Question

a man can see clearly up to 3 M prescribe a lens for his spectacles so that he can see clearly up to 12 metres

Answer 1

Answer:

The man need -4 D power convex lens to see up to 12 m

Explanation:

According to the problem the the man can see up to 3 M

therefore V = - 3 M

now to see the objects present up to 12 M

therefore U = - 12 M

We need to find the focal length ,f

1/f = 1/V - 1/U

   = 1/(-3) - 1/(-12)

   = - (4-1)/12

  = - (1/4)

Now we need to calculate the power

P = 1/f

   = 1 / - (1/4)

   = - 4 D