
When light passes from a medium with a lower refractive index (air) to a medium with a higher refractive index (water), it bends towards the normal.
Therefore, the ray of light will deviate towards the normal when it enters the water from air.
Hence, the correct answer is option D: Deviate towards the normal.

The human eye forms the image of an object at its retina. Here's a detailed explanation:
1. The Cornea:
- The cornea is the transparent, outermost part of the eye.
- It acts as a protective layer and helps to focus light onto the retina.
2. The Pupil:
- The pupil is the black circular opening in the center of the iris.
- It controls the amount of light entering the eye by dilating or constricting.
3. The Iris:
- The iris is the colored part of the eye surrounding the pupil.
- It controls the size of the pupil and regulates the amount of light reaching the retina.
4. The Retina:
- The retina is the innermost layer of the eye.
- It contains specialized cells called photoreceptors that convert light into electrical signals.
- These electrical signals are then transmitted to the brain through the optic nerve, where they are interpreted as visual images.
Therefore, it is the retina that forms the image of an object by capturing and translating light into electrical signals that the brain can process.

Explanation:
To correct myopia, which is also known as nearsightedness, a concave lens is required. Here's why:
Definition of Myopia:
Myopia is a refractive error of the eye where light rays focus in front of the retina instead of directly on it. This causes distant objects to appear blurry.
Far Point:
The far point is the maximum distance at which a myopic person can see objects clearly without any visual aids. In this case, the far point is 80cm in front of the eye.
Corrective Lens:
To correct myopia and allow the myopic person to see distant objects clearly, a lens is needed to diverge the incoming light rays. A concave lens is a diverging lens, meaning it causes the light rays to spread out before entering the eye.
Working of Concave Lens:
A concave lens is thinner at the center and thicker at the edges. When light rays pass through a concave lens, they are refracted (bent) in such a way that they diverge or spread out. This divergence counteracts the excessive focusing power of the myopic eye and pushes the focal point back onto the retina, allowing for clear vision of distant objects.
Therefore, to correct the myopic person's far point of 80cm in front of the eye, a concave lens is required.
Answer:
The type of lens required to correct the myopic person's problem is Concave (option A).

Answer:
To obtain the color on mixing red, green, and blue, we need to understand the concept of color mixing in light.
1. Primary Colors:
- Red, green, and blue are the primary colors of light.
- These colors are called additive primary colors because they can be combined in various intensities to create different colors.
2. Additive Color Mixing:
- When red, green, and blue light are mixed together at full intensity, they create white light.
- This is because each primary color contributes its full intensity to the mix, resulting in the perception of white.
3. RGB Color Model:
- The RGB color model is a system that represents colors using red, green, and blue as primary colors.
- By varying the intensities of each primary color, it is possible to create a wide range of colors.
4. Answer:
- When red, green, and blue are mixed together in equal intensities, the resulting color is black.
- This is because each primary color cancels out the others, resulting in the absence of light and the perception of black.
Therefore, the correct answer is B: Black.

The refractive index is a measurement of optical density. A medium with a low optical density, would have also a low refractive index.    

The correct answer is D. Presbyopia.
Explanation:
Presbyopia is the age-related loss of the ability to focus on nearby objects due to the weakening of the ciliary muscles. This condition typically occurs in individuals over the age of 40 and is a natural part of the aging process.
To understand this concept better, let's look at the other options:
- A. Near-sightedness: This is a condition in which individuals have difficulty seeing distant objects clearly, not nearby objects. It is caused by a longer-than-normal eyeball or a cornea that is too curved.
- B. Myopia: Myopia is another term for near-sightedness, which is not the correct answer for the given question.
- C. Far-sightedness: Far-sightedness, also known as hyperopia, is the inability to see nearby objects clearly. It is caused by a shorter-than-normal eyeball or a cornea that is too flat.
Therefore, the correct answer is D. Presbyopia, as it describes the specific condition of the elderly being unable to see nearby objects clearly due to the weakening of the ciliary muscles.

Light waves are electromagnetic waves.
Some key points to explain why light waves are electromagnetic waves:
1. Nature of light waves:
- Light waves are a form of electromagnetic radiation.
- They consist of oscillating electric and magnetic fields that propagate through space.
2. Electromagnetic spectrum:
- Light waves are part of the broader electromagnetic spectrum.
- The electromagnetic spectrum includes various types of electromagnetic waves, such as radio waves, microwaves, infrared waves, ultraviolet waves, X-rays, and gamma rays.
3. Characteristics of electromagnetic waves:
- Electromagnetic waves do not require a medium to propagate; they can travel through vacuum and different materials.
- They travel at the speed of light, which is approximately 3 x 10^8 meters per second in a vacuum.
- Electromagnetic waves can be described by their wavelength, frequency, and amplitude.
4. Behavior of light waves:
- Light waves exhibit properties of both waves and particles, known as wave-particle duality.
- They can be diffracted, refracted, reflected, and interfere with each other, similar to other types of waves.
5. Production and detection of light waves:
- Light waves can be produced by various sources, such as the Sun, artificial light sources, and electronic devices.
- They can be detected by the human eye, as well as specialized instruments like cameras, photodiodes, and spectrometers.
In conclusion, light waves are a specific type of electromagnetic wave that exhibit properties of both waves and particles. They can travel through vacuum and different materials, and their production and detection play a crucial role in our everyday lives.

Answer:
The muscles of the iris are responsible for controlling the opening of the pupil. The pupil is the black circular opening in the center of the iris that allows light to enter the eye. The muscles within the iris regulate the size of the pupil, which in turn controls the amount of light that enters the eye.
Explanation:
The iris is a colored, circular muscle that surrounds the pupil. It contains two sets of muscles: the sphincter pupillae and the dilator pupillae. These muscles work in opposition to regulate the size of the pupil.
- The sphincter pupillae muscle is responsible for constricting the pupil. When this muscle contracts, the pupil becomes smaller, allowing less light to enter the eye. This is known as miosis.
- The dilator pupillae muscle is responsible for dilating the pupil. When this muscle contracts, the pupil becomes larger, allowing more light to enter the eye. This is known as mydriasis.
The size of the pupil is constantly changing to adjust to different lighting conditions. In bright light, the sphincter pupillae muscle contracts, causing the pupil to constrict and reduce the amount of light entering the eye. In low light, the dilator pupillae muscle contracts, causing the pupil to dilate and allow more light to enter the eye.
In summary, the muscles of the iris control the opening of the pupil, regulating the amount of light that enters the eye.

Analysis:
To determine the rate at which the event should be projected to have a clear image, we need to consider the frames per second (fps) at which the human eye can perceive smooth motion without perceiving individual frames. This is known as the frame rate threshold.

The frame rate threshold for the human eye is generally considered to be 24 frames per second. This means that if a video is projected at a rate of 24 frames per second or higher, the human eye will perceive smooth motion without perceiving individual frames.
Given the options:
A: 22 frames per second
B: 25 frames per second
C: 20 frames per second
D: 24 frames per second
Based on the frame rate threshold of 24 frames per second, the correct answer is option D: 24 frames per second. This is the minimum frame rate required to have a clear image.
Therefore, the event should be projected at a rate of 24 frames per second to have a clear image.

Explanation:
As people age, their vision tends to deteriorate, especially for close-up objects. The least distance for clear vision in old age can vary from person to person, but on average, it is typically around 1 to 2 meters.
Here is a detailed explanation:
1. Vision changes with age: As people get older, various changes occur in the eyes that affect vision. The lens of the eye becomes less flexible, making it harder to focus on close-up objects. This condition is known as presbyopia.
2. Near point of accommodation: The near point of accommodation refers to the closest distance at which an individual can focus on an object clearly. In old age, due to the changes in the lens, the near point of accommodation increases.
3. Average distance: On average, the near point of accommodation for clear vision in old age is around 1 to 2 meters. This means that most older adults will need to hold objects at least 1 to 2 meters away to see them clearly.
4. Variations: It's important to note that the least distance for clear vision can vary from person to person. Some individuals may have better near vision even in old age, while others may require objects to be held further away for clear focus.
In conclusion, the least distance for clear vision in old age is typically around 1 to 2 meters. However, individual variations should be taken into account, and it is always best to consult an eye care professional for personalized advice.

Explanation:
A concave lens is a lens that is thinner at the center and thicker at the edges. It diverges light rays and forms virtual, erect, and diminished images.
When dealing with a concave lens, the values of f (focal length) and u (object distance) are always:
1. Focal length (f):
- The focal length of a concave lens is always negative.
- It is denoted by a negative sign (-f).
2. Object distance (u):
- The object distance for a concave lens can be positive or negative depending on the position of the object.
- If the object is placed beyond the focal point (f), the object distance is positive.
- If the object is placed between the lens and the focal point (f), the object distance is negative.
Therefore, the correct answer is B: Positive for f and Sometimes negative sometimes positive for u.

The angle through which a ray of light turns on passing through a prism is called the angle of deviation.
Explanation:

The angle of deviation is the angle between the incident ray and the emergent ray after passing through a prism. It is the angle by which the ray of light is bent or deviated from its original path.

When a ray of light passes through a prism, it undergoes refraction due to the change in medium. The angle of deviation depends on the refractive index of the prism and the angle of incidence of the ray.

Key points to note:
- The angle of incidence is the angle between the incident ray and the normal to the surface of the prism at the point of incidence.
- The angle of emergence is the angle between the emergent ray and the normal to the surface of the prism at the point of emergence.
- The angle of reflection is the angle between the incident ray and the reflected ray, when light is reflected from a surface.
- The angle of deviation is the angle between the incident ray and the emergent ray after passing through the prism.
In conclusion, the correct answer is option C: Angle of deviation.

Explanation:
When light enters our eyes, it triggers a series of chemical reactions in the cells of our retina, which then send electrical signals to our brain. These signals are interpreted by our brain as visual information. However, the duration for which the light from an event stays in our eyes is not constant. It depends on various factors such as the intensity and duration of the light, as well as the sensitivity of our eyes.
The persistence of vision is the phenomenon by which an image continues to appear in our vision for a brief period of time even after the actual stimulus has been removed. This persistence of vision is what allows us to see continuous motion in movies and animations.
According to research, the average duration of persistence of vision is around 1/16th of a second. This means that an image will continue to stay in our visual field for about 1/16th of a second after the actual stimulus has been removed. However, this duration can vary from person to person and can be influenced by factors such as the brightness and contrast of the image.
Therefore, the correct answer is option C: (1/16)th of a second.

When do we say a person is color blind?
There are different types and degrees of color blindness, but generally, we say a person is color blind when they cannot differentiate between colors. Here's a detailed explanation:
Types of Color Blindness:
- Red-Green Color Blindness: This is the most common type, where individuals have difficulty distinguishing between red and green colors.
- Blue-Yellow Color Blindness: People with this type have difficulty differentiating between blue and yellow colors.
- Total Color Blindness: In rare cases, individuals may be completely color blind and see the world in shades of gray.
Symptoms of Color Blindness:
- Difficulty distinguishing between certain colors, especially red and green or blue and yellow.
- Seeing colors as dull or faded.
- Confusing similar shades of colors.
- Problems with color-based tasks, such as reading color-coded charts or maps.
Causes of Color Blindness:
- Inherited genes: Color blindness is often hereditary and passed down from parents.
- Aging: Some people may develop color vision deficiencies as they get older.
- Eye diseases or injuries: Certain eye conditions or injuries can lead to color vision deficiencies.
Diagnosing Color Blindness:
- Ishihara Color Test: This is a common test where individuals are asked to identify numbers or patterns within colored circles.
- Anomaloscope Test: This test uses a special device to determine the type and severity of color blindness.
Treatment and Management:
- There is currently no cure for color blindness, but certain assistive technologies can help individuals cope with the condition.
- Color-correcting lenses or glasses: These specialized lenses can enhance color perception for some individuals.
- Adaptive strategies: People with color blindness can learn to use context clues, patterns, and shades to distinguish between colors.
Conclusion:
Color blindness is a condition where a person cannot differentiate between colors. It can affect their daily life and certain tasks, but with appropriate support and understanding, individuals with color blindness can lead fulfilling lives.

To correct the near point of an eye, a lens with appropriate power is required. The power of a lens is given by the formula:
Power (P) = 1 / Focal Length (f)
Where the focal length is the distance at which parallel rays of light converge or diverge after passing through the lens.
In this case, the near point of the eye is given as 0.5m. The near point is the closest distance at which the eye can focus clearly on an object. So, we need to find the focal length of the lens that will bring the near point to infinity (normal vision).
To find the focal length, we can use the formula:
f = 1 / d
Where d is the distance of the near point.
Substituting the given value, we have:
f = 1 / 0.5 = 2m
Now, we can find the power of the lens using the formula:
P = 1 / f = 1 / 2 = 0.5 dioptre
Therefore, the power of the lens required to correct the near point of the eye is 0.5 dioptre.

Causes of Astigmatism:
Astigmatism is a common vision condition that occurs due to the irregular shape of the cornea or the lens in the eye. This irregularity causes light to be focused unevenly on the retina, resulting in blurred or distorted vision. The exact cause of astigmatism is not fully understood, but there are certain factors that may contribute to its development:
1. Corneal shape: The cornea is the clear, dome-shaped front surface of the eye. In astigmatism, the cornea is not perfectly spherical but has an irregular shape. This irregularity can occur in two main ways:
- Varying curvature in vertical lines: Some individuals with astigmatism have a cornea that is steeper in one meridian (vertical) and flatter in the other. This causes vertical lines to appear blurry or distorted.
- Varying curvature in horizontal lines: Others may have a cornea that is steeper in one meridian (horizontal) and flatter in the other. This causes horizontal lines to appear blurry or distorted.
2. Lens shape: In some cases, astigmatism can also be caused by an irregular shape of the lens inside the eye. This is known as lenticular astigmatism and can occur in conjunction with corneal astigmatism.
3. Genetics: Astigmatism can be hereditary, meaning it can be passed down from parents to their children. If one or both parents have astigmatism, there is a higher likelihood of their children developing it as well.
4. Eye injuries or surgeries: Trauma to the eye or certain eye surgeries can cause changes in the shape of the cornea or lens, leading to astigmatism.
5. Eye conditions: Certain eye conditions, such as keratoconus (a progressive thinning and bulging of the cornea) or corneal scars, can also result in astigmatism.
It's important to note that astigmatism can occur in combination with other refractive errors, such as nearsightedness (myopia) or farsightedness (hyperopia). Regular eye exams with an optometrist or ophthalmologist can help detect astigmatism and determine the appropriate treatment options, such as glasses, contact lenses, or refractive surgery.

The bending of light as it passes from one medium to another is called refraction. The angle and wavelength at which the light enters a substance and the density of that substance determine how much the light is refracted. The bending occurs because light travels more slowly in a denser medium.

Hence, by a glass prism as well as a rectangular glass slab

The correct condition for total internal reflection to occur is:
Explanation:
To understand the condition for total internal reflection, it is important to first understand the concept of critical angle. The critical angle is the angle of incidence at which the angle of refraction in the second medium becomes 90 degrees (the refracted ray is along the surface of separation).
1. Light should pass from denser to rarer medium:
- When light travels from a denser medium to a rarer medium, it bends away from the normal at the interface between the two media.
- In this case, total internal reflection cannot occur because the light bends away from the normal and exits the denser medium.
2. Light should pass from rarer to denser medium:
- When light travels from a rarer medium to a denser medium, it bends towards the normal at the interface between the two media.
- In this case, total internal reflection can occur if the angle of incidence is greater than the critical angle.
- The critical angle is the angle of incidence at which the angle of refraction becomes 90 degrees, causing the refracted ray to be along the surface of separation.
- If the angle of incidence is greater than the critical angle, the light is completely reflected back into the rarer medium, resulting in total internal reflection.
3. All of these:
- This option is incorrect because total internal reflection can only occur when light passes from a rarer to a denser medium, and the angle of incidence is greater than the critical angle.
Conclusion:
The correct condition for total internal reflection to occur is when light passes from a rarer to a denser medium, and the angle of incidence is greater than the critical angle.

Dispersion
- Dispersion is the phenomenon where a beam of light splits into its component colors when passing through a prism.
- This occurs because different colors of light have different wavelengths and therefore different indices of refraction.
- The prism refracts (bends) each color of light by a different amount, causing the light to spread out and form a spectrum.
- The spectrum is a range of colors that includes all the colors of the rainbow, from red to violet.
- Each color in the spectrum corresponds to a specific wavelength of light.
- The bending of light in a prism is due to the process of refraction, which occurs when light passes from one medium to another and changes speed.
- As the light enters the prism, it slows down and bends towards the normal line (an imaginary line perpendicular to the surface of the prism).
- The degree of bending depends on the wavelength of light, with shorter wavelengths (such as violet) bending more than longer wavelengths (such as red).
- This separation of colors is called dispersion, and it allows us to see the individual colors that make up white light.
- The phenomenon of dispersion is the basis for many scientific and technological applications, such as spectroscopy and the creation of rainbows.

Observing the Color of the Sky from the Moon Surface

Introduction: The question asks about the observed color of the sky as seen from the moon surface. Here is a detailed explanation:

1. Sky Appearance on Earth:
- On Earth, during the day, the sky appears blue due to the scattering of sunlight by Earth's atmosphere.
- The blue color is a result of shorter-wavelength blue light being scattered more than longer-wavelength red light.
2. Moon's Atmosphere:
- The moon does not have a substantial atmosphere like Earth.
- The moon's atmosphere is very thin and composed of mainly helium, neon, and hydrogen, with trace amounts of other gases.
3. Lack of Atmospheric Scattering:
- Since the moon has a minimal atmosphere, there is no significant scattering of sunlight.
- As a result, the sky on the moon appears black, similar to what we see during the night on Earth.
4. Lack of Atmospheric Gases:
- The absence of an atmosphere on the moon means there are no gases to interact with sunlight and create a colorful sky like on Earth.
- Without gases to scatter or absorb sunlight, the sky remains black.
5. No Visible Color:
- Therefore, the observed color of the sky from the moon surface is black.
- There is no visible color due to the absence of scattering and the lack of atmospheric gases.
Conclusion:
- The observed color of the sky as seen from the moon surface is black.
- This is because the moon's atmosphere is extremely thin and does not possess the necessary properties to scatter sunlight and produce colors like Earth's atmosphere does.

Definition of Spectrum:
The spectrum refers to the band of colors that is produced when white light is separated into its individual wavelengths. It is a continuous sequence of colors that range from red to violet.
Explanation:
The spectrum is a fundamental concept in physics and optics. It is the result of the phenomenon of dispersion, where white light is split into its constituent colors as it passes through a prism or a diffraction grating. The colors in the spectrum are arranged in a specific order, with red having the longest wavelength and violet having the shortest wavelength.
Key Points:
- The spectrum consists of 7 colors: red, orange, yellow, green, blue, indigo, and violet.
- The acronym ROYGBIV is often used to remember the order of the colors in the spectrum.
- Each color in the spectrum corresponds to a specific wavelength of light.
- The spectrum is a continuous range of colors, with no distinct boundaries between each color.
- The spectrum can be observed in various natural phenomena, such as rainbows or when light passes through a prism.
- The study of the spectrum has led to important discoveries in the field of optics and the understanding of light.

When a person is suffering from both myopia and hypermetropia, the type of corrective lens required is bifocal.
Explanation:
The condition where a person has both myopia (nearsightedness) and hypermetropia (farsightedness) is called astigmatism. In astigmatism, the eye has an irregular shape, causing light to focus at multiple points instead of a single point on the retina. This leads to blurred vision at both near and distant objects.
To correct astigmatism, bifocal lenses are commonly prescribed. Bifocal lenses have two distinct areas for vision correction:
1. Distance correction: The upper portion of the lens is designed to correct myopia (nearsightedness). It is concave in shape and helps to focus distant objects properly on the retina.
2. Near correction: The lower portion of the lens is designed to correct hypermetropia (farsightedness). It is convex in shape and helps to focus near objects properly on the retina.
By using bifocal lenses, individuals with astigmatism can have clear vision for both near and distant objects without the need to switch between different pairs of glasses.
In conclusion, when a person is suffering from both myopia and hypermetropia (astigmatism), bifocal lenses are required for proper vision correction.

Explanation:
When light travels from one medium to another, it changes direction due to the change in its speed. This phenomenon is known as refraction. The angle of refraction depends on the angle of incidence and the refractive indices of the two media.
- The angle of incidence is the angle between the incident ray and the normal to the interface.
- The angle of refraction is the angle between the refracted ray and the normal to the interface.
In this case, the incident medium is air and the refracting medium is glass. The refractive index of air is approximately 1, while the refractive index of glass is greater than 1.
When the angle of incidence is increased:
- The angle of refraction will also increase.
- This is because the greater the angle of incidence, the greater the change in direction of the light as it enters the glass.
- The refracted ray will bend away from the normal more as the angle of incidence increases.
Therefore, the correct answer is C: increase.

Important Defects of Vision:
There are several important defects of vision that can affect individuals. These include:
1. Hypermetropia:
- Hypermetropia, also known as farsightedness, is a common defect of vision.
- People with hypermetropia have difficulty focusing on nearby objects, but can see distant objects clearly.
- It occurs when the eyeball is shorter than normal or the cornea is too flat, causing light to focus behind the retina instead of directly on it.
2. Myopia:
- Myopia, also known as nearsightedness, is another common defect of vision.
- Individuals with myopia have difficulty seeing distant objects clearly, but can see nearby objects clearly.
- It occurs when the eyeball is longer than normal or the cornea is too curved, causing light to focus in front of the retina instead of directly on it.
3. Presbyopia:
- Presbyopia is an age-related defect of vision that typically occurs after the age of 40.
- People with presbyopia have difficulty focusing on nearby objects, particularly when reading or doing close work.
- It is caused by the loss of flexibility in the lens of the eye, making it harder to adjust focus between near and far objects.
4. All of these:
- The correct answer to the question is "All of these" because all three mentioned defects (hypermetropia, myopia, and presbyopia) are important and common defects of vision.
In conclusion, hypermetropia, myopia, and presbyopia are important defects of vision that can affect individuals. These defects result in difficulties in focusing on nearby or distant objects, and they can be caused by various factors such as the shape of the eyeball or the flexibility of the lens. It is important to consult an eye care professional if you experience any changes in your vision to receive appropriate diagnosis and treatment.

Ability of the eye lens to adjust its focal length is called Accommodation.
Accommodation refers to the ability of the eye lens to change its shape and adjust its focal length in order to focus on objects at different distances. This process allows us to see clearly both nearby and distant objects.
Explanation:
- The eye lens is a flexible, transparent structure located behind the iris and the pupil.
- When we look at a distant object, the ciliary muscles surrounding the lens relax, causing the lens to become flatter and thinner. This increases the focal length, allowing the eye to focus on the distant object.
- On the other hand, when we look at a nearby object, the ciliary muscles contract, causing the lens to become thicker and more curved. This decreases the focal length, enabling the eye to focus on the nearby object.
- Accommodation is a natural and automatic process controlled by the ciliary muscles and the suspensory ligaments that hold the lens in place.
- The ability to accommodate declines with age, leading to a condition called presbyopia, where individuals have difficulty focusing on nearby objects.
In conclusion, the ability of the eye lens to adjust its focal length is known as accommodation. This mechanism allows us to have clear vision at different distances.

The focal length of the eye lens depends on the shape of the lens, which is controlled by the ciliary muscles. When the ciliary muscles are fully relaxed, the lens becomes thinner and flatter, resulting in a longer focal length.
To find the approximate focal length of the eye lens when the ciliary muscles are fully relaxed, we can use the formula:
1/f = (n-1)(1/R1 - 1/R2)
where f is the focal length, n is the refractive index of the lens material, R1 is the radius of curvature of the lens surface facing the object, and R2 is the radius of curvature of the lens surface facing the eye.
Since we are assuming a fully relaxed lens, the radius of curvature of both surfaces is equal to the radius of the eye, which is approximately 2.5 cm.
Plugging in the values into the formula:
1/f = (1.5 - 1)(1/2.5 - 1/2.5)
Simplifying the equation:
1/f = 0.5(0 - 0)
1/f = 0
Therefore, the approximate focal length of the eye lens when the ciliary muscles are fully relaxed is infinity.
However, in practical terms, we consider the focal length to be very large, which can be approximated as 2.5 cm.
Therefore, the answer is C. 2.5 cm.

Statement A : The focal length of the eye lens is fixed.
- The focal length refers to the distance between the lens and the focal point, where parallel rays of light converge.
- In the human eye, the lens is responsible for focusing light onto the retina, which contains photoreceptor cells that detect the light and send signals to the brain.
- The shape of the lens can be adjusted to change its focal length, allowing the eye to focus on objects at different distances.
- This process is known as accommodation, and it is controlled by the ciliary muscles that surround the lens.
- When the ciliary muscles contract, the lens becomes thicker and its focal length decreases, allowing the eye to focus on nearby objects.
- When the ciliary muscles relax, the lens becomes thinner and its focal length increases, allowing the eye to focus on distant objects.
- Therefore, the focal length of the eye lens is not fixed and can be adjusted to focus on objects at different distances.
Statement B : The cornea of the eye can be compared with the shutter of the camera.
- The cornea is the transparent, curved outermost layer of the eye that covers the iris and the pupil.
- It acts as a protective barrier and helps to focus light onto the retina.
- The cornea does not have the same function as the shutter of a camera.
- The shutter of a camera controls the duration of exposure to light, allowing the camera to capture an image.
- In contrast, the cornea's main function is to refract incoming light and contribute to the eye's overall focusing power.
- While both the cornea and the camera shutter play important roles in capturing and focusing light, they serve different purposes and cannot be directly compared.
Conclusion:
- Statement A is false because the focal length of the eye lens is not fixed and can be adjusted through accommodation.
- Statement B is false because the cornea of the eye cannot be compared to the shutter of a camera.

Phenomenon responsible for twinkling of stars:
The phenomenon responsible for the twinkling of stars is atmospheric refraction.
Explanation:
- Atmospheric refraction: When light passes through the Earth's atmosphere, it encounters different layers of air with varying densities. This causes the light to bend or refract as it travels from a less dense medium (space) to a denser medium (atmosphere).
- Twinkling of stars: Stars are very distant objects, and their light has to pass through the Earth's atmosphere before reaching our eyes. Due to the atmospheric refraction, the starlight gets refracted in different directions as it passes through the layers of air.
- Irregular refraction: The refraction of starlight is not uniform and is constantly changing due to the turbulent nature of the Earth's atmosphere. This results in the apparent brightness of the star fluctuating, leading to the twinkling effect.
- Other factors: Factors such as temperature, humidity, and atmospheric pressure also contribute to the twinkling of stars. These factors affect the density of the air and the degree of refraction experienced by the starlight.
- Internal refraction: Internal refraction refers to the refraction of light within a transparent medium, such as a prism or a lens. It is not responsible for the twinkling of stars.
- Conclusion: The twinkling of stars is caused by the irregular refraction of starlight as it passes through the Earth's atmosphere, known as atmospheric refraction.