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On placing a transparent glass cube on the printed page of a book, it was found that the covered printed words are not visible from any of the lateral sides of the cube. Explain why? |
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Answer» Solution :The REFRACTIVE index of glass relative to AIR, `mu = 1.5.` So the critical angle for the two media, `theta_(c) = sin^(-1)""((1)/(mu)) = sin^(-1)""((1)/(1.5))` `= 42^(@) ("nearly")` There is always a thin layer of air between the glass cube and the paper [Fig. 2.79]. If any ray from printed words is INCIDENT on the lower surface of the cube even with the maximum angle of `90^(@)`, then it enters glass with a maximum angle of refraction `42^(@)`. Then, the minimum angle of incidence of the ray on the side face of the cube becomes `(90^(@) - 42^(@)) or 48^(@)`. A total internal reflection OCCURS as this angle is greater than the critical angle, and no ray can emerge out of the side face. So, the printed words are not visible to an eye placed outside any of the side faces. 
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