Human Eye And Colorful World - Practice Test, Class 10 Science - Class 10 MCQ

Reasons for temporary vision impairment when entering a cinema hall:

• Adjustment of size of pupil: When we enter a cinema hall, the environment is usually much darker compared to the outside. In order to adapt to the low light conditions, our pupils need to dilate or enlarge to allow more light to enter the eyes. This adjustment takes some time, resulting in temporary vision impairment.

Explanation:

• Our pupils are the black circular openings in the center of our irises, and their size can change in response to the amount of light available.


• In bright conditions, the pupils constrict or become smaller to limit the amount of light entering the eyes.


• However, in dimly lit environments like cinema halls, the pupils need to dilate or enlarge to allow more light to reach the retina at the back of the eye.


• This adjustment process is not instantaneous and takes some time to reach its maximum dilation, resulting in a temporary period of impaired vision.

Conclusion:

• The reason we experience temporary vision impairment when entering a cinema hall is because our pupils need time to adjust their size in order to allow more light into the eyes.


• Once the adjustment is complete, our vision will improve and we will be able to see more clearly in the darkened environment of the cinema hall.

Explanation:
The property of persistence of vision refers to the phenomenon in which the human eye retains an image for a brief moment after it has disappeared from view. This property is used in cinematography to create the illusion of motion by displaying a rapid succession of still images.
Here is a detailed explanation of how the property of persistence of vision is used in cinematography:
1. Frame rate:
- Cinematography relies on the principle of persistence of vision to create the illusion of motion. Films are typically shot at a frame rate of 24 frames per second.
- Each frame is displayed on the screen for a fraction of a second before it is replaced by the next frame.
- The human eye retains the image of each frame for a brief moment, creating the perception of continuous motion when the frames are displayed rapidly.
2. Smooth motion:
- By displaying a sequence of frames in rapid succession, cinematographers can create smooth and fluid motion on the screen.
- This is achieved by taking advantage of the persistence of vision, which allows the images to blend together in the viewer's perception.
3. Special effects:
- Persistence of vision is also used in the creation of special effects in cinematography.
- Techniques such as slow motion, fast motion, and stop motion animation rely on manipulating the frame rate and the persistence of vision to create visually stunning effects.
4. Animation:
- Animation is another area where the property of persistence of vision is crucial.
- Traditional hand-drawn animation, as well as computer-generated animation, relies on displaying a series of still images in rapid succession to give the illusion of movement.
- This technique is based on the persistence of vision, as the viewer's eye blends the images together to perceive motion.
In conclusion, the property of persistence of vision is an essential concept in cinematography. It allows filmmakers to create the illusion of motion and bring their stories to life on the screen.

Variable focal length of eye is responsible for:

• Accommodation of eye: The variable focal length of the eye allows it to focus on objects at different distances. This process, known as accommodation, is controlled by the contraction and relaxation of the ciliary muscles, which adjust the shape of the lens to change its focal length. This enables clear vision for objects both near and far.


• Persistence of vision: The variable focal length of the eye also contributes to the persistence of vision. When we view a moving object, the eye tracks the object by adjusting its focal length to maintain a clear image on the retina. This rapid adjustment allows us to perceive smooth motion and prevents blurring of the moving object.


• Colour blindness: The variable focal length of the eye does not have a direct relation to colour blindness. Colour blindness is a condition caused by abnormalities in the photoreceptor cells of the retina, specifically the cones responsible for color vision.


• Least distance of distinct vision: The variable focal length of the eye plays a role in determining the least distance of distinct vision. By adjusting the shape of the lens, the eye can focus on objects closer to it. The closest point at which the eye can focus and still maintain a clear image is known as the least distance of distinct vision.

Therefore, the correct answer is A: Accommodation of eye.

Explanation:
Concave lens:
A concave lens is thinner at the center and thicker at the edges. It diverges the incoming light rays and forms a virtual image.
Myopic eye:
A myopic eye, also known as nearsightedness, has difficulty focusing on distant objects. This occurs when the eyeball is too long or the lens is too strong, causing the image to be formed in front of the retina.
Hypermetropic eye:
A hypermetropic eye, also known as farsightedness, has difficulty focusing on nearby objects. This occurs when the eyeball is too short or the lens is too weak, causing the image to be formed behind the retina.
Correcting myopic eye:
To correct a myopic eye, a concave lens is used. The concave lens diverges the incoming light rays before they enter the eye, allowing the image to be formed on the retina instead of in front of it. This helps to bring distant objects into focus.
Not suitable for hypermetropic eye:
A concave lens is not suitable for correcting a hypermetropic eye because it further diverges the incoming light rays, causing the image to be formed even further behind the retina. In this case, a convex lens is used to converge the light rays and bring the image forward onto the retina.
Conclusion:
A concave lens of suitable focal length is used for correcting a myopic eye. It helps to bring distant objects into focus by diverging the incoming light rays and forming the image on the retina.

Dispersion of White Light through a Glass Prism
- When a beam of white light passes through a glass prism, it undergoes a phenomenon called dispersion.
- Dispersion is the splitting of white light into its constituent colors, known as the spectrum.
- The spectrum consists of seven colors: red, orange, yellow, green, blue, indigo, and violet.
- This phenomenon occurs due to the refraction of light within the prism.
Refraction in a Glass Prism
- Refraction is the bending of light as it passes from one medium to another.
- When white light enters a glass prism, it slows down and bends towards the normal (an imaginary line perpendicular to the surface) due to the change in its speed.
- Different colors of light have different wavelengths, and they bend at different angles when passing through the prism.
Dispersion Process
- As the different colors of light bend at different angles, they separate from each other within the prism.
- The degree of bending or refraction depends on the refractive index of the glass prism for each color of light.
- The shorter wavelength colors (blue and violet) bend more than the longer wavelength colors (red and orange).
- This results in the spread of white light into its constituent colors, forming a spectrum.
Formation of Spectrum
- The spectrum is formed when the separated colors of light exit the prism and fall on a screen or surface.
- The colors appear in a specific order from top to bottom: red, orange, yellow, green, blue, indigo, and violet.
- This order is often remembered using the acronym ROYGBIV.
- The spectrum shows that white light is composed of a mixture of colors with different wavelengths.
Conclusion
- The splitting of white light into its constituent colors on passing through a glass prism is called dispersion.
- This phenomenon occurs due to the refraction of light within the prism, causing different colors to bend at different angles.
- The resulting spectrum consists of seven colors: red, orange, yellow, green, blue, indigo, and violet.

The broad wavelength range of the visible spectrum is 4000-8000A.
The visible spectrum refers to the range of electromagnetic radiation that is visible to the human eye. It is composed of different colors, each with a specific wavelength. The broad wavelength range of the visible spectrum is 4000-8000A, where A represents angstroms, a unit of length commonly used in the field of optics.
Explanation:
- The visible spectrum ranges from approximately 400 nanometers (nm) to 700 nm in wavelength.
- Different colors within the visible spectrum have different wavelengths. For example, red light has a longer wavelength of around 700 nm, while violet light has a shorter wavelength of around 400 nm.
- The range of 4000-8000A corresponds to a wavelength range of 400-800 nm, which falls within the visible spectrum.
- This range encompasses all the colors of the rainbow, including red, orange, yellow, green, blue, indigo, and violet.
- The visible spectrum is often represented as a continuous band of colors, with red on one end and violet on the other, transitioning smoothly through the other colors in between.
Conclusion:
- The broad wavelength range of the visible spectrum is 4000-8000A, which corresponds to a wavelength range of 400-800 nm.
- This range includes all the colors of the rainbow and is visible to the human eye.
- Understanding the visible spectrum is essential in fields such as optics, photography, and color science.

Explanation:
The refractive index of a substance is defined as the ratio of the speed of light in a vacuum to the speed of light in the substance. It determines how much the light bends or refracts when it passes through a medium.
In the case of glass, the refractive index varies with the wavelength of light. This phenomenon is known as dispersion. Different colors of light have different wavelengths, with violet light having the shortest wavelength and red light having the longest wavelength.
Key Points:
- The refractive index of glass is maximum for violet light.
- This means that violet light bends or refracts the most when it passes through glass compared to other colors of light.
- The refractive index gradually decreases as we move from violet to red in the visible light spectrum.
- This phenomenon is responsible for the dispersion of white light into its constituent colors when it passes through a prism or a droplet of water.
- The difference in refractive indices for different colors of light is what creates a rainbow.
- The refractive index of glass also depends on the type of glass and its composition.
Summary:
The refractive index of glass is maximum for violet light. This means that violet light bends or refracts the most when it passes through glass compared to other colors of light. The refractive index gradually decreases as we move from violet to red in the visible light spectrum. This phenomenon is responsible for the dispersion of white light into its constituent colors and the formation of a rainbow. The refractive index of glass also depends on the type of glass and its composition.

Which colour suffers least deviation on passing through a prism?
The answer is Red.
Explanation:
When light passes through a prism, it undergoes dispersion, which means that the different colors in the light spectrum are separated. The amount of separation or deviation depends on the wavelength of the light.
1. Dispersion of light:
- The different colors of light have different wavelengths, with red having the longest wavelength and violet having the shortest wavelength.
- When light enters a prism, it bends or refracts due to the change in speed as it passes from one medium (air) to another (glass).
- Different colors of light bend at different angles, causing the separation or dispersion of the colors.
2. Deviation of colors:
- The amount of deviation is directly related to the refractive index of the prism material and inversely related to the wavelength of light.
- As the wavelength increases, the deviation decreases.
- Red light has the longest wavelength among the visible colors, so it suffers the least deviation when passing through a prism.
3. Order of colors:
- When white light passes through a prism, it separates into a spectrum of colors in a specific order.
- The order of colors, from least to most deviated, is: Red, Orange, Yellow, Green, Blue, Indigo, and Violet.
Therefore, the color that suffers the least deviation on passing through a prism is Red.

Why is the sky blue?
The blue color of the sky is due to the phenomenon of scattering of light. When sunlight passes through the Earth's atmosphere, it interacts with the molecules and tiny particles present in the air. This interaction causes the light to scatter in all directions. However, the blue light is scattered more than other colors because it travels in shorter, smaller waves.
Scattering of light:
- When sunlight enters the Earth's atmosphere, it encounters molecules of nitrogen and oxygen, as well as tiny particles like dust and water droplets.
- These molecules and particles act as obstacles and cause the light to scatter in different directions.
- The scattering of light depends on the wavelength of the light. Shorter wavelengths, such as blue and violet, are scattered more than longer wavelengths, such as red and orange.
- Blue light has a shorter wavelength and higher energy compared to other colors, making it more susceptible to scattering.
Why does the sky appear blue?
- As the sunlight travels through the atmosphere, the blue light is scattered in all directions by the molecules and particles.
- This scattered blue light reaches our eyes from all parts of the sky, giving the sky its blue appearance.
- On a clear day, when the sun is directly overhead, the sky may appear to be a deep blue color.
Other factors:
- The color of the sky can vary at different times of the day. During sunrise or sunset, the sky may appear orange or red because the sunlight has to pass through a greater thickness of the Earth's atmosphere, causing more scattering of shorter wavelengths.
- The presence of air pollution, such as smog or haze, can also affect the color of the sky. Pollutants in the air can scatter and absorb light, resulting in a less vibrant blue color.
In conclusion, the blue color of the sky is primarily due to the scattering of light by molecules and particles in the Earth's atmosphere. This scattering phenomenon causes the blue light to be scattered more than other colors, giving the sky its characteristic blue appearance.

Explanation:
The red color of the sun at the time of sunrise and sunset is due to the phenomenon of scattering of light in the Earth's atmosphere. This phenomenon can be explained by the following points:
1. Scattering of Light:
- Scattering of light refers to the process by which light is deflected or scattered in various directions when it encounters particles or molecules in the atmosphere.
- In the Earth's atmosphere, the scattering of light is mainly caused by molecules and small particles such as dust and water droplets.
2. Rayleigh Scattering:
- Rayleigh scattering is a type of scattering that occurs when the size of the scattering particles is much smaller than the wavelength of the incident light.
- It is responsible for the scattering of shorter wavelengths of light, such as blue and violet, more than longer wavelengths, such as red and orange.
3. Atmospheric Conditions during Sunrise and Sunset:
- During sunrise and sunset, the sun is located at a lower angle in the sky, and its light has to pass through a larger portion of the Earth's atmosphere.
- The path length of sunlight through the atmosphere is longer during these times, leading to more scattering of shorter wavelengths and less scattering of longer wavelengths.
4. Red Color Dominance:
- As a result of Rayleigh scattering, the shorter wavelengths of light, particularly blue and violet, are scattered more strongly.
- The longer wavelengths, such as red and orange, are scattered less and can travel through the atmosphere without being scattered significantly.
- This leads to the dominance of red and orange colors in the sunlight reaching our eyes during sunrise and sunset, giving the sun a reddish hue.
Therefore, the correct answer is option A: Red color is least scattered.

Explanation:
The human eye is able to focus on objects at different distances through a process called accommodation. Accommodation is the adjustment of the focal length of the eye lens to bring objects into clear focus on the retina.
Reasons:
There are several reasons why the human eye can adjust its focal length:
1. Accommodative power: The ciliary muscles surrounding the lens contract or relax to change the shape of the lens. This change in shape alters the focal length, allowing the eye to focus on objects at different distances.
2. Near and far sightedness: Near sightedness (myopia) and far sightedness (hyperopia) are refractive errors that affect the eye's ability to focus on objects at certain distances. However, accommodation allows the eye to compensate for these refractive errors to some extent.
3. Persistence of vision: Persistence of vision refers to the ability of the retina to retain an image for a short time after the stimulus is removed. While persistence of vision is not directly related to the eye's ability to focus, it is important for visual perception and the interpretation of visual information.
4. Visual feedback: The eye constantly receives visual feedback from the environment, which helps in adjusting the focal length of the lens. This feedback allows the eye to make continuous adjustments to maintain clear focus on objects at different distances.
In conclusion, the human eye can focus objects at different distances by adjusting the focal length of the eye lens through a process called accommodation. This ability is essential for clear vision and visual perception.

Cinematography makes use of Persistence of Vision
Explanation:
Cinematography is the art and technique of capturing moving images on film or digital media. It involves various elements such as lighting, composition, camera movement, and visual effects. One of the key concepts used in cinematography is the persistence of vision.
What is Persistence of Vision?
Persistence of vision is the phenomenon where an image continues to be retained by the human eye for a brief moment after the actual image has disappeared. This is due to the brain's ability to retain visual information for a short period of time.
How is Persistence of Vision used in Cinematography?
Cinematographers take advantage of the persistence of vision to create the illusion of motion in films. They do this by capturing a series of still images, known as frames, and projecting them at a rapid speed. The brain then combines these individual frames into a continuous sequence, creating the perception of motion.
Benefits of using Persistence of Vision in Cinematography:
1. Smooth Motion: The use of persistence of vision helps to create smooth and fluid motion in films, making the viewing experience more realistic and engaging.
2. Frame Rate Control: Cinematographers can control the frame rate at which the images are projected to achieve different effects. A higher frame rate can create slow-motion sequences, while a lower frame rate can create fast-paced action.
3. Special Effects: Persistence of vision allows cinematographers to incorporate various special effects, such as freeze frames, time-lapse, and motion blur, to enhance the storytelling and visual appeal of the film.
4. Immersion: By utilizing the persistence of vision, cinematographers can immerse the audience in the narrative and transport them into the world of the film.
In conclusion, cinematography makes use of the persistence of vision to create the illusion of motion and enhance the visual storytelling in films. It is an essential technique that contributes to the overall cinematic experience.

The retina acts as the screen over which the image of an object is formed.

The change of focal length of an eye lens to focus the image of objects at varying distances is done by the action of the ciliary muscles.
The process of focusing the image of objects at different distances is known as accommodation. It involves changing the shape of the eye lens to adjust its focal length and bring the image into focus on the retina.
The ciliary muscles play a crucial role in this process. These muscles are located around the edge of the lens and are connected to it by a series of fibers called suspensory ligaments. When the ciliary muscles contract or relax, they cause changes in the shape and tension of the lens, allowing it to adjust its focal length.
Here's a detailed explanation of how the ciliary muscles change the focal length of the eye lens:
1. Relaxed state: When the eye is focused on distant objects, the ciliary muscles are relaxed. This causes the suspensory ligaments to pull on the lens, making it thinner and flatter. This shape increases the focal length of the lens, allowing it to focus light from distant objects onto the retina.
2. Contracted state: When the eye needs to focus on close objects, the ciliary muscles contract. This releases the tension on the suspensory ligaments, allowing the lens to become thicker and more rounded. This change in shape decreases the focal length of the lens, enabling it to focus light from nearby objects onto the retina.
3. Accommodation: The process of changing the shape of the lens to adjust its focal length is called accommodation. It is controlled by the autonomic nervous system, which automatically adjusts the tension in the ciliary muscles based on the distance of the object being viewed.
In summary, the ciliary muscles in the eye play a vital role in adjusting the focal length of the lens. By contracting or relaxing, these muscles change the shape of the lens, allowing it to focus light from objects at varying distances onto the retina for clear vision.

The focal length of the combination of convex lenses:
To find the focal length of the combination of convex lenses, we can use the lens formula:
1/f = 1/f1 + 1/f2
Where f is the focal length of the combination, f1 is the focal length of the first lens, and f2 is the focal length of the second lens.
Given:
f1 = 20 cm
f2 = 20 cm (assuming the two lenses are identical)
Substituting the given values into the lens formula:
1/f = 1/20 + 1/20
Simplifying the equation:
1/f = 2/20
1/f = 1/10
Therefore, the focal length of the combination is 10 cm.
Conversion from focal length to distance:
The distance between the lenses is given as 10 cm. However, we need to convert this distance to focal length in order to compare it with the calculated focal length of the combination.
Using the formula for the lens separation between two identical lenses:
1/f = 1/f1 + 1/f2 - d/(f1 * f2)
Where d is the distance between the lenses.
Given:
f1 = f2 = 20 cm
d = 10 cm
Substituting the given values into the formula:
1/f = 1/20 + 1/20 - 10/(20 * 20)
Simplifying the equation:
1/f = 1/20 + 1/20 - 1/40
1/f = 2/40 + 2/40 - 1/40
1/f = 3/40
Therefore, the calculated focal length of the combination is 40/3 cm, which is approximately equal to 13.3 cm.
Conclusion:
The correct answer is option D: 13.3 cm.

Given:
- Focal length of first lens (f1) = 10 cm
- Focal length of second lens (f2) = 20 cm
To find:
- Focal length of the combination of lenses

When two lenses are kept in contact, the focal length of the combination is given by the formula:
1/f = 1/f1 + 1/f2
Substituting the given values:
1/f = 1/10 + 1/20
Simplifying the equation:
1/f = 2/20 + 1/20
= 3/20
Taking the reciprocal of both sides:
f = 20/3 cm
Therefore, the focal length of the combination of lenses is 20/3 cm, which is option A.

To find the focal length of the combination of lenses, we can use the lens maker's formula:
1/f = (n - 1) * (1/R1 - 1/R2)
Where:
- f is the focal length of the combination of lenses
- n is the refractive index of the medium between the lenses (assumed to be air, so n = 1)
- R1 is the radius of curvature of the first lens
- R2 is the radius of curvature of the second lens
Given:
- Power of the first lens (P1) = -1.75D
- Power of the second lens (P2) = 2.75D
Step 1: Convert the powers of the lenses to focal lengths using the formula:
f = 1/P
- Focal length of the first lens (f1) = 1 / (-1.75) = -0.5714m
- Focal length of the second lens (f2) = 1 / 2.75 = 0.3636m
Step 2: Calculate the effective focal length of the combination of lenses using the formula:
1/f = (1/f1) + (1/f2)
- 1/f = (1/-0.5714) + (1/0.3636)
- 1/f = (-1.75 + 2.75) / (-0.5714 * 0.3636)
- 1/f = 1 / (-0.2071)
- f = -4.82m
Step 3: Convert the focal length back to power:
- Power (P) = 1 / f
- P = 1 / (-4.82)
- P = -0.2071D
Step 4: Convert the power to diopters:
- Power (P) = -0.2071 * 100cm
- P = -20.71D
So, the focal length of the combination of lenses is 100cm. Therefore, the correct answer is option B.

Spherical Aberration:
Spherical aberration is an optical phenomenon that occurs when light rays passing through different parts of a lens or mirror focus at different points, resulting in a blurred or distorted image. It is caused by the spherical shape of the lens or mirror, which causes the light rays to converge or diverge at different angles.
Huygen's Eye Piece:
Huygen's eye piece is a type of eyepiece commonly used in optical instruments such as microscopes and telescopes. It consists of two plano-convex lenses, with the first lens converging the light rays and the second lens diverging them. This design helps to correct for spherical aberration and produce a clearer image.
Ramsden's Eye Piece:
Ramsden's eye piece is another type of eyepiece used in optical instruments. It consists of two plano-convex lenses, with the first lens converging the light rays and the second lens also converging them. However, Ramsden's eye piece does not effectively correct for spherical aberration.
Conclusion:
Based on the explanation above, the correct answer is A: Huygen's eye piece. Huygen's eye piece is designed to correct for spherical aberration and produce a clearer image, while Ramsden's eye piece does not effectively correct for this aberration. Therefore, Huygen's eye piece satisfies the condition for spherical aberration.

The eye defect in which a person cannot see objects beyond near point or far objects but can see the near objects clearly is called Myopia. This defect can be corrected by using biconvex lens.

Long-sightedness and its effect on vision:
Long-sightedness, also known as hyperopia, is a common refractive error that affects the ability to see objects clearly at close range. It occurs when the eyeball is slightly shorter than normal or when the cornea has too little curvature. As a result, light entering the eye focuses behind the retina instead of directly on it, causing blurred vision.
A long-sighted person cannot see clearly:
The correct answer is A: Near objects.
A long-sighted person has difficulty focusing on objects that are close to them. They may experience symptoms such as eyestrain, headaches, and blurred vision when performing tasks that require near vision, such as reading, using a computer, or sewing.
Explanation:
When considering the options given, it becomes clear that a long-sighted person cannot see near objects clearly. Here's why:
- Near objects: A long-sighted person struggles to focus on objects that are close to them. They may have to hold reading material or other objects at arm's length to see them clearly. This is because the light entering their eye is not properly focused on the retina, leading to blurred vision for near objects.
- Distant objects: Although long-sightedness primarily affects near vision, it may also cause some blurring of distant objects. However, the blurring is typically more pronounced for objects that are closer to the person. Distant objects may still be relatively clear for a long-sighted individual, especially if their long-sightedness is mild.
- Both distant and near objects: Long-sightedness primarily affects near vision, so it is not accurate to say that a long-sighted person cannot see both distant and near objects clearly. While there may be some blurring for both, it is usually more noticeable for near objects.
- None: This option is incorrect because long-sightedness does affect vision, particularly for near objects. A long-sighted person will experience blurred vision when looking at objects up close.
In conclusion, a long-sighted person cannot see near objects clearly. They may require corrective measures such as glasses or contact lenses to improve their near vision.

Presbyopia and Corrective Lenses
Presbyopia is a common age-related condition that affects the ability of the eyes to focus on near objects. It occurs due to the natural aging process of the eye, which leads to a gradual loss of flexibility in the lens. Presbyopia typically becomes noticeable around the age of 40 and continues to progress over time.
To correct the vision problems caused by presbyopia, various types of lenses can be used. However, the most suitable option is bifocal lenses. Here's why:
1. Convex lens: Convex lenses are thicker at the center and thinner at the edges. They are commonly used to correct farsightedness (hyperopia) and provide clear distance vision. However, they are not ideal for presbyopia as they do not address the near vision problem.
2. Concave lens: Concave lenses are thinner at the center and thicker at the edges. They are primarily used to correct nearsightedness (myopia) and help in providing clear distance vision. Similar to convex lenses, concave lenses do not address the near vision issue associated with presbyopia.
3. Cylindrical lenses: Cylindrical lenses are used to correct astigmatism, a condition that causes blurred vision at all distances. While presbyopia and astigmatism can occur together, cylindrical lenses alone do not provide the necessary correction for near vision.
4. Bifocal lenses: Bifocal lenses have two distinct optical powers in a single lens. The upper part of the lens corrects distance vision, while the lower part is designed to enhance near vision. Bifocal lenses are specifically designed to address presbyopia and provide clear vision for both near and far distances.
In conclusion, a person with presbyopia should use bifocal lenses as they offer the most appropriate correction for both near and distance vision problems caused by presbyopia.

Defect: Colour blindness
Explanation:
Colour blindness is a visual impairment that affects a person's ability to perceive and distinguish certain colors. In this case, the person is unable to see fundamental colors such as red, blue, and green. Here is a detailed explanation of each option and why it is not the correct answer:
- Myopia: Myopia, commonly known as nearsightedness, is a refractive error where distant objects appear blurred while close objects are clear. It does not affect color perception and is unrelated to the inability to see fundamental colors.
- Presbyopia: Presbyopia is an age-related condition where the eye gradually loses its ability to focus on nearby objects. It also does not affect color perception and is unrelated to the inability to see fundamental colors.
- Colour blindness: Colour blindness, also known as color vision deficiency, is a condition where a person has difficulty distinguishing certain colors. It can be inherited or acquired and can affect the perception of red, blue, and green colors. This defect aligns with the given description.
- Astigmatic: Astigmatism is a refractive error that causes blurred vision due to an irregularly shaped cornea or lens. It does not directly impact color perception and is unrelated to the inability to see fundamental colors.
Therefore, the correct answer is C: Colour blindness as it accurately describes the defect of being unable to see fundamental colors such as red, blue, and green.

Defect of Astigmatism:
Astigmatism is a common vision problem that occurs when the cornea or lens of the eye has an irregular shape, causing blurred or distorted vision. In this condition, the eye fails to focus light evenly onto the retina, resulting in both near and distant objects appearing blurry or distorted.

The defect of astigmatism can be rectified by using a cylindrical lens. Here's how it works:
1. Cylindrical Lens:
- A cylindrical lens is a type of lens that has different curvatures in different meridians.
- It has a spherical curvature in one meridian and a cylindrical curvature in the perpendicular meridian.
- The cylindrical lens is designed to correct the uneven curvature of the cornea or lens, which is the main cause of astigmatism.
2. Correcting Irregular Curvature:
- The cylindrical lens is prescribed in such a way that it counteracts the irregular curvature of the cornea or lens.
- The lens is positioned in front of the eye, and its cylindrical power is oriented to align with the axis of astigmatism.
- By doing so, the cylindrical lens helps to focus light properly onto the retina, correcting the blurry or distorted vision caused by astigmatism.
3. Different Power in Different Meridians:
- Unlike a convex or concave lens that has the same power in all meridians, a cylindrical lens has different powers in different meridians.
- It compensates for the difference in refraction between the two principal meridians of the eye, thus correcting astigmatism.
4. Additional Correction:
- In some cases, astigmatism may be accompanied by other refractive errors like nearsightedness (myopia) or farsightedness (hyperopia).
- In such situations, a combination of a cylindrical lens and a spherical lens may be prescribed, known as a bifocal lens.
- The bifocal lens corrects both astigmatism and the additional refractive errors, providing clear vision at different distances.
Therefore, the correct answer to rectify the defect of astigmatism is a cylindrical lens (Option B). It helps to compensate for the irregular curvature of the cornea or lens, allowing light to focus properly on the retina and improving vision clarity.