Question

प्रयाग / Experimen उत्तल लेंस की सहायता से एक उत्तल दर्पण की फोकस दूरी ज्ञात करें। Find the focal lengh of a convex mirror using a convex lens.​

Answer 1

Answer:

In an experiment a convex lens of focal length 15cm is placed coaxially on an optical bench in front of a convex mirror at a distance of 5cm from it. It is found that an object and its image coincide, if the object is placed at a distance of 20cm from the lens. The focal length of the convex mirror is: 

A

27.5cm

B

20.0cm

C

25.0cm

D

30.5cm

Correct option is

A

27.5cm

for convex lens:

u=−20cm,f=15cm

v1−u1=f1

v1=f1+u1

v1=151+(−201)

v=−20+15−15×20=5300=60cm

for mirror:

u=55cm,f=?

for the mirror to form image at '0':

u=R=2f

f=2

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