Question

the image of an object as formed by a lens is of magnification -1.If distance between the object and image is 60cm,then find the focal length of the lens? if the object is moved 20cm towards the lens , where would the image be formed? 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 a 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.