A small coin is resting on the bottom of a beaker filled with a liquid. A ray of light from the coin travels upto the surface of the liquid and moves along its surface (see figure). [AIPMT (S) 2007] |
How fast is the light travelling in the liquid? |
A) \[1.8\times {{10}^{8}}m/s\]
B) \[2.4\times {{10}^{8}}m/s\]
C) \[3.0\times {{10}^{8}}m/s\]
D) \[1.2\times {{10}^{8}}m/s\]
Correct Answer: A
Solution :
Key Idea : Critical angle is the angle of incidence in denser medium for which the angle of refraction in rarer medium is . |
As shown in figure, a light ray from the coin will not emerge out of liquid, if . |
Therefore, minimum radius R corresponds to . In |
or |
or |
Given, |
Hence, |
or or |
But or |
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