The human eye and the colourful world Worksheet-4

The human eye and the colourful world Worksheet-4

 

  1. The far point of a myopic eye is at 50 cm. Calculate the power of the lens to correct his vision.

 

  1. The far point of a myopic eye is 80 cm in front of the eye. What is the power of the lens required to enable him to see very distant objects clearly?

 

  1. The near point of a hypermetropic eye is at 75 cm from the eye. What is the power of the lens required to enable him to read clearly a book held at 25 cm from the eye ?

 

  1. A person cannot see the objects distinctly, when placed at a distance less than 100 cm. What is the power of the spectacles that he should use to see clearly the objects placed at 25 cm?

 

  1. A short sighted person cannot see clearly beyond 2 m. Calculate power of the lens required to correct his eye to normal vision.

 

  1. A myopic person can see things clearly only when they lie between 10 cm and 100 cm from the eye. Which lens will enable him to see the moon clearly ?

 

  1. The distance of distinct vision of a person is 50 cm. He wants to read a book placed at 25 cm. What should be the focal length of the spectacles?

 

  1. A person wants to read a book placed at 20 cm, whereas near point of his eye is 30 cm. calculate the power of the lens required.

 

  1. The far point of a myopic person is 4 meter. Calculate the power of the lens he requires to look at the stars.

 

  1. A myopic person uses specs of power –0.5D. What is the distance of far point of his eye ?

 

Answer:

  1. Here, distance of far point x = 50 cm, power P = ?

As f = –x            ∴ f = –50 cm

As         

Negative sign indicates that the lens used is concave.

 

  1. Distance of far point x = 80 cm, 

power, P = ?

          f = –x        f = –80 cm

         

 

  1. Here, distance of near point, x' = 75 cm 

distance of book,

power P = ? & focal length f = ?

We know that

   

As f is positive in this case so the corrective lens is convex.

 

  1. Given :  distance of near point, x' = 100 cm

distance of object, d = 25 cm.

power, P = ?, focal length, f = ?

As    

         

As    

         

As f is positive in this case so the corrective lens is convex.

 

  1. distance of far point, x = 2m., power P = ?

As     f = –x

          ∴ f = –2 m = –200 cm

As    

         

 

  1. Distance of far point, x = 100 cm.

To see the moon clearly, he has to use spects with lens of focal length,

f = –x = –100 cm

 

  1. x' = 50 cm, d = 25 cm, f = ?

From = 50 cm.

  1. d = 20 cm, x = 30 cm, P = ?

          = 60 cm.

          = 1.67 D.

 

  1. x = 4m, P = ?

f = –x = –4m

 

  1. P = –0.5 D, x = ?