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Asked by adjacentcalliber10 | 10 May, 2019, 11:28: AM
answered-by-expert Expert Answer
I consider the maximum angle of incidence θmax is the angle of incidence at glass surface perpendicular
to the plastic thin film as shown in figure.
It is not possile to get θmax at the glass surface parallel to plastic thin film, as explained later.
 
If total internal reflection is taking place at glass-plastic film interface, then we have  1/sinθC = 1.5/1.2  ................(1)
 
where θC is the critical angle at glass-plastic  film interface.
 
From eqn.(2) we get θC = 53° ,  then as shown in figure, angle of refraction θr corresponding to the ray
undergoing total internal reflection is 37°.
 
( Since this critical angle θC = 53° , if the incident beam enters through glass surface parallel to plastic film,
we require angle of refraction also 53°, which is impossible to get in glass-air interface with refractive index of glass
as 1.5. Even for grazing incidence ( angle of incidence 90°), we get angle of refraction 42° )
 
As per snell's law at glass surface as mentioned earlier,  ( sin θmax / sin θr ) = 1.5  ................(2)
 
from eqn.(2) , using θr = 37° , we get θmax = 65° .
Answered by Thiyagarajan K | 11 May, 2019, 21:20: PM

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