Physics 11

Name: __________________________________

Refraction Wave speed depends on the media. What happens when waves travel from one medium into another? Consider some waves moving from the open ocean to shore. As the water gets more and more shallow, the waves slow down (velocity decreases).

Waves traveling perpendicular to the new medium (Ɵi= 0o) continue in the same direction.  Velocity _________, but frequency ______________  Therefore wavelength _______________ When waves are not perpendicular they will also ________. Refraction: Optical density:

(vacuum < air < water < glass < diamond) Snell’s law

Consider a laser shining into a piece of quartz: Angle of Incidence:

n= ϴi = ϴR =

Angle of Refraction:

When light travels from:  less dense to more dense it _______ _______ and bends ____________ the normal  more dense to less dense medium it _________ ________ and bend _______ ________ the normal.

Ex: A ray of light traveling in air strikes a block of quartz at a angle of 15o. Find the angle of refraction. Draw a diagram.

The index of refraction for any substance is:

Where: n= c= v=

Medium Vacuum Air Water Ethanol

n

Medium Crown glass Quartz Flint Glass Diamond

n

Ex: A ray of light travels from underwater into air. It travels in the air at an angle of 65o, find the incident angle. Draw a diagram.

Ex: What is the speed of light in water?

Total Internal Reflection When passing from a more dense to a less dense medium, light refracts away from the normal.

Critical angle: Total Internal Reflection:

Ex: Find the critical angle for light traveling from water into air. Draw a diagram. If the angle is large enough then the angle of refraction will be parallel to the medium boundary. (i.e. Ɵr = 90o)

Total Internal Reflection We know that when light hits a barrier between media, some of the light is reflected and some is refracted into the new medium. Consider now light that is going from a high index of refraction to one with a lower index of refraction, as seen in the figure below. As we know, some light is partially reflected while the rest is refracted away from the normal line. Notice that as the angle of incidence θi increases, the angle of refraction θR increases even more rapidly.

Since θR > θi in this situation, as the angle of incidence increases the angle of refraction will reach 90o before the angle of incidence does. Obviously, the angle of refraction can never be greater than 90o if the refracted ray is to remain in the refracting medium. The angle of incidence for which the angle of refraction is 90o is known as the critical angle, θc.

If the angle of incidence exceeds the critical angle, there can be no angle of refraction. In this case, all of the light is reflected. This is known as total internal reflection and can be seen in the figure below. Notice that the law of reflection still holds; in other words, the angle of incidence is equal to the angle of reflection.

When total internal reflection occurs, none of the light enters into the optically less dense (lower index of refraction) material; the interface behaves as a perfect mirrored surface. In fact, more light is reflected in this situation than when using a mirrored surface such as a shiny metal. To summarize, there are two conditions required for total internal reflection to occur: 1. The light must be travelling from a higher index of refraction to a lower index of refraction. 2. The angle of incidence must be greater than the critical angle, θc, associated with the two materials.