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Q.

A bulb is placed at a depth of27 m in water and floating opaque disc is placed over the bulb so that the bulb is not visible from the surface. The minimum diameter of the disc is

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a

42 m

b

6 m

c

27 m

d

12 m

answer is D.

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Detailed Solution

given that

A bulb is placed at the bottom of water tank of depth 27m

Question Image

A floating opaque disc of radius r is kept on the surface to make the bulb not visible

From BOQ

OBQ=C  critical angle

OB=27   OQ=r

We know

μwater  air =μwaterμair=1sin C

μw=4/3

43=1sin C

sin C=34

Question Image

We can have from BOQ

tan C=OQOB

37=r27

r=6 m

Diameter of disc d=2r=12 m

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