Question

If the power of a lens is -4.0 D, then it
means that the lens is a

A. concave lens of focal length -50 m

B. convex lens of focal length +50 cm

C. concave lens of focal length -25 cm

D. convex lens of focal length -25 m​

Answer 1

Answer:

C. Concave lens of focal length -25 cm

Answer 2

Given:- P= -4.0D

To find:- Type of lens and focal length

Solution:-  

According to the formula and optics the power of the lens is given as follows:-

P = 1/f  

where P is the power of the lens

f is the focal length of the lens.

Now as P = -4D

P = 1/f

-4 = 1/f

f = -1/4

f = -0.25  

focal length is generally expressed in m therefore focal length is 25cm.

As the focal length of the given problem is negative the type of lens is concave.

A convex lens has a positive focal length and positive power of the lens.

concave lens of focal length -25 cm is the right answer.