NCERT Solutions For Class 10 Science Chapter 10 The Human Eye and The Colourful World

When the ciliary muscles are relaxed, the eye lens becomes thin. This results in the increase of the focal length, and distant objects are clearly visible to the eyes. 

To see the nearby objects clearly, the ciliary muscles contract which makes the eye lens thicker. This results in reduction of focal length of the eye lens, and the nearby objects become visible to the eyes.

Therefore, the focal length can be adjusted by the human eye lens to view both distant and nearby objects on the retina. This ability of the eye refers to the power of accommodation of the eye.

The person can clearly see the objects which are near, but he cannot see the objects beyond 1.2m. This arises as the image of an object beyond 1.2m

is obtained in front of the retina and not at the retina, as displayed in the figure given below.

To correct this defect of vision, a concave lens should be used. The image will be brought back to the retina by the concave lens as shown in the given figure.

The near point of the eye refers to the minimum distance of the object from the eye that can be seen clearly without any strain to the eye. This distance is 25cm, for a normal human eye.The far point of the eye refers to the maximum distance to which the eye can clearly see the objects, without getting any strain to the eye. The far point of the normal human eye is at an infinite distance.

A student has difficulty in reading the blackboard while sitting in the last row. It means that he is unable to see distant objects clearly.

The student is suffering from myopia. This defect can be corrected using a concave lens.

The correct answer is (b) accommodation, 

The focal length of the eye lens is changed by the human eye to see objects situated at various distances from the eye. This is possible due to the power of accommodation of the eye lens.

(d) The image of an object is formed by the human eye at its retina.

 (c) The least distance of distinct vision refers to the minimum distance to see a clear and distinct image. This distance is 25cm for a young adult with normal vision.

(c) The curvature of the eye lens is changed by the relaxation or contraction of ciliary muscles.

The focal length of the eyes is changed due to the change in curvature of the eye lens. Therefore, the change in focal length of an eye lens occurs due to the action of ciliary muscles.

The size of eyes cannot increase or decrease because of which the image distance remains constant.

When the object’s distance from the eye is increased, the image distance in the eye does not change. The increase in the object distance is balanced by the change in the focal length of the eye lens. The focal length of the eye changes in such a manner that the image is always formed at the retina of the eye.

Stars emit their own light and they shine due to the atmospheric refraction of light.

Stars are situated very far away from the earth. Therefore, they are considered as point sources of light. When the light coming from stars enters the earth’s atmosphere, it gets refracted at different levels because of the variation in air density present at different levels of the atmosphere. 

When the star light refracted by the atmosphere comes more towards the surface of earth, it appears brighter than when it comes less towards the earth. Clearly, it appears as if the stars are twinkling at night.

 Planets do not twinkle because they appear larger in size than the stars as they are comparatively closer to earth. Planets are a collection of many point-sized sources of light. The various regions of these planets produce either brighter or dimmer effect in such a way that the resultant of brighter and dimmer effect is zero.

Clearly, the twinkling effects of the planets are nullified and that is why they do not twinkle.

During sunrise, the light rays from the sun travel a greater distance in the earth’s atmosphere before reaching our eyes. In this journey, the shorter wavelengths of lights are spread out and only the longer wavelengths reach our eyes. 

Since blue colour has a short wavelength and red colour has a long wavelength, red colour can reach our eyes after the atmospheric scattering of light. Thus, the sun appears to be reddish early in the morning.

Since there is no atmosphere in outer space that can scatter the sunlight, the sky appears dark instead of blue to an astronaut. 

Since the sunlight is not scattered in space, no scattered light reaches the astronaut’s eyes and the sky appears black to them instead of blue

A person who suffers from hypermetropia can see distinct objects clearly but faces difficulty in seeing nearby objects clearly. This happens because the eye lens focuses the incoming divergent rays beyond the retina.

This defect of vision can be fixed using a convex lens. A convex lens of a suitable power converges the incoming light in such a manner that the image is formed on the retina, as shown in the given figure.

The person will be able to clearly see the object kept at 25cm (near point of the

normal eye), if the image of the object is formed at his near point, which is given as 1m

Object distance, u=−25cm

Image distance,

v=−1m=−100cm

Focal length,

F