Question

pls help me.


A real object is placed on the principal axis at a distance of 20 cm from a convex lens. If the size of the image is the twice of the object , then read the following and choose the answer

1. the power of the lens is 7.5 D if the image is real.

2. the power of the lens is 2.5 D if the image is real.

3. the power of the lens is 7.5 D if the image is virtual.

4. the power of the lens is 2.5 D if the image is virtual.​

Answer 1

Object distance, u = -20cm

Given, Size of the image = 2 × size of object

=> h' = 2h

We know that,

m = h'/h = -v/u

=> 2h/h = -v/-20

=> 2 = v/20

=> v = +40cm (virtual)

Applying lens formula:

1/v - 1/u = 1/f

=> 1/40 - 1/-20 = 1/f

=> 1/40 + 1/20 = 1/f

=> 3/40 = 1/f

=> f = 40/3 = +13.34cm

We know that,

Power, p = 1/f

=> p = 1/13.33

=> p = +0.75 D

Since the image formed behind the mirror (i.e positive)

So, the power of the lens is 7.5 D if the image is virtual.

Answer 2

Answer:

opt 2

image is real

bcz in lenses magnification is v/u and not -v/u