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Laser light is shown passing into a hemi-cylindrical dish filled with water. The light enters the water (at the curved side of the dish) along the normal line; no bending occurs upon entry. The light continues through the water along a straight line until it reaches the boundary with air (at the flat side of the dish). The angle of incidence in the water is 50°. Since the angle of incidence is greater the air-water critical angle of about 48°, it undergoes total internal reflection (TIR). Rather than refract out of the dish of water at the flat side, the laser light is seen reflecting and exiting along the curved side of the dish.


For more information about the physics of total internal reflection and the critical angle, visit The Physics Classroom Tutorial -

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Taken on October 13, 2010