Human Eye and Colorful World
What is meant by power of accommodation of the eye? [2010,2012,2014,2015]
The images of objects at different distances from the eye are brought to focus on retina by changing the focal length of the eyelens. This is known as power of accommodation of the eye.
What is the near point and the far point of a normal human eye? [2011, 2012, 2014]
Near point is 25 cm and far point is infinity.
A student has difficulty reading the blackboard while sitting in the last row. What could be the defect the child is suffering from? How can it be corrected? 
The child is suffering from myopia. It can be corrected by using spectacles with concave lenses of suitable focal length.
A person with a myopic eye cannot see objects beyond 1.2 m distinctly. What should be the type of the corrective lens used to restore proper vision?
A concave lens of focal length 1.2 m should be used to restore proper vision.
Chapter End Questions
The human eye can, focus objects at different distances by adjusting the focal length of the eyelens. This is due to
The human eye forms the image of an object at its
The least distance of distinct vision for a young adult with normal vision is about 
(a) 25 m
(b) 2.5 cm
(c) 25 cm
(d) 2.5 m
(c) 25 cm
The change in focal length of an eyelens is caused by the action of the 
(c) ciliary muscles
(c) ciliary muscles
A person needs a lens of power -5.5 D for correcting his distant vision. For correcting his near vision, he needs a lens of power +1.5 D. What is the focal length of the lens required for correcting
(a) distant vision?
(b) near vision?
The far point of a myopic person is 80 cm in front of the eye. What is the nature and power of the lens required to correct the problem?
Given: Distance of far point = 80 cm, P= ?
For viewing far-off objects, the focal length of corrective lens,
Make a diagram to show how hypermetropia is corrected. The near point of a hypermetropic eye is 1 m. What is the power of the lens required to correct this defect? Assume that the near point of the normal eye is 25 cm. 
The required diagram is shown below:
Why is a normal eye not able to see clearly the objects placed closer than 25 cm? 
A normal eye is not able to see clearly the objects placed closer than 25 cm because thefocal length of eyelens cannot be decreased below a certain minimum limit.
What happens to the image distance in the eye when we increase the distance of an object from the eye? 
In order to focus the eye on objects situated at varying distances, the focal length of the eye lens is changed by the action of ciliary muscles.
Why do stars twinkle? [2011,2012,2015]
The rays of starlight pass through many layers of atmosphere. These layers have different densities and hence, different refractive indices. So the directions of rays reaching the eye keep changing causing the image of the star to shift in random directions or eyen disappear for an instant. Thus, the brightness of the star seems to change.
Explain why the planets do not twinkle. [2011,2012,2015]
Planets are comparatively much closer to the earth and are thus seen as extended sources of light. Being of larger size, planets can be taken as a collection of a number of point-sized sources of light which nullify the twinkling effect of each other.
Why does the sun appear reddish early in the morning? [2011,2012]
At the time of shnrise, the rays of the sun have to travel a larger atmospheric distance. As the wavelength of red colour is the largest of all the colours of sunlight, most of the blue colour and other colours are scattered away. Only red . colour which is,,least scattered, enters into our eyes. Hence, the sun appears reddish at the time of sunrise.
Why does the sky appear dark instead of blue to an astronaut? [2011,2012]
This is because there is no scattering of light due to the absence of atmosphere in the free space.