Question

Two thin lenses of power +5D and -3D are in contact. What is the focal length of the combination ?

Answer 1

total power after combination=+5-3=+2

power=1/f(in metre)
2=1/f
f=+0.5

Answer 2

Answer: 0.5

Explanation:

Power 1= P1= 5D

Power 2= P2= -3D

Power of combination= P1+P2

=5D-3D

=2D

Now we have to find the focal length of the combination

Thus,

Focal length is given by f

Where f =focal length

Thus, f=1/power

Thus

f= 1/2D

=0.5

Here, the power of a lens is defined as the reciprocal of its focal length.

That is D = 1/f

The focal length is defned as the distance between the centre of a mirror or a lens band its focus.