Electromagnetic Waves

Physics 202Professor Vogel Lecture 13

Reflection and Refraction

Angles measured from normal !! Angle of incidence (1)

Angle of reflection (1’)

Angle of refraction (2)

Law of Reflection 1’ = 1

Law of Refractionn2 sin 2 = n1 sin 1

General Cases

n2 > n1 Light is bent towards the

normal 2 < 1 e.g. air to glass

n2 < n1 Light is bent away from the

normal 2 > 1 e.g. glass to air

Index of Refraction Every material has an index of

refraction that determines its optical properties Indices of refraction determine

bending, by law of refraction Index of refraction also determines the

speed of light within the material v=c/n generally, v<c

Consequences of Laws Law of Reflection

The angle of reflection is equal to the angle of incidence (1’ = 1)

Light changes direction when it reflects but our vision expects light to travel on

a straight line we see images where light appears to

come from

Plane Mirror 1’ = 1

we see images where light appears to come from Consider how light from

your eye reflects from a mirror to get to your eye, so you can see it

repeat for light from your feet

Images in Plane Mirror Upright Same distance behind mirror as object

is in front Generally “front” is side light comes from.

Same size as object Left and right are reversed.

Cardinal directions aren’t reversed.

Images in Plane Mirror

Based on depth perception.

Where do both the purple and the green rays appear to come from?

Images in 2-D corner Reflector

Can see an image in each mirrorLight can also reflect from both mirrors, then back to you.You see image of image 1Also see image of image 2Both should be same place

Images in 2-D corner Reflector

Image of imagereflected twiceleft and right exchanged twiceIf you are right-handed, so is that image!

Images in Curved MirrorsConcave mirror

think “cave”like bowl of spoonimage is upright and larger – if object is closeimage is inverted and smaller – if object is far

Convex mirrorlike back of spoonimage is upright and smaller – any distance

Consequences of Laws Law of Refraction

n2 sin 2 = n1 sin 1

Light changes direction when it refracts but our vision expects light to travel on a

straight line we see images where light appears to come

from

Demo

Chromatic Dispersion

The index of refraction depends on the wavelength of light In general, n is larger for

shorter wavelengths Blue light bent more than

red

Incident white light is spread out into its constituent colors

Chromatic dispersion with raindrops causes rainbows

Chromatic Dispersion