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Light Reflection and Refraction
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24/08/2021 11:58 am
Topic starter
A convex lens of focal length 25 cm and a concave lens of focal length 10 cm are placed in close contact with one another.
(a) What is the power of this combination?
(b) What is the focal length of this combination?
(c) Is this combination converging or diverging?
1 Answer
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24/08/2021 12:06 pm
\(f_1\) = 25 cm = 0.25 m
\(P_1\) = \(\frac{1}{f_1}\) = \(\frac{1}{0.25}\) = 4D
\(f_2\) = -10 cm = -0.1 m
\(P_2\) = \(\frac{1}{f_2}\)
= \(\frac{1}{-0.1}\) = -10D
(a) Power of the combination:
P = \(P_1 + P_2\)
= 4D + (-10D)
= -6D
(b) Focal length of the combination:
f = \(\frac{1}{P}\) = \(\frac{1}{-6}\)
= -0.1666 m = -16.66 cm
(c) The combination has negative focal length, so it is diverging.