Question

factorization questions are 4x square subtract 12 x plus 9​

Answer 1

Answer:

19. Three lenses in contact have a combined focal length of 12 cm. When the third lens is removed, the combined focal length is 60/7 cm. The third lens is

(1) A converging lens of focal length 30 cm

(2) A converging lens of focal length 60 cm

(3) A diverging lens of focal length 30 cm

(4) A diverging lens of focal length 60 cm

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Physics

Ray Optics and Optical Instruments

Combination of Lenses

Three lenses in contact hav...

PHYSICS

Asked on December 30, 2019 byRahamath Mudrabettu

Three lenses in contact have a combined focal length of 12cm. When the third lens is removed the combined focal length becomes 60cm. The third lens is :

A

a converging lens of focal length 30 cm

Your Answer

B

a converging lens of focal length 60 cm

C

a diverging lens of focal length 30 cm

Correct Answer

D

a diverging lens of focal length 60 cm

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VIDEO EXPLANATION



ANSWER

Let f1, f2 and f3 be three focal lengths of the lenses.

f=12

f1=f11+f21+f31

When the third lens is removed, f′1=f11+f21

f′=

f′=760

121=7601+f31

⇒f3=−30

Since f3 is negative, it is a diverging lens having focal length 30cm.