When two lenses are in contact, the power of the combination
(P‌combined ‌) is given by the sum of the powers of the individual lenses
(P1. and
P2 ). The power of a lens is the reciprocal of its focal length in meters
(P=‌., where
P is in diopters and
f is in meters).
The given powers of the lenses are:
The first lens:
+1DThe second lens:
−2DWe calculate the combined power as follows:
P‌combined ‌=P1+P2=+1D+(−2D)=−1DThe negative sign of the combined power indicates that the combination acts as a diverging lens. The focal length of the combined lens system can be found using the formula
f‌combined ‌=‌ :
f‌combined ‌‌‌=‌=−1m=−100‌cm The negative focal length confirms that the lens acts as a diverging lens. However, we usually refer to the magnitude of the focal length for lenses. Thus, the focal length of the combination is
100‌cm, but since it's a diverging lens, the correct interpretation is:
Option A: diverging lens of focal length
100‌cm