Question

two thin concave lenses each of focal length 30 cm are placed coaxially in contact.if a point object is placed 15 cm from combination then final image is formed at a distance from lens?​

Answer 1

Answer:

7.5 cm

Explanation:

The combined focal length of the lenses can be given by

$1/F=1/f1+1/f2$

since both lenses are concave (diverging)

$1/F=-(1/30+1/30)\\1/F=-1/15$

Acc to the lens formula

$1/u+1/v+1/F\\1/15+1/v=-1/15\\1/v=-(1/15+1/15)\\1/v=-2/15$

v=-7.5cm ("-" represents the distance from the lens)

therefore the image is formed 7.5 cm away lenses.