Question 9
Numeric Type
A point source of light is kept on the principal axis of a concave mirror at a distance of 5cm from the pole. This results in a real image. Now, the mirror is moved along
axis away from the source by distance of 15cm, the new image formed is such that the gap between it and the source is exactly same as what is used to be before the
movement. What is the focal length of the mirror? (in cm)
Answer:
Formula for refraction through the lens is,
u=−15cm and f=±10cm
v
1
+
15
1
=
10
1
v=30cm
As v is positive,the image I
1
,is formed to the right of lens.
The convex mirror is to be placed to the right of lens in such a way hat light rays fall on the mirror normally.
Then only the light rays will retrace their path and form final image on the object itself.
This shows that image I
1
formed by the lens should be at the center of curvature of the mirror i.e,PI
1
=R=2f=24cm
As ∣v∣=OI
1
=30cm. ,Hence OP=30−24=6cm
Thus,the convex mirror should be placed at a distance of 6cm to the right of lens