Question

Two thin lenses of focal length +10cm and -5 cm are kept in contact. What is the focal length and power of the combination ?

Answer 1

Hey friend, Here the focal lengths are f1 and f2
so , f1=10 cm=0.1 m
f2=-5 cm= -0.05m
Power of combination =P=P1+P2
P1=1/f1=1/0.1m
P1=10D
P2=1/0.05
P2= -20D

P=10D-20D
P= -10D

Hope it will help you friend

Answer 2

The focal length and power of a lens are related as follows:

Power[in diopters(D)] = 1/[focal length(f) in m]

The focal length of the two lenses are given as:

f1 = 10cm = 0.1m

f2= -5cm = 0.05m

Therefore, the powers can be determined as:

P1 = 1/0.1 = 10 D

P2 = 1/-0.05 = -20 D

Therefore, since powers are additive we get the power of the given combination as:

P = P1 + P2 = 10+(-20) D = -10 D

The focal length of the given combination is: Reciprocal of the power of the combination

f = 1/-10 = -0.1m or 10cm

Answer: The focal length and the power of the given combination are -0.1m/10cm and -10D respectively.