refraction. light travels in straight lines. we have examined how light can reflect off an object....
TRANSCRIPT
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Refraction
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Lighthttp://arrowheadradio.com/images/fiber-optics.jpg
Travels in straight lines.
We have examined how light can reflect off an object. Today we examine Refraction
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Speed of Light (c)
Light moves very fast. 300 million m/s (in a vacuum)
Light moves more slowly the denser the medium is.
Speed of light through air is slower than a vacuum. Speed through glass is slower than air.
Refraction is based on the Speed of light.
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Refraction
Reflection occurs when light bounces. Refraction occurs when light transmits through an
object. (or medium)
The different densities of the media will make the light bend.
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Refractionhttp://thescienceroom.pbworks.com/f/brokpen.jpg
Below the water, the pencil is bent.
As the light rays move from water(dense) to air(not dense) they bend.
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Refraction
Refraction is most common with water and air, but occurs with any clear substances. (glass, plastic, diamond etc)
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Refraction in a Pool
Ever lose your key in a pool?
Refraction makes it look like the key is somewhere else.
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Refraction in a Pool
The light bends when it leaves the water and makes the object appear to be “above” the actual position.
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Why does light bend?
The light travels more slowly in dense media. Some of the light is slowed down and pulls the rest with it.
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Index of Refraction
Every substance has an index of refraction to tell us how much light will bend when passing through it. (pg 437 Table 11.5)
Higher number = greater refraction.
We will be using these values for
calculations later
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How did we come up with all those indices?
Index of refraction (n) =
Speed of light in a vacuum(c)
Speed of light in medium(v)
Or
n = c/v
The refractive index of a medium, n, is calculated by comparing the speed of light in the medium, v, with the speed of light in a vacuum, c.
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Predicting Refraction
We use a normal with refraction, just like we did with reflection.
Remember: Less dense = low refractive index More dense = higher refractive index
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Predicting Refraction
How does the light bend when going from a less dense to more dense medium?
From low index to high index, bend toward normal.
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High to Low Refractive Index
How does light bend when going from a more dense to less dense media
High index to low index, bend away from normal.
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Practice
Read example problems page 438
Do all 6 calculations page 438
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Fibre Optics
Light can be bent and controlled with fibre optic cables. (glass tubes as thin as a human hair)
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Fibre Opticshttp://static.howstuffworks.com/gif/fiber-optic-fiber-edit.jpg
Core - Thin glass centre of the fibre where the light travels
Cladding – glass that reflects light back into core
Buffer coating - Plastic coating that protects the fibre moisture
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Dispersionhttp://www.educationalelectronicsusa.com/l/images/light-XVa.gif
White light can be split into component colours. This dispersion is a type of refraction.
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Rainbows
Light refracts on the way into the raindrop, disperses into colour, reflects off the back, and refracts on the way out...... yikes.
Rainbows are easy...
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Work
Pg. 916 #1, 2, 3, 5, 7
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Snell’s Law
A formula that uses the index of refraction to calculate the change in angle of the light ray between two media
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Calculations with Snell’s Law
n1 sin 1= n2 sin 2
Let the indices of refraction be n1 and n2
Let the angle of incidence be 1
Let the angle of refraction be 2
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Practice
Read example problems page 441 - 442
Do all 6 calculations page 441 - 442
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Total Internal Reflectionhttp://www.hsc.csu.edu.au/senior_science/core/info_systems/9_4_6/Laserpath.gif
Sometimes light does not pass from one medium to another, but stays within the medium
This happens in the case of fibre optics
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Total Internal Reflection http://labspace.open.ac.uk/file.php/6211/t305_002i.jpg
Light reflects completely off the inside wall of a denser medium (higher index of refraction) rather than passing through.
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RECALL!
When light passes from a denser material into a less dense material, the light refracts away from the normal
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The Critical Angle
As the angle of Oi, the Oref also increases At the critical angle, the refracted ray of light
follows a path exactly along the surface of the water
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Past The Critical Angle
If the Oi is increased past the critical angle, the light ray is reflected not refracted
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Mirages
Both total internal reflection and refraction play a role in forming a mirage
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Mirages
Light rays pass through layers of air with progressively lower indices of refraction, eventually the light is totally internally reflected
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Mirageshttp://www.islandnet.com/~see/weather/graphics/photos/infmirg2.gif
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The End
Questions: pg 447 # 11, 12, 13, 16 (Calculator) 17, 18, 19, 21 (No Calculator)