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Light Reflection and Refraction
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24/08/2021 12:13 pm
Topic starter
Two lenses A and B have focal lengths of +20 cm and, -10 cm, respectively.
(a) What is the nature of lens A and lens B?
(b) What is the power of lens A and lens B?
(c) What is the power of combination if lenses A and B are held close together?
1 Answer
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24/08/2021 12:18 pm
\(f_A\) = +20 cm = +0.2 m
\(f_B\) = -10 cm = -0.1 m
(a) Lens A is a convex lens (positive focal length) and lens B is a concave lens (negative focal length).
(b) \(P_A\) = \(\frac{1}{f_A}\) = \(\frac{1}{+0.2}\)
= +5D
\(P_B\) = \(\frac{1}{f_B}\) = \(\frac{1}{-0.1}\)
= -10D
(c) Power of combination
P = \(P_A\) + \(P_B\)
= +5D + (-10D)
= -5D