Question

A concave lens of focal length 20 cm is placed 20 cm in front of a plane mirror as shown below. Light from a very far
away source is falling on the lens. The position of final image formed by combination is (Consider the size of concave
lens is sufficiently large)​

Answer 1

Explanation:

From lens formula,

f

1

=

v

1

−

u

1

⇒

v

1

=

f

1

+

u

1

=

20

1

−

12

1

=

4

1

(

5

1

−

3

1

)

=−

4

1

(

15

2

)

Image distance for convex lens, v=−30cm

Distance of image formed by lens from mirror =40cm

∴ Final image formed behind the mirror =40cm

Answer 2

Given:

A concave lens of focal length 20 cm is placed 20 cm in front of a plane mirror as shown below. Light from a very far away source is falling on the lens.

To find:

Position of final image ?

Calculation:

Since the light rays are coming from infinity , after refraction through the concave lens, a point sized image will be produced at the focus of the lens.

So, 1st image will be 20 cm in front of concave lens.

Now, this image will act as the virtual object for the plane mirror placed 20 cm from the concave lens.

So, distance of the virtual object from plane mirror is (20 + 20) = 40 cm.

Now, we know that image is formed by a plane mirror at a distance equal to object distance. So the final image will be produced 40 cm from the plane mirror.

$\boxed{ \bf final \: image \: at \: 40 \: cm \: from \: plane \: mirror}$