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Refraction of light

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Why does this straw look broken? Light reflecting off the straw above the water goes straight to your eye. When light crosses a boundary between materials, it may change direction through refraction. Refraction Light reflecting off the straw below the water bends—refracts—as it passes into the air, so it appears to have come from a different direction.

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Light may reflect at the boundary between two materials, staying in the original medium. Light may refract as it crosses a boundary between two materials, changing its direction. Reflection and refraction

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When you look out a window at night, you can see your reflection in the glass. Since the light also refracts, someone standing outside can see you too. Reflection and refraction In this window you can see items inside the store AND the woman’s reflection. This is because the light reflects and refracts at the same time.

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Refraction is a property of all waves. Refraction occurs at a boundary between two materials. Light refracts because it travels at different speeds in different materials. What causes refraction?

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Angle of incidence Angle of refraction Visualizing refraction The angle of incidence lies between the incident ray and the normal. The angle of refraction lies between the refracted ray and the normal.

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When light slows down it bends towards the normal. When light speeds up it bends away from the normal line. The direction of refraction

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The effects of refraction Light from this fish bends away from the normal as it passes from water into air. Where does the observer THINK the fish is located?

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The observer thinks the fish is located farther to the left. The effects of refraction

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The amount of refraction depends on the combination of materials. Which combination of materials results in greater refraction? air to glass air to water The amount of refraction

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The amount of refraction depends on the combination of materials. Which combination of materials results in greater refraction? air to glass Notice the greater deflection of the refracted ray in the glass. The amount of refraction

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The index of refraction Every light medium has an index of refraction n that determines how much it will refract light. The value of n is never less than 1, and has no units.

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On the diagram below, label the incident angle, incident ray, refracted angle, refracted ray, and the normal. Assessment

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On the diagram below, label the incident angle, incident ray, refracted angle, refracted ray, and the normal. Assessment

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If a light beam shines from glass ( n = 1.5) into air ( n = 1.003), does it deflect towards the normal or away from it?

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If the glass is replaced with diamond ( n = 2.4), does the beam deflect more or less? Assessment Glass has a higher index of refraction than air so the beam bends away from the normal.

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If a light beam shines from glass ( n = 1.5) into air ( n = 1.003), does it deflect towards the normal or away from it? If the glass is replaced with diamond ( n = 2.4), does the beam deflect more or less? The difference in the index of refraction is greater so the beam deflects more. Assessment Glass has a higher index of refraction than air so the beam bends away from the normal.

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Snell’s law and the critical angle

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The refracted angle depends on: the angle of incidence the index of refraction of each material The angle of refraction can be calculated using Snell’s law. Snell’s law

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n i = index of refraction of 1 st material n r = index of refraction of 2 nd material θ i = angle of incidence θ r = angle of refraction sin is the sine function Snell’s law

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A light beam travels from glass into air. Set n i = 1.52 (glass) Set n r = 1 (air) Engaging with the concepts Angle of refraction If θ i = 40º what is θ r ?

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Engaging with the concepts Angle of refraction A light beam travels from glass into air. Set n i = 1.52 (glass) Set n r = 1 (air) If θ i = 40º what is θ r ? 77.7º

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The critical angle Light passing from high to low index of refraction ( n i > n r ) bends away from the normal. As the angle of incidence increases, the angle of refraction approaches 90º.

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The critical angle Light passing from high to low index of refraction ( n i > n r ) bends away from the normal. As the angle of incidence increases, the angle of refraction approaches 90º. The critical angle is the angle of incidence that results in an angle of refraction of 90º.

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What do you think happens if you exceed the critical angle? The critical angle Light passing from high to low index of refraction ( n i > n r ) bends away from the normal. As the angle of incidence increases, the angle of refraction approaches 90º. The critical angle is the angle of incidence that results in an angle of refraction of 90º.

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Total internal reflection This can ONLY happen if the incident material has the higher index of refraction. If you exceed the critical angle the refracted ray disappears. The result: total internal reflection.

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Calculate the critical angle θ c How do you calculate the critical angle? Start with Snell’s law:

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The critical angle is θ i when θ r = 90º: How do you calculate the critical angle? Start with Snell’s law: Calculate the critical angle θ c

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How do you calculate the critical angle? Start with Snell’s law: Calculate the critical angle θ c The critical angle is θ i when θ r = 90º: The sine of 90º is 1.

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How do you calculate the critical angle? Start with Snell’s law: Rearrange to get the critical angle formula: Calculate the critical angle θ c The critical angle is θ i when θ r = 90º: The sine of 90º is 1.

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Set: n i = 1.52 (glass) n r = 1 (air) Engaging with the concepts What is the critical angle for light passing from glass ( n = 1.52) into air ( n = 1.0)?

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Engaging with the concepts The critical angle is 41° What is the critical angle for light passing from glass ( n = 1.52) into air ( n = 1.0)? Set: n i = 1.52 (glass) n r = 1 (air)

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Assessment Calculate the critical angle between acrylic ( n = 1.49) and water ( n = 1.33).

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Asked: θ c Given: n i = 1.49, n r = 1.33 Relationships: Assessment so Solution: use the critical angle formula.

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