Physics Optics

A combination of two thin lenses in contact behave as converging lens of focal length 80 cm if one of the lense is convrging lens of focal length 20 cm,  the power of other lens will be

Explanation.
f= focul length of combination of two lenses
f1= focul length of ist lens
f2= focul length of 2nd lens
As 1/f =1/f1  + 1/f2
1/80 = 1/20 + 1/f2
1/f2 = 1/80 - 1/20
f2 = -26.66cm
 = -0.266 m

As Power of lens P= 1/f (where f will be in meters)

So
P=1/-0.266
= -3.75D