Reflection and Refraction

Chapter 29

Reflection

When a wave reaches a boundary between two media, some or all of the wave bounces back into the first medium.

Reflection

The angle of incidence is equal to the angle of reflection" is one way of stating the law of reflection for light in a plane mirror.

Reflection

Law of Reflection:

the angle of incidence equals the angle of reflection

Reflection

Consider the diagram at the right. Which one of the angles (A, B, C, or D) is the angle of incidence? ______ Which one of the angles is the angle of reflection? ______

Reflection

A ray of light is incident towards a plane mirror at an angle of 30-degrees with the mirror surface. What will be the angle of reflection?

Reflection

Perhaps you have observed the image of the sun in the windows of distant buildings near the time that the sun is rising or setting. However, the image of the sun is not seen in the windows of distant building during midday. Use the diagram below to explain, drawing appropriate light rays on the diagram.

A ray of light is approaching a set of three mirrors as shown in the diagram. The light ray is approaching the first mirror at an angle of 45-degrees with the mirror surface. Trace the path of the light ray as it bounces off the mirror. Continue tracing the ray until it finally exits from the mirror system. How many times will the ray reflect before it finally exits?

Reflection

Reflecting Mirrors

In order to see the image of an object in a mirror, you must sight at the image; when you sight at the image, light will come to your eye along that line of sight.

The image location is thus located at that position where observers are sighting when viewing the image of an object

It is the location behind the mirror where all the light appears to diverge from

Reflecting Mirrors

Reflecting Image Location

Image Formation in Plane Mirrors

But what if the man stood a different distance from the mirror? Wouldn't that cause the man to need a different amount of mirror to view his image? Maybe less mirror would be required in such an instance?

Image Formation in Plane Mirrors

Image Formation in Plane Mirrors

Diffuse Reflection

Rough surface, diffuse reflection

Diffuse Reflection

Rough surfaces consist

of a large number

of different specularly-

reflecting planes.

Visible light reflects

diffusely from

paper.

Diffuse Reflection

Diffuse Reflection

Rough surfaces seem

smooth

for very long-wavelength

electromagnetic waves.

This parabolic dish reflects

long wavelength radio

waves

to the detector, but reflects

most of the shorter

wavelength

radiation away from the

detector.

Reflection of Sound

Reflected Wave

Incident Wave

Reflection of Sound

spherical wavefronts reflected as if there were an "image" of the sound source at the same distance on the other side of the wall.

Reflection from Concave Surface

Any concave surface will tend to focus the sound waves which reflect from it

Refraction

Refractionis the bending of light as it passes between materials of different optical density.

Index of Refraction of a material is the ratio of the speed of light in vacuum to the speed of light in that material:

n = c/v

where v is the speed of light in the material.

Refraction

the more dense the material, the slower the speed of light in that material. Thus n > 1 for all materials, and increases with increasing density.

n = 1 in vacuum.

n = c/v

The frequency of light does not change when it passes from one medium to another. According to

the formula v = f , the wavelength must change.

The index of refraction can therefore be written in terms of wavelengths as:

n=

where 0 is the wavelength of the light in the

vacuum and is the wavelength of the light in the

medium.

Refraction

Refraction

Light in air incident on glass surface where it is partly reflected at the interface and partly transmitted into the glass.

The direction of the transmitted ray is changed at the air/glass surface. The angle of refraction r is less than the angle of incidence i.

Refraction

The velocity of visible light in different media. The value of 100% refers to the velocity of light in vacuum.

Refraction

Light waves of wavelength incident on glass change direction and wavelength when transmitted into the glass.

Refraction of Light

The greater the change in velocity and wavelength, the greater the change in direction

Refractive Index

For light, the index of refraction n equals the ratio of the velocities of light in vacuum (c) to that in the medium (v), that is n = c/v.

Medium Refractive Index

Air 1.0003Water 1.33

Linseed

Oil

1.48

Co Green 2.00

Diamond 2.42

Ti White 2.5

Light in Glass

Light incident on a glass plate. The reflected part of the ray is shown along with the light path for the refracted component.

Total Internal Reflection

If light is inside a material such as glass with a larger refractive index n2 than that n1 of the material outside such as air, there is an angle, the critical angle of incidence, beyond which the light is reflected back into the material and does not escape

sin ic = n1 / n2

Critical Angle

For light exiting glass,

n2 = 1.5, the relation becomes

sin ic = 1.0 / 1.5

the value of the critical angle of incidence is 41

Total Internal Reflection

The internal reflectance at an air/glass interface for light rays from a point source

in glass. Light rays incident at angles to normal at greater than the critical angle

(here, 41 for glass to air) do not leave the material and are reflected at the

glass/air interface

Refraction of Sound

Refraction of Sound