The human eye and the colourful world Worksheet-5

The human eye and the colourful world Worksheet-5

 

  1. The near point of a hypermetropic eye is 50 cm. Calculate the power of the lend to enable him to read a book at 40 cm ?   

 

  1. A person with a myopic eye cannot see objects beyond 1.2 m distinctly. What should be the type of corrective lens to restore proper vision ?

 

  1. What is the far point and near point of the human eye with normal vision ?

 

  1. 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?

 

  1. 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, distant vision and near vision ?

 

  1. 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?

 

  1. Why is a normal eye not able to see clearly the objects placed closer than 25 cm ?

 

  1. What happens to the image distance in the eye when we increase the distance of an object from the eye ?

 

  1. Why does the sky appear dark instead of blue to an astronaut?

 

  1. What is far point of a normal human eye ?

 

Answer:

  1. x' = 50 cm, D = 40 cm, P = ?

 

  1. distance of far point, x = 1.2 m

For viewing distant objects, focal length of corrective lens,

f = –x = –1.2m

          P = = – 0.83 D

The lens should be concave.

 

  1. For human eye with normal vision far point is at infinity and near point is at 25 cm from the eye.

 

  1. Since the child is having difficulty in reading the blackboard, he is suffering from myopia or short sightedness. In order to correct this defect, he should use spectacles with concave lens of suitable focal length.

 

  1.  For distance viewing,  f = ?,  P = –5.5 D

As    

For near vision section, correction = + 1.5 D

P = 1.5 D

From    = 66.7 cm

 

  1. Distance of far point, x = 80 cm,  P = ?

For viewing distant objects, focal length of corrective lens,

f = –x = –80 cm

The lens is concave.

 

  1. Because the focal length of eye lenses cannot be decreased below a certain minimum limit.

 

  1. For a normal eye the image distance in the eye is fixed and it is equal to distance of retina from the eye lens. When we increase the distance of the object from the eye, focal length of eye lens is changed on account of accommodating power of the eye so as to keep image distance constant.

 

  1. At such huge heights of the astronaut, there is nothing or hardly anything which can scatter the sunlight. Hence the sky appears dark.

 

  1. Infinity.