refraction of light when can light “bend”?. review light travels in a straight line…. most of...
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Refraction of Light
When can light “bend”?
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Review
• Light travels in a straight line….
• most of the time.
• The speed of light? 3.0 x108 m/s
• This is only true in a vacuum.
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Concept Check
• You run faster in air than you do in water….
• Some things, you can’t run through if they are dense enough (like walls)
• Light is the same way!
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How “c” changes
• The more dense a medium is, the slower light travels.
• Note: Some stuff you can’t run through, light can still go through, like really thick glass.
• The “ocular density” of an object we will call n (index of refraction).
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To find n…
• You can calculate the value of n by the equation:
• n will always be bigger than 1.• A table of values of n can be found on pg 490 of
your book.
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Why the car crashes
• When a speeding car hits water with the tires on one side, that side slows down, causing the car to spin out.
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What happens when light slows down?
• When light switches mediums, it takes a turn as well….
Water tank
AirӨi
Өr
Normal Line
Өi - Angle of incidence
Өr -Angle of refraction
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Key Points
• The air and the water each have their own density, so the values of “n” (index of refraction) are different too.
• You can see the light bends, so the angle of incidence and the angle of refraction ARE NOT EQUAL.
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How the light bends
• If the light slows down, it bends toward the normal line (glass < air).
• If it’s going the other way, it speeds up and bends away from the normal (air > glass ).
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Refraction
• Where does the cat see the fish?
• Where does the fish see the cat?
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Snell’s Law
• Def: Snell’s Law relates the indices of refraction and the two angles as:
• Make sure your calculator is in “degrees” mode, or things will be much harder…..
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How come water bends light and reflects it too???
• You can see reflections in water and glass, but you can also see through them.
• If the angle of incidence is big enough, light will mostly reflect, and you can see images.
• If the angle of incidence if small, most of the light will go into the new medium, and you can see through it.
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Water tank
Air
Өi
Өr
Normal LineThe angle when light switches from bouncing off water to going into it is called the critical angle
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Why is the sky blue?• Each color of light turns a different amount when it
goes from the medium of space to the medium of the atmosphere.
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Example
• A light ray of wavelength 589 nm traveling through air strikes a smooth, flat slab of crown glass at an angle of 30.0˚ to the normal. Find the angle of refraction, θr.
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Practice
• Complete Page 493 Practice A in your textbooks using Snell’s Law.