When two lenses of focal length 10 centimetre and -5 cm placed in contact then find the net power

The net power is -0.1.

Explanation:

Power of lenses is defined as the ability of lenses to converge or diverge the incident light. Mathematically, the power of any lens can be determined by the reciprocal of the focal length of the lens.

Power = 1/f

As here two lenses are given, so the power for first lens with focal length 10 cm is P1 = 1/10=0.1.

Similalrly, for the second lens have the focal length of -5cm, so the power of this lens will be P2 = -1/5=-0.2.

Then the net power = P1+P2=0.1-0.2=-0.1.

Thus, the negative net power of the connecting lens indicate that the incident light will be diverged. So the net power is -0.1.