Class 10 Science Chapter 10 Previous Year Questions - The Human Eye and the Colourful World

Ans: (d)
The lens system of the human eye forms an image on a light-sensitive screen called the retina. The retina captures the light and sends the visual information to the brain through the optic nerves, allowing us to see.

Ans: (i) Hypermetropia or Far-sightedness.
Reason – Image is formed behind the retina. Near point for the person is farther away from the normal near point (25 cm)  
(ii)

• Focal length of the eye lens is too long. 
• The eyeball has become too small.

(iii)
N = Near point of a hypermetropic eye
N’= Near point of a normal eye

Ans: (a)
The correct statements about the human eye are (A) and (B). The diameter of the eyeball is indeed about 2.3 cm, and the iris is a dark muscular part that controls the size of the pupil, regulating the amount of light entering the eye. While (C) and (D) have some truth, they are not entirely accurate: most light refraction occurs at the cornea, not primarily at the crystalline lens, and the ciliary muscles adjust the shape of the lens rather than the distance between the lens and retina.

Ans: (c)
The assertion (A) states that a myopic eye, or nearsighted eye, cannot see distant objects clearly, which is true. However, the reason (R) claims that converging lenses are used to correct myopia, which is false; instead, diverging lenses are prescribed for myopia to help the eye focus on distant objects. Thus, the answer is (c) because (A) is true, but (R) is false.

Ans: When he cannot see nearby objects distinctly but can see far object clearly.
2 causes: 
(i) Focal length of the eye lens is too long.
(ii) Eyeball becomes too small.
Convex or Converging lens

Ans: (a) Ability of the eye lens to adjust its focal length.  
Ciliary muscles  
(i)  While focusing on nearby objects ciliary muscles contract, eye lens becomes thick and its focal length decreases.  (ii) While focusing on distant objects ciliary muscles relax, eye lens becomes thin and its focal length increases.
OR
(b)

Two conditions:  
(i)  Presence of tiny water droplets in the atmosphere.
(ii) Position of Sun at the back(behind) the observer. 

Ans: 

• Scattering of light / Tyndall effect.
• When a beam of light strikes fine particles of smoke, it is reflected diffusely and the path of the light becomes visible. 
• Very fine particles scatter mainly blue light/short wavelength colours while the particles of larger size scatter longer wavelength colours.

Ans: (c)
When a beam of white light passes through an area with fine dust particles, the color of light that gets scattered the most is blue. This happens because shorter wavelengths of light, like blue, scatter more easily than longer wavelengths, like red or orange, according to Rayleigh scattering. Therefore, blue light is more visible in such conditions.

Ans:

• Ability of the eye lens to adjust its focal length.
• Image distance remains unchanged.
• Ciliary muscles: While focusing on distant objects ciliary muscles relax, eye lens becomes thin and its focal length increases.

Ans: (a) (i) 

• Hypermetropia 
• Ciliary muscles/ eye lens 

(ii)

• Focal length of the eye lens is too long. 
• Eyeball becomes too small.

(iii) Converging lenses/ convex lens They provide the additional focussing power required for forming the  image on the retina./ Helps to decrease the focal length of the eye lens.
OR
(b) The splitting of white light into its constituent colours is called dispersion.
Cause: Different colours of white light bend through different angles with respect to incident ray.

Ans: (c)

The assertion (A) is true because a rainbow is indeed a natural spectrum of sunlight seen in the sky. However, the reason (R) is not entirely accurate; a rainbow typically forms when the sun is low in the sky, not directly overhead, and there are water droplets present. Therefore, the correct answer is (b) because both statements are true, but (R) is not the correct explanation of (A).

Ans: (c)
The assertion (A) is true because red light signals are indeed used to indicate that vehicles should stop. However, the reason (R) is false; red light is actually scattered the least among the visible colors, which is why it can be seen from a distance without much scattering. Therefore, the correct answer is (c), as (A) is true, but (R) is false.

Ans: Student is suffering from myopia.
The two possible reasons due to which the defect of vision arises are : excessive curvature of the eye-lens and elongation of the eye-ball.
A student with myopia has the far point nearer than infinity, thus, the image of a distant object is formed in front of the retina.

Correction of myopia: This defect can be corrected by using a concave lens of suitable power as it brings the image back on to the retina, thus the defect is corrected.

Ans: (a) Hypermetropia
(b) Hypermetropia is caused due to following reasons:
(i) Shortening of the eyeball
(ii) Focal length of crystalline lens is too long
It is corrected by using a convex lens which converges and shifts the image to the retina from behind.

Ans: 
(i) The phenomenon of the splitting up of the white light into its constituent's colours is called dispersion of light. Dispersion of light is caused due to, different constituents and colours of light after different refractive indices to the material of the prism.
(ii) The formation of a rainbow is caused by the dispersion of the white sunlight into its constituent colours.
(iii) Based on the dispersion of white light into its constituent's colours, we can conclude that
(a) The white light consists of seven colours.
(b) The violet light suffers maximum deviations and the red light suffers minimum deviation.

Ans: After a rain-shower, the sunlight gets dispersed by tiny droplets, present in the atmosphere. The water droplets act like small glass prisms. They refract and disperse the incident sunlight, then reflect it internally, and finally refract it again when it comes out of the raindrop. Due to dispersion or light and internal reflection, different colours reach the observer's eye, which is called a rainbow.

Ans: As our earth has an atmosphere and space has no atmosphere, the sunlight scatters by the particles in the atmosphere. The amount of scattering of light is inversely proportional to wavelength thus violet, and blue colours scatter more than red and hence the colour of the sky appears blue from the earth.

Ans: (c)
Sol: Red colour is used to paint the danger signals. As live know, visible spectrum has violet indigo, blue, green, yellow. orange and red colour and these colours are arranged with red on the top and violet at the bottom (near base of the prism) on the basis of wavelength and frequency, and among these colours red colour has largest wavelength (≈ 750 nm). Thus, here Y will be red.

Ans: (b)
Sol: The blue colour of clear sky is due to Rayleigh elastic scattering of sun light

Ans: The phenomenon of scattering of light by the colloidal particles gives rise to the Tyndall effect. When a beam of light strikes colloidal particles, the path of the beam becomes visible. This is known as the Tyndall effect. This phenomenon can be observed when
(i) Sunlight passes through a canopy of dense forest when tiny water droplets in the mist scatter light.
(ii) torch light is switched on in a foggy environment, light rays are visible after being scattered by the fog particles in the surrounding air.
(iii) a fine beam of sunlight enters a smoke-filled room through a small hole.
(iv) Shining a flashlight beam into a glass of dilated milk produces a Tyndall effect.

Ans: Accommodation power is the property of the eye lens to adjust its focal length to focus objects situated at different distances from the eye on the retina.
When the ciliary muscles are relaxed, the eye lens becomes thin and its focal length is maximum and equal to the diameter of the eyeball. In this condition, one can see distant objects.
At the time of looking at nearby objects, the ciliary muscles of the eye contact and the eye lens become thicker. Consequently, the focal length of the eye lens decreases and nearby objects are focussed at the retina.
There are definite limits to accommodation power for a healthy normal eye. The farthest distance, up to which an eye can see objects clearly, is called the far point of the eye and its value is infinity. The minimum distance, up to which an eye can see distinctly, is known as the near point of the eye and its value is 25 cm for a normal eye.

Ans: A person is said to have a myopic vision if he can see objects situated near the eye clearly but cannot see distant objects. If a person can see clearly up to a distance ‘x’ from the eye then it means that the far point of the eye has shifted from infinity to a point O situated at distance V from the eye.
Naturally light rays coming from a distant object (u = ∞) are focussed in front of the retina of the eye.
Two possible causes of myopia are :
(i) Either the power of the eye lens has become more than its normal value due to excessive curvature of the cornea (focal length of eye lens has decreased) or
(ii) Elongation of the eyeball due to some genetic defect.
To rectify this defect a concave lens of focal length f is used, which may form the virtual image of the distant object at the far point of the defective eye (i.e., u = - ∞ and v = - x ) so that now the defective eye may form the image at the retina.
Obviously, by using the lens formula, we have
⇒ f = -x
The ray diagram of the defective eye and its rectification are shown here.

Ans: Two main defects of vision, their cause and correction are as follows :
(1) Myopia - In myopia or near-sightedness (or short-sightedness), a person can see nearby objects clearly but cannot see distant objects distinctly. For a myopic eye, the far point is not at infinity but has shifted nearer to the eye.
Myopia arises due to either (i) excessive curvature of the cornea, or (ii) elongation of the eyeball.
Myopia can be corrected by using a concave lens whose focal length has the same numerical value as the distance of the far point of the defective eye.
(2) Hypermetropia - In hypermetropia or long-sightedness, a person can see distant objects distinctly but cannot see nearby objects so clearly. For a longsighted eye, the near point is not at 25 cm but has shifted away from the eye.
Hypermetropia arises either because
(i) the focal length of the eye lens is too large, or
(ii) contraction of the eyeball.
Hypermetropia can be corrected by using a suitable convex lens, which forms a virtual image of the object situated at 25 cm at the near point of the defective eye so that the eye lens can focus it on the retina.

Ans: A rainbow is a natural spectrum appearing in the sky after a rain shower. A Rainbow is caused by the dispersion of sunlight by tiny water droplets hanging in the atmosphere after a rain shower. The water droplets act like small prisms. As shown in the figure, the water droplets refract and disperse the incident sunlight. These rays are then reflected internally and finally refracted again and come out of raindrops. Due to the dispersion and internal reflection of light different colours reach the observer’s eye and a rainbow is seen.
An important point to be noted here is that a rainbow is always formed in a direction opposite to that of the Sun.

Ans: The atmospheric air layer just near the earth's surface is comparatively denser and the upper layers of the atmosphere are successively rarer and more rarer. Hence, a light ray passing through atmospheric air undergoes refraction. Since the physical conditions of air are not stationary, the apparent position of the distant object, as seen through the air, fluctuates. It is known as an effect of atmospheric refraction.
Light coming from a distant star entering into the earth’s atmosphere gradually bends towards the normal on account of atmospheric refraction. Hence, the star appears slightly higher than its actual position when viewed near the horizon.

Ans: As we go up and up in earth’s atmosphere, it goes on becoming rarer and more rarer. As a result, the atmospheric layer near the earth’s surface has a maximum refractive index and the refractive index gradually decreases with an increase in height.
When a light ray from a star enters into earth’s atmosphere, it travels from a rarer to denser medium and hence continues to bend towards the normal. As a result, an observer on Earth considers the apparent position of a star to be at a higher altitude.

Ans: There is no change in the image distance in the eye. The eye lens can adjust its focal length called accommodation. When object distance decreases, ciliary muscles contract and lens becomes thick and its focal length decreases. It facilitates near vision.

Ans: 

• The defect of vision is Presbyopia.
• A bifocal lens is required by such persons to improve their vision.

Structure and function of Bifocal lens 

• To view far-off objects, the upper part of the bifocal lens is a Concave or Diverging lens. 
• To facilitate or view nearby objects, the Lower part of the bifocal lens is Convex or Converging lens.

Ans: (a) The function of the given parts is stated below:
(i) The cornea is the outer bulged-out thin transparent layer of the eye and provides most of the refraction for the light entering into the eye.
(ii) The iris controls the size of the pupil of the eye.
(iii) The crystalline lens provides the finer adjustment of the focal length required to focus objects situated at different distances in front of the eye on the retina.
(iv) The ciliary muscles help in controlling the curvature of the crystalline lens and thus can change the power of the crystalline lens.

(b) In the early morning, the Sun is situated near the horizon. Light from the Sun passes through thicker layers of air and covers a larger distance in the Earth's atmosphere before reaching our eyes. While passing through atm osph blue light is mostly scattered away and the Sun appear reddish as shown in Fig.

The phenomenon is not observed by an observer on the Moon because the Moon has no atmosphere of its own and hence no scattering of light is possible.

Ans: (a) Power of accommodation: The ability of an eye lens to adjust its focal length to form a sharp image of the object at varying distances on the retina is called the power of accommodation. The ciliary muscles of the eye are responsible for changing the focal length of the eye lens.
(b) 

• The student is suffering from myopia shortsightedness or nearsightedness.
• When a student is seated in the back seat.

In this case, the student is suffering from myopia and has a short focal length of the eye lens.
(ii) When a student is seated in the front seat.

Causes:
(i) Excessive curvature of the eye lens
(ii) Elongation of eyeball.
The defect is corrected by using a concave lens of suitable power placed in front of the eye as shown below. It diverges the rays and forms a virtual image of a distant object at the far point of the myopic eye. These diverged rays enter the eye and form the image on the retina. Thus, the concave lens shifts the image back onto the retina instead of in front of it and the defect is corrected.

Ans:  (a) (i) Presbyopia 

• Presbyopia is a condition that occurs as a part of normal ageing.
  Due to loss of power of accommodation of the eye, with age, objects at a normal near working distance will appear blurry. The near point gradually recedes away. This defect of the eye is called Presbyopia. 
• Sometimes, a person may suffer from both myopia and hypermetropia. 

(ii) Presbyopia is caused due to

• weakening of ciliary muscles, and 
• eye lens becomes less flexible and elastic, i.e. reducing the ability of the eye lens to change its curvature with the help of ciliary muscles. 

(iii) A bifocal lens will be required to see nearby as well as distant objects. For myopic defects, the upper part of the bifocal lens consists of a concave lens used for distant vision and to correct hypermetropia, the lower part of the bifocal lens consists of a convex lens. It facilitates near vision.
(b) At the time of sunrise/sunset, the sun is near the horizon, so the sun's rays have to travel through a larger atmospheric distance. The fine particles of the atmosphere scatter away the blue component and other shorter wavelengths of the sunlight. As λ_b < λ_r, only red colour having a longer wavelength and is least scattered, reaches our eyes. Hence, the sun appears red at sunrise or sunset.

Ans: Hypermetropia or long-sightedness is that defect of vision in which the defective eye can see distant objects distinctly but is unable to see distinctly an object placed near his eye. For a nearby object, the image is formed behind the retina.
Two possible causes of this defect of vision are :
(i) The power of the eye lens is less (or the focal length of the eye lens is more) due to less curvature of the cornea.
(ii) The size of the eyeball is shortened.
The hypermetropia defect can be corrected by using a converging (convex) lens of appropriate power.
Ray diagrams showing the defect and its correction are

Ans: When a beam of white light passes through a glass prism it splits up into its constituent seven colours. The splitting of white light into its constituent colours when light passes through a dispersive medium is called “dispersion of light”. The seven colours, usually expressed as ‘VTBGYOR’ constitute the spectrum of white light.
The ray diagram showing dispersion is given here
Cause of Dispersion: When a beam of white light enters a glass prism (or any other dispersive medium), the light ray bends towards the normal one entering into the glass. However, different colours of light bend through different angles concerning the incident ray The red light bends the least while the violet light bends the most. So, rays of different colours emerge along different paths and, thus, become distinct. Hence, dispersion is caused and a spectrum is formed.

Ans: Cause of dispersion: From Snell’s law of refraction, the angle of refraction of light in a prism depends on the refractive index of the prism material. Moreover, the refractive index of the material varies inversely with the speed of light and also varies inversely with the wavelength of light. Hence, different colours of white light bend through different angles concerning the incident light, as they pass through the glass prism.
Newton Experiment: Consider a prism A. When a beam of white light falls obliquely on one of the faces of this prism, it splits up into seven constituent colours. The violet colour deviates the most and the red colour deviates the least.
If another identical prism B is placed in an inverted position concerning the first prism A, the constituent coloured rays that emerge out of prism A will be made to merge to come out as a beam of white light, as shown below.

Ans: The defect of vision is hypermetropia.
Focal length of lenses,

Ans: Recombination of Colours: The colours of white light split by a glass prism can be recombined to get white light by another identical glass prism. Newton; demonstrated this phenomenon of recombination of the coloured rays of a spectrum to get back white light.
(а) A triangular prism ABC is placed on its base BC.
(b) A similar prism A 'B 'C ' is placed alongside its refracting surface in the opposite direction, i.e. in an inverted position concerning the first prism as shown in the figure.
(c) A beam of white light entering the prism ABC undergoes refraction and is dispersed into its constituent seven colours.
(d) These constituent seven coloured rays are incident on the second inverted prism A'B'C' and get further refracted.
(e) The second prism recombines them into a beam of white light that emerges from the other side of the second prism and falls on the screen.
(f) This is because the refraction or bending produced by the second inverted prism is equal and opposite to the refraction or bending produced by the first prism. This causes the seven colours to recombine.
(g) A white patch of light is formed on the screen placed beyond the second prism. This proves the phenomenon of recombination of the spectrum of white light.

Ans. The retina behaves like a light-sensitive screen, on which real and inverted image of any object situated in front of the eye is formed.

Ans: It provides the fine adjustment of the focal length of the eye lens system to focus images of objects situated at different distances on the retina.

Ans: Iris controls the size of the pupil

Ans: Adjust/modify the shape (curvature) of the eye lens so that its focal length can be increased or decreased.

Ans: (i ) 25 cm, (ii ) infinity.

Ans: (i) Excessive curvature of the eye lens.
(ii) Elongation of the eyeball.

Ans: The old person is suffering from presbyopia.

Ans: Priya is suffering from myopia (near-sightedness).

Ans: (e) Red light is deviated the least and (ee) Violet light is deviated the most.

Ans: Black (dark).

Ans: Red light is least scattered by fog or smoke and can be easily seen from a distance.

Ans: (a) The iris controls the size of the pupil. It adjusts in size, and therefore, helps in regulating the amount of light entering the eye through a variable aperture ‘pupil’. When the fight is very bright, the pupil becomes very small. However, in dim fight, it opens up completely through the relaxation of the iris.
(b) The crystalline eye lens consists of a fibrous, jelly-like material. Its curvature can be modified to some extent by the ciliary muscles. The change in the curvature of the eye lens can change its focal length. When the muscles are relaxed, the lens is thin and its focal length is more (about 2.5 cm). When the ciliary muscles contract the eye lens becomes thicker. Consequently, the focal length of the eye lens decreases.

Ans: An optical prism has two triangular bases and three rectangular lateral surfaces, which are inclined to each other. The augle between its two lateral faces is called the angle of the prism.
The labelled diagram has been shown in Fig in which
∠PEN = ∠i = angle of incidence, ∠N'EF = ∠r = angle of refraction and ∠MFR = ∠e = angle of emergence
Whenever refraction of light takes place through a prism, the emergent ray bends towards the base of the prism. The angle between the directions of the incident ray and the emergent ray is called the angle of deviation D.

Ans: Stars are very far away from the earth and behave as almost a point object. The atmosphere is made of several layers and their refractive indices keep on changing continuously. So the light rays coming from the star keep on changing their paths continuously. As a consequence, the number of rays (or the light energy) entering the pupil of the eye changes with time and the stars appear twinkling.
A planet is comparatively nearer to the Earth and subtends a larger angle at the eye. So, it may be considered a collection of a large number of point-sized objects. Due to atmospheric refraction, the quantity of light coming from any one point-sized object changes with time but the total light entering the observer’s eye due to all these point objects remains almost the same. As a result, the planet does not twinkle.

Ans: The air becomes rarer as its height above the earth increases. Its refractive index decreases. A ray of light from the Sun when it enters the atmosphere at the horizon gets refracted from a rarer to a denser medium. The rays, therefore gradually bend towards the normal and the Sun appears to be raised.
As a result, the Sun is visible to an observer nearly two minutes before actual sunrise at the horizon. Similarly, even after actual sunset, the Sun is seen by us for about 2 minutes. Thus, in effect, the Sun is seen for 4 minutes more. It means that the apparent duration of the day (from sunrise to sunset) has increased by 4 minutes more than its actual duration.

Ans: (a) Dispersion: The splitting up of white light into its component colours is called dispersion.
Cause of dispersion: From Snell's law of refraction, the angle of refraction of light in a prism depends on the refractive index of the prism material. Moreover, the refractive index of the material varies with the speed of light. The different constituent colours of white light have different speeds in the transparent material of the prism. Hence for each colour/wavelength, the refractive index of prism material is different. Therefore, each colour bends (refracted) through a different angle concerning incident rays, as they pass through the prism. The red colour has maximum speed in the glass prism, so it is least deviated, while the violet colour has minimum speed so its deviation is maximum. Thus, the ray of each colour emerges along different paths and becomes distinct.
(b) Rainbow: It is an optical natural spectrum, produced by nature in the sky, in the form of a multicoloured arc. The rainbow is formed due to the dispersion of sunlight by water droplets suspended in the atmosphere after rainfall. These water droplets act like small prisms. The Sunlight enters the water droplets. At the point of incidence, it refracts and disperses then gets reflected internally and finally gets refracted again at the point of emergence as it comes out of the rain-drop.

Therefore, due to refraction, dispersion and internal reflection of the sunlight, different colours reach the observer’s eye along different paths and become distinct. It creates a rainbow in the sky.
Hence “Rainbow is an example of dispersion of sunlight.”
Necessary conditions for the formation of a rainbow.
(i) The presence of water droplets in the atmosphere, and
(ii) The sun must be at the back of the observer, i.e. the observer must stand with his back towards the sun.

Ans: Atmospheric Refraction: The refraction of light caused by the earth’s atmosphere due to gradual change in the refractive indices of its different layers by the varying conditions of it, is called atmospheric refraction.
(a) Twinkling of stars 
The hot layers (low densities) of air at a high altitude, behave as an optically rarer medium for the light rays, whereas the cold dense layers (high densities) of air near the earth’s surface, behave as an optically denser medium for the light rays. So, when the light rays (starlight) pass through the various layers of the atmosphere, they will deviate and bend toward the normal. As a result, the apparent position of the star is slightly different from its actual position. Thus, the stars appear slightly higher (above) than their actual positions in the sky.
The fluctuation in the positions of the stars occurs continuously due to the changing amount of light entering the eye. The stars sometimes appear brighter and at some other times, they appear fainter. This causes the twinkling of stars.(b) Advanced sunrise and delayed sunset The sun is visible 2 minutes before sunrise and 2 minutes after sunset because of atmospheric refraction. This can be explained below.

The figure shows the actual position of the sun S at the time of sunrise or sunset, just below the horizon while the apparent position S) above the horizon appears to us.
When the sun is slightly below the horizon, the light rays move through the different layers of varying refractive indices of air and get bent towards the normal. These rays appear to come from S) which is the apparent position of the sun. That is why, the sun is visible to us when it has - been actually below the horizon or before the actual crossing of the horizon by the sun at the time of sunrise or sunset. So, due to the atmospheric refraction, the phenomenon of advanced advance sunrise and delayed sunset is observed.

Ans: The pupil is a small circular transparent aperture whose size is controlled by the iris.

Ans: Cataract

Ans: Dispersion of light is the splitting of light into its component colours on passing through a dispersive medium e.g., a prism.

Ans: The scattering of light depends on the size of the scattering particle and the wavelength of light.

Ans: Myopia or Short-sightedness or Nearsightedness: The defect of vision is due to which an eye cannot see distant objects distinctly but can see nearby objects. This defect of the eye is called myopia.
Short-sightedness is caused due to 
(i) excessive curvature of the eye lens or
(ii) elongation of eyeball.

The image, in this case, forms in front of the retina, so the distant object looks blurred. For every myopic eye, there exists a far point beyond which a clear image cannot be seen. When the object lies at the far point, the image formed is focussed on the retina by the eye lens and therefore the object is visible to us.

The far point (F) of a myopic eye is less than infinity: The image is formed on the retina
The short-sightedness is corrected by using a concave lens of suitable power placed in front of the eye. It diverges the rays and forms a virtual image of a distant object at the far point of the myopic eye. These diverged rays enter the eye and form the image on the retina. Thus, the concave lens shifts the image back onto the retina instead of in front of it and the defect is corrected.
Ray diagrams to show the image formation in the case of:
(i) Defective eye In a myopic eye, the image of a distant object is formed in front of the retina (and not on the retina).
A concave lens is placed in front of the eye which forms a virtual image of a distant object at a far point (F) of the myopic eye.