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CBSE Class 12-science Answered

show that no ray can pass through the prism whose refracting angle A is greater than twice the critical angle for the material of the prism.   (2008 cbse)
Asked by rohit dalal | 17 Feb, 2015, 10:19: AM
answered-by-expert Expert Answer
 
 
The figure above shows the refraction of light ray through a prism.
 
For a prism, we know that
 
begin mathsize 14px style straight r subscript 1 plus straight r subscript 2 equals straight A end style
We want that no ray should emerge out from the prism. So, the angle of emergence should be equal to 90º or it should be totally internally reflected into the prism.
This will happen when r2 = critical angle C
 
So, (r2)min = C
 
Now, we want that angle of prism should be 2C. So, (r1)min = C
 
Now, this will happen when the incident light ray is equal to grazing angle = 90º.
 
Hence, we can conclude that when the angle of the prism is greater than or equal to 2C, then no ray comes out of the prism.
Answered by Romal Bhansali | 19 Feb, 2015, 03:42: PM

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