Science Ncert Exemplar for Class 10 Science Chapter 11 - The Human Eye And The Colourful World

Answer:

The person is suffering from myopia. He needs a concave lens and hence power would be negative.
Object distance, $u=-\infty$
Image distance, $v=-2 \mathrm{m}$
$$\begin{gathered} \mathrm{Using} \mathrm{lens} \mathrm{formula} \\ \frac{1}{v}-\frac{1}{u}=\frac{1}{f} \\ \frac{1}{f}=\frac{1}{-2}-\frac{1}{\infty } \\ f=-2 \mathrm{m} \end{gathered}$$

$P=\frac{1}{f}=-\frac{1}{2 \mathrm{m}}=-0.5 \mathrm{D}$

Hence, the correct answer is option B.

Answer:

The near point of his eye has receded away.
In hypermetropia, the near point of the eye moves away from 25 cm. Due to this, the person needs to keep the book at more than 25 cm to read it properly.
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Hence, the correct answer is option A.

Answer:

If Prism is kept with base BC at the bottom, then the emerged band of colour would show violet at the bottom. 
If Prism is kept with base BC at the top, then violet would be at the top, followed by indigo and blue.

Hence, the correct answer is option  B.

Answer:

At noon, the Sun is nearly overhead, so the light from the Sun would travel relatively shorter distance. At noon, the Sun appears white as only a little of the blue and violet colours are scattered.
Hence, the correct answer is option  A.

Answer:

A rainbow is a natural spectrum of seven colours appearing in the sky after a rain shower. It is caused by dispersion of sunlight by tiny water droplets, present in the atmosphere. The water droplets act like small prisms. They refract and disperse the incident sunlight, then reflect it internally, and finally refract it again when it comes out of the raindrop.

Hence, the correct answer is option  C.

Answer:

Due to the refraction of light by different layers of varying refractive indices, the apparent position of the source of light keeps on changing.
Due to this, stars appear to twinkle.

Hence, the correct answer is option  B.

Answer:

Violet and blue light get scattered more than lights of all other colours by the atmosphere.

Hence, the correct answer is option C.

Answer:

 All the colours of white light move at the same speed.

Hence, the correct answer is option  C.

Answer:

The red light is scattered the least by smoke or fog and hence it is visible clearly from large distances.

Hence, the correct answer is option B.

Answer:

Red colour scatters the least and hence travels the farthest. During sunset or sunrise, light has to travel a longer distance to reach us. 
Hence, only red light reaches us and the sky appears reddish.

Hence, the correct answer is option B.

Answer:

The bluish colour of water in deep sea is due to scattering of light, which is maximum for blue colour.
​Hence, the correct answer is option C.

Answer:

When the light rays enter the eye through a thin membrane, it enters through the transparent bulge on the front surface of the eyeball, called the cornea. Most of the refraction for the light rays entering the eye occurs at this outer surface of the cornea.

Hence, the correct option is B.

Answer:

The focal length of the eye lens increases when the eye muscles are relaxed and becomes thinner.

Hence, the correct answer is option A.

Answer:

Nearsightedness or myopia is the inability of the eye to see the far located objects clearly.

Hence, the correct option is C.

Answer:

(i) Myopic eye: In a myopic eye, the image of a distant object is formed in front of the retina and not at the retina itself. Here O is the far point of myopic eye and O' is the far point of normal eye.

(ii) Hypermetropic eye: In hypermetropic eye, the light rays from a closeby object are focussed at a point behind the retina. Here, N is the near point of a hypermetropic eye and N’ is the near point of a normal eye. 

Answer:

Since, the student sitting at the back of the classroom cannot read clearly the letters written on the blackboard, it means the student is suffering from Myopia. The doctor will suggest her, a concave lens of suitable power which will bring the image back on retina and therefore the defect will be corrected.
The ray diagram for the correction of this defect is as below:


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Answer:

Eyes have the ability to adjust its focal length which is known as the power of accommodation and we can see nearby and also the distant objects clearly. Relaxation of muscles makes the lens thinner and its focal length increases to make us see the distant objects clearly. The contraction of ciliary muscles increases the curvature of the eye lens and makes the eye lens thicker. Consequently, the focal length of the eye lens decreases. This enables us to see nearby objects clearly.

Answer:

(a) The negative power of the lens suggests that she must be suffering from myopia.

(b) Power, P = ─4.5 D, focal length, f = ?

Now,    P = 1/ f

⟹       f = 1/ P = 1/ −4.5 = −0.222 m = − 22.2 cm

(c) Myopia is corrected using a concave or divergent lens.

Answer:

When the second identical prism placed in an inverted position with respect to the first prism, the condition is fulfilled.

Answer:

Answer:

No, the light of stars enters the earth’s atmosphere and undergoes refraction continuously before reaching the earth. The atmospheric refraction takes place under a gradually changing refractive index. Since, starlight bends towards the normal; the apparent position of the star is slightly different from its actual position.

Answer:

Raindrops present in the atmosphere after rainfall serve as a prism and refract, disperse and internally reflect the incident sunlight followed by final refraction. The whole sequence of dispersion and internal reflection leads to the formation of the rainbow after rainfall in the sky.

Answer:

When sunlight passes through the atmosphere, the fine particles in air scatter the blue colour (shorter wavelengths) more strongly than red. The maximum scattered blue light enters our eyes. Hence, the colour of sky appears blue.

Answer:

During sunrise/sunset, the sun looks reddish because, at this stage, Sunrays travel longer distance in the atmosphere and the red light having the largest wavelength is scattered the least. During noon, the sun is directly overhead and sunlight travels relatively shorter distance and almost all colours of light are scattered equally. Thus, the sun appears white at noon.

Answer:

Structure of the human eye: Cornea is the transparent window of the eye that serves in the refraction of most of the light rays entering the retina. Iris is the coloured part of the eye that controls the amount of light by regulating the pupil size. The pupil is a hole shaped opening in the middle of the eye. The retina is the innermost layer of the eye and contains an outer pigmented layer and an inner nervous layer. It has photoreceptors (rods and cones). The optic nerve consists of nerve fibres from the innermost layer of the retina and serves to transmit the impulse to the brain. The eye lens is made up of fibrous, jelly-like material and transparent structure. It serves to form an inverted real image of the object on the retina. Vitreous humour is clear, a semi-solid structure that supports the eyeball.


Functioning of the human eye:  Light enters the eye through the pupil and then the eye lens converges these light rays on the retina. An inverted real image of the object is formed on the retina and image-electric signals are sent to the brain via the optic nerves. The brain reconstructs the erect image of objects and we can see the objects.

We are able to see nearby as well as distant objects due to the eye’s ability to adjust its focal length which is known as the power of accommodation. Relaxation of muscles makes the lens thinner and its focal length increases to make us see the distant objects clearly. Contraction of ciliary muscles increases the curvature of the eye lens and makes the eye lens thicker. Consequently, the focal length of the eye lens decreases. This enables us to see nearby objects clearly.

 

Answer:

Myopia: When a person can see nearby objects clearly but cannot see distant objects distinctly then the person is suffering from myopia. A person with this defect has the far point nearer than infinity. Myopia is also known as near-sightedness. 



Correction of Myopia: Wearing of concave lens diverges the light rays to help the image to be focused on the retina, hence myopia is corrected using the concave lens of suitable focal length.



Hypermetropia: When a person can see distant objects clearly but cannot see nearby objects distinctly, then the person is suffering from hypermetropia. The near point, for the person, is farther away from the normal near point (25 cm). Hypermetropia is also known as far-sightedness.



Correction of hypermetropia: Wearing a convex lens in front of a hypermetropic eye converges the light rays to help the image to be focused on the retina, hence this defect is corrected using the convex lens of suitable focal length.



 

Answer:

Here PE is the incident ray, EF is the refracted ray and FS is the emergent ray. A ray of light is entering from air to glass at the first surface AB. The light ray on refraction has bent towards the normal. At the second surface AC, the light ray has entered from glass to air. Hence it has bent away from normal. The specific shape of the prism makes the emergent ray bend at an angle to the direction of the incident ray. This angle is called the angle of deviation. In this case ∠D is the angle of deviation. The angle between the incident ray and emergent ray is called angle of deviation.

Answer:

Light from the Sun near the horizon passes through thicker layers of air and larger distance in the earth’s atmosphere before reaching our eyes. Near the horizon, most of the blue light and shorter wavelengths are scattered away by the particles. Therefore, the light that reaches our eyes is of longer wavelengths (reddish). This gives rise to the reddish appearance of the Sun.




At noon, the Sun is nearly overhead, and hence the light would  travel relatively shorter distance. At noon, the Sun appears white as only a little of the blue and violet colours are scattered.

Answer:

Splitting of light into its constituent colors is referred to as dispersion. When a narrow beam of white light falls on a triangular glass prism, the light of different colors have different refractive indices in the glass. However, the speed of light is the same irrespective of its colors. Different refractive indices of different colors of light lead to their different bending pattern. This causes splitting of white light into light of seven colors called as VIBGYOR. V= violet, I= indigo, B=blue, G=green, Y= yellow, O=orange, R=red.


The light of red color bends the least on passing through the prism and appears at the top while violet color bends through the maximum angle and appears at the bottom.

Answer:

There are several layers present in the atmosphere. The upper layer is optically rare, but the bottom layer is optically denser. When light passes through thicker and denser layers of the atmosphere, it bends towards the normal. As the stars are present very far away from us, the light coming from them get refracted multiple times and that results in the twinkling of the stars.