31. The drawing shows a crown glass slab with a rectangular cross section. As illustrated, a laser beam strikes the upper surface at an angle of 60.0°. After reflecting from the upper surface, the beam reflects from the side and bottom surfaces. (a) If the glass is surrounded by air, determine where part of the beam first exits the glass, at point A, B, or C. (b) Repeat part (a), assuming that the glass is surrounded by water instead of air.

 

w1377

 

Snell’s law giving us the critical angle for refraction is

 

 

 

If medium is air

 

 

If medium is water

 

 

So for the Glass Air case, the first angle of incidence is 60° > 41°, so light will be totally internally reflected.  The second angle of incidence by geometry will be 30°<41° so some light will pass into the air, so B

 

For the Glass water case the first angle is still 60°<61.1°  so some light will pass and A

 

For Glass-Air, Point B is the first point light leaves glass!

For Glass-Water, Point A is the first point light leaves glass!

 

 

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This page last updated on July 1, 2018