A convex lens form a real image of a point object at a distance of 50cm from the convex lens - Physics - Ray Optics And Optical Instruments

Question

A convex lens form a real image of a point object at a distance of
50cm from the convex lens is placed 10cm behind the convex lens on
the image side On placing a plane mirror on the image side and
facing the concave lens ,it is observed that the final Image now
coincides with the object itself The focal length of the concave
lens is
1)50cm 2)20cm 3)40cm 4)25cm

Govind · Accepted Answer

Hi,

Dear student, you question is incomplete However, I am trying to
solve your problem
From the last part of
your question which says image coincides with the object
It means light rays retraces
its path So the rays are falling perpendicular to the plane mirror
This means that the images is formed at infinity by the concave
lens

The image for the first lens will be
the virtual object for the second lens
So, for concave lens,
u = + 40 cm
v = ∞

<img alt="" src=
"https://s3mn%20mnimgs%20com/img/shared/ck-files/ck_56f46573827e0%20png"
style="width: 323px; height: 252px;">
Using the lens formula we have

1f=1v-1u⇒1f=1-∞-140⇒f=-40 cm

Thus, the focal length of concave mirror is 40 cm