# When there is a change of medium, a part of the light wave gets transmitted. However, the transmitted part travels with different speed (v) and in a different.

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When there is a change of medium, a part of the light wave gets transmitted. However, the transmitted part travels with different speed (v) and in a different direction. This phenomena is called refraction. Refraction

ΘiΘi Incoming Reflection Refraction Θ refraction Applet: http://www.walter-fendt.de/ph11e/refraction.htm

Index of refraction Index of refraction (medium specific) Index of the refraction, n, is defined as n = c/v : v: velocity of light in medium Examples Vacuum, n = 1 Air, n = 1.0003, very close to vacuum! Water, n=1.33 Glass, n = 1.5 Diamond, n = 2.4

Law of refraction When light goes from a medium with smaller index of refraction to a medium with larger index of refraction, it bends toward the normal. When light goes from a medium with larger index of refraction to a medium with smaller index of refraction, it bends away from normal.

ΘrΘr ΘrΘr Air Water Air Light is closer to the normal in a dense medium

Snell’s Law n i Sin  i = n f Sin  f From air to water,  air = 30   then  water = 22  From water to air, if  water = 22   then  air = 30  , on the other hand,  water = 30   then  air = 42 

Straw illusion: A straight straw appears to have bent in water due to refraction!

We always get cheated in a bar, because the refraction in a thick walled beer glass makes the walls look thin!

How to spear a fish? We think we see exactly where the fish is. Actually, the fish is lower than we perceive. It appears closer to the surface than it actually is. If you put the tip of your spear in the water, it will seem bent.

Remember that when the light goes from a dense (large n) to a thin medium (small n), the angle of transmission is larger…. From water to air, if the incident angle is ~50 o, the transmitted angle ~ 90 o. If the incident angle is larger, there is NO transmission at all!

Incoming Reflection Refraction ΘrΘr http://www.phy.ntnu.edu.tw/java/light/flashLight.html

The incident angle at which the total internal reflection just occurs is called the critical angle. From glass to air, 42° From water to air, 50° From diamond to air, 24. 5° Examples of total internal reflection: To change the direction of a beam by 90°or a 180° without energy loss. View from the underwater.

Fiber Optics Use thin, flexible glass or plastic fibers as light pipes, which have a larger index of refraction than the surroundings. If the light hits the surface at an angle larger than the critical angle, it is totally internally reflected: No loss of light from escaping! Thus the light can be used to transmit information through the optical fibers.

Why is the fiber optical transmission is better? Light has high frequency of oscillation, thus can have store much more information! Other uses: See and photograph the inside of vital organs.

What is a mirage? The index of refraction of the air is close, but not equal to 1, and it changes with temperature too. A continuous change of the index of refraction makes a light bend from its original direction. Thus we see the sun even after it has set. A hot, dark asphalt road can cause the light to bend, producing a mirage.

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