Question

The image of an object formed by a lens is of magnification -1. If the distance between the object
and its image is 60cm, what is the focal length of the lens? If the object is moved 20cm towards
the lens, where would the image be formed? State reason and also draw a ray diagram in support
of your answer​

Answer 1

Answer:

f= 15cm

Image will be formed at 6cm on left side of the lens.

Explanation:

Since image is not magnified, the object is placed at 2f. So, u=v i.e. object distance = Image distance. (-1= v/u. )

-ve sign indicates that the lens is concave lens.

Given: v+u=60cm.

Substitue u=v.

u= 30cm.

Now we also know that the obejct is at 2f

so , 2f= u

Therefore f= 15cm

Now, new object distance= u2 = u -20 cm.

= 30 -20

=10 cm.

Using lens formula,

1/f=1v- 1/u.

f= - 15cm for concave lens.

u= - 10 cm

Find v.

v comes out to be -6cm. Which means it is on the same side as the obejct.

Accordingly draw the ray diagram.