Reflection and

Refraction

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Reflection• Reflection is when light hits

a boundary and bounces off• The law of reflection states

that the angle of incidence is equal to the angle of reflection

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Reflection

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Refraction• Refraction is the

change in direction of waves crossing a boundary between two different media

• Angle would be measured with a protractor!

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Refraction

Incident rayR

efracted ray

No

rmal

θi

θr

Medium 1

Medium 2

boundary

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Parts of a refraction diagram• Normal – line drawn at a right

angle to the barrier between the media

• Boundary – barrier between the media

• Angle of incidence – angle of ray in first media

• Angle of refraction – angle of ray in second media

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Index of refraction• Measure of how much light

bends when it passes into a medium from a vacuum

• nsubstance = c / vsubstance

• Index of refraction = speed of light in a vacuum

speed of light in the medium• c = 3.0 x 108 m/s 7

If you didn’t get that…

• index of refraction = noun Optics. a number indicating the speed of light in a given medium as the ratio of the speed of light in a vacuum to that in the given medium Symbol: n Also called index, refractive index.

• SO, NO UNITS!

Some Common values for n…

Vacuum 1

CrownGlass 1.52

Air 1.0003

Salt 1.54

Water 1.33

Asphalt 1.635

Ethyl Alcohol 1.36

Heavy Flint Glass 1.65

FusedQuartz 1.4585

Diamond 2.42

Whale Oil 1.460

Lead 2.6

Sample problems1. The speed of light in one type

of plastic is 2.00 X 108 m/s. What is the index of refraction of plastic?

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• n = c/v

• n = 3.0 x 108 m/s / 2.0 x 108 m/s

• nplastic = 1.5

2. Find the speed of light in water (n = 1.33).

• Vwater = 2.3 x 108 m/s

Snell’s Law• Used to determine the

angles of incidence or refraction.

• nisini = nrsinr

• Index of refraction of 1st medium x sine of angle of incidence = Index of refraction of 2nd medium x sine of angle of refraction

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Sample Problems3. If a light beam in air (n =

1.0003) hits a sheet of crown glass (n = 1.52) at 30.0, at what angle was it refracted?

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•nisini = nrsinr

• (1.0003)sin30 = (1.52)sinθ

• θ = 19.21°

4. A ray of light in air is incident upon a diamond at 45.0 (ndiamond = 2.42) What is the angle of refraction?

• 16.99°

Sample Problems5. A ray of light passing through

ethyl alcohol (n = 1.36) strikes a solid at an angle of 75º. If the angle of refraction is 40º, what is the index of refraction of the solid?

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• nr = 2.04

Sample Problems6. Find the speed of light in a

substance if the ray of light enters the substance from air at an angle of 75º to the surface and the angle of refraction is 80º to the surface.

• 2 steps

• Use Snell’s Law and then n = c/v

• v = 2.0 x 108 m/s