Question

What will be the focal length of the lens combination when a lens of power 5D is kept in contact with a lens oj power -2D?
(a) 45cm
(b) 33.3 cm
(c) 25cm
(d) 16.5cm

Answer 1

Answer:

The answer is 33.3 cm

Explanation:

Given data:

Power P1 = 5D

Power P2 = -2D

Now combine the power of the system:

Combined power = P1+P2

                          = 5 D - 2D = 3D

we know that power is equal to the inverse of focal length.

Power P=1/f in meters

Focal length = 1/P

As we know  

1 m = 100 cm

Therefore in centi meters it will be.

100/3 =33.3 cm

The focal length is 33.3 cm.