Question

A student focused the image of a candle flame on a screen using convex lens as:
Position of candle- 11.8 cm
Position of convex lens-51 cm
Position of screen-90.2 cm
Calcu
late the focal length and draw a ray diagram for the formation of the image for this case

Answer 1

Given:
Object distance:u=
=position of convex lens - position of object
=51-11.8
=39.2 cm
= - 39.2cm by sign convention

IMAGE distance = V=position of screen- position of lens
=90.2-51
=39.2cm

Lens formula:
1/f = 1/v-1/u
= 1/39.2 + 1/39.2
=2/39.2
f= 39.2/2
=18.1 cm
Therefore focal length of convex is 18.1cm and object is placed at 2f (C) that is on 39.2 cm.Hence image also will be formed on C.

Nature of image:
Real
Inverted
Same size
Refer attachment for diagram