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Lecture

# Critical angles, rainbows, thin film interference

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School
Department
Physics
Course
PH 122
Professor
Brian Jones
Semester
Spring

Description
25 April Light of wavelength 500 nm in air enters a glass block with index of refraction n=1.5. When the light enters the block, which property of light does not change? Frequency If energy is conserved, the frequency must remain the same What Determines Color? The wavelength of a light wave is 700 nm in air; this light appears red. If this wave enters a pool of water, its wavelength becomes: λ = λair.33 = 530 nm If you are swimming underwater, this light still appears red. Given this, what property of a light wave determines its color? Frequency Critical Angles n2 θ = sin (1 ) c n1 What are the critical angles for Water (n=1.33) – 48° Diamond (n=2.42) – 24° Mirages are caused by total internal reflection. Making a Rainbow The sun is behind your back when you see a rainbow. Dispersion causes different colors to refract inside water droplets at different angles. Most of the light refracts into the air at the back of the droplet, but a little reflects back into the drop. Dispersion separates the colors even more as the rays refract back into the air. Seeing a Rainbow You see a rainbow with red on the top, violet on the bottom. Red light is refracted predominantly at 42.5°. The red light reaching your eye
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