Reflection & Refraction When waves are incident on the boundary between to media, some of the wave will be reflected back into the first medium, some of the wave will be transmitted into the second medium.

Answer Me!!! Can you always see yourself in a mirror? Why or why not?

Representation of Waves A ray is used to represent one wavelet and show the direction of the wave along that ray. Barrier Wave fronts ray Wave fronts ray

Law of Reflection If an incident ray strikes a reflective surface, the ray will be reflected at the same angle as measured from the normal. Angle of incidence (  I ) = Angle of reflection (  R )  RR Normal Incident ray Reflected ray

Diffuse Reflection regular reflection. diffuse reflection.

Virtual images If an object is placed in front of a mirror. An image of that object is created behind the plane of the mirror. The location of this image can be found by tracing reflected rays back until they intersect. object image

Practice Problem 1 Which of the following is the appropriate reflected ray? Incident ray A B C D

Answer Me!!! What do you notice about the straw in the glass on Mr. Scholz’s desk? Why do you think the straw appears this way?

Refraction of Light Light travels through different media at different speeds. As these wavefronts travel slower, rays of light bend toward the normal. The bending of light as it travels through different media is called refraction.

Refraction Example 1 AIR: less dense, Light travels faster WATER: more dense, light travels slower Boundary Bends toward normal

Refraction Example 2 AIR: less dense, Light travels faster WATER: more dense, light travels slower Boundary Bends away from normal

Refraction Example 3 AIR: less dense, Light travels faster WATER: more dense, light travels slower Boundary Ray is not refracted when parallel to normal

Practice Problem A man is trying to catch a fish using a spear. The water is clear so he can se the fish swimming by. As one get close, he sends the spear exactly where he sees the fish. Does he catch his fish? Why or Why not?

Where is the Sun? As the light from the sun travels through our atmosphere, it is refracted. So, where we see the sun is actually an optical illusion Actual sun Where we see the sun

Index of Refraction An object’s index of refraction (n) is a measure of how fast light will travel through that object. speed of light in a vacuum (c) speed of light in the given medium (v). The Physics Reference Tables have a list of common indices of refraction.

Practice Problem 2 What is the speed of light through Lucite?

Practice Problem 3 What material allows light to pass through it at a speed of 2.25 x 10 8 m/s?

Practice Problem 4 The speed of light in a transparent material is 0.75 times its speed in air. What is the index of refraction of the material?

Index of Refraction Since the frequency of light does not change as it passes through materials: n 1 1 = n 2 2 OR n 1 v 1 = n 2 v 2

Practice Problem 5 As a monochromatic beam of light passes obliquely from flint glass into water, how does the characteristics of the light beam change?  Wavelength will _____________  Frequency will ______________  Speed will _______________  Angle will _____________

Snell’s Law Snell’s Law describes the relationship between index of refraction and the angle light will travel at. 11 22 Index: n 1 Index: n 2

Practice Problem 7 A beam of light is incident on a Lucite block in air at an angle of 33 o. Calculate the angle of refraction.

Practice Problem 8 A ray of light passes from air into a block of transparent material. The angle between the incident ray and normal is 45 o. Then angle between the refracted ray and normal is 26 o. Calculate the index of refraction of the material.

Practice Problem 9 Find the angle of the light beam in the Lucite. What material is the light traveling slowest in? Air Lucite Medium Y Air

Practice Problem 10 A beam of light passes through parallel sections of glycerol, medium X and medium Y. What could medium X and Y be made from? Glycerol Medium X Medium Y 40 o 35 o 40 o

Practice Problem 11 A monochromatic beam of yellow light is incident upon a Lucite block in air at an angle of 33 o. Calculate the angle of refraction and draw the incident and refracted rays.