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CBSE Class 12-science Answered

please help
Asked by arpitarout132 | 29 Apr, 2022, 01:12: AM
Expert Answer
Figure shows a glass cube of edge 5 cm is placed on a point source so that the point source is act centre of bottom surface of cube.
 
As shown in figure , when light incident on side surface , total internal reflection takes place when angle of incidence exceeds the critical angle C.
 
Hence light will not come out of surface above the point where total internal reflection takes place .
 
We need to cover the area below the point where total internal reflection is taking place to block the light coming out of surface.
 
Critical angle C is given as ,  sin (C) = 1/μg  = 1/√2
 
where μg is refractive index of glass
 
 we get C = 45o .  Hence total internal reflection starts at half of height of cube i.e 2.5 cm at the side of face .
 
Hence minimum area to be covered = 4 × ( 2.5 × 5 ) = 50 cm2
Answered by Thiyagarajan K | 29 Apr, 2022, 09:39: AM
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