Today’s Topic:

The Law of Reflection Learning Goal:

Students will be able to explain the laws

that govern the reflection of light. Do Now:

Jackie wears a green shirt. White light hits her

shirt from the sun.

What happens to the green portion of the

white light once it hits her shirt?

What happens to the violet portion of the white

light once it hits her shirt?

Homework

Complete The Law of Reflection packet

(Due Tuesday 5/19)

Complete the Doppler Effect Worksheet

(Two Days Late)

Recap The color an object appears depends on

the colors of light it reflects.

For example, a red book only reflects red

light:

White

light

Only red light is reflected

Recap A white hat would reflect all seven colors:

A pair of purple pants would reflect purple light

White

light

Purple light

Recap

Objects can only reflect light of

frequencies present in the illuminating

light source.

Not all light sources are created equal –

some sources reflect frequencies more

intense than other sources.

Recap The color of a transparent object is the

color of the light it transmits – or allows to

pass through.

Summary Questions Based on what we learned yesterday,

answer and discuss the answers to these

questions with your table partners:

A student looks through their see-through

purple ruler. Everything the student sees

has a purple color to it. Why is this?

Summary Questions Mr. Gutterman paints a picture by

candlelight. When he moves the painting

into the sunlight, the colors look weird.

Why does

his painting

look strange?

Reflection What is the definition of a

reflection in terms of Physics?

When a wave reaches a boundary

between two media, usually some or all

of the wave bounces back into the first

medium.

Wave Boundary

(wall)

Reflection

Reflection

Why is light “weird”?

Because it behave with wave-like and

particle-like properties.

As a result, light will reflect when it hits

the boundary between two media.

When light hits a reflective surface, (like

metal or a mirror) the reflected wave has

almost the full intensity of the incoming

wave.

Reflection Let’s think about objects bouncing.

This tennis ball represents a

photon/waves when they reflect off of an

object.

When the object travels in one direction,

how do we expect the object to reflect?

Reflection How will the ball reflect when moving in two

directions?

Using these tennis balls, I would like you to

test your prediction.

Does your prediction match? What happens to

the reflection angle if you change the angle in

which you toss the ball?

The Law of Reflection The direction of light movement can be

thought of as a straight line – or rays.

If you create a straight line perpendicular

from a reflecting surface, it is called the

“normal”.

Mirror

Normal

The Law of Reflection The ball you just tossed can be thought of as

the “incident ray” and you have tossed it with a

certain angle of incidence from the normal.

Mirror

Normal

Incident ray

Angle of

incidence

The Law of Reflection How does this angle you toss it compare to

the angle the ball makes when it bounces up

from the table?

Mirror

Incident ray

They’re equal!

Reflected ray

Angle of

incidence

Angle of

reflection

The Law of Reflection The Law of Reflection states that the angle of

incidence and the angle of reflection are equal

to each other.

Mirror

Incident ray Reflected ray

Angle of

incidence

Angle of

reflection

The Law of Reflection Angle of Incidence = Angle of Reflection

θi = θr

Mirror

Incident ray Reflected ray

Angle of

incidence

Angle of

reflection

Specular Reflection When we have more than one light ray striking

a flat reflective surface, we get the following

situation:

This is called Specular Reflection.

A clear reflection is produced, because all of

the light is hitting one spot.

Other Reflection However, this is not very realistic, as

most surfaces – up close – are not flat.

Realistically, surfaces look more like this.

However, the Law of Reflection still holds

true for incoming waves!

Other Reflection

How is this different than specular

reflection?

The reflected waves are all over the

place!

This is called diffuse reflection.

Diffuse Reflection

Although each ray obeys the law of

reflection, the many angles of the surface

cause the incident light rays to reflect in

many different directions.

Diffuse Reflection

Here are two pieces of glass.

Which piece will produce normal

reflections and which will produce diffuse

reflection?

Diffuse Reflection All rough objects diffuse light. Even smooth

objects, like a piece of paper, diffuse light.

Under a microscope, paper looks like that.

Now you can understand why light rays will

diffuse when they contact that.

Diffusion Uses Identify what common object in your house

diffuses light.

Hint: it is located very close to light sources in

your house.

Lampshades!

Wet/Dry Asphalt at Night Another instance of diffuse reflection is seen

at night after rain has fallen.

Normally, light will diffuse when a car’s

headlights hit the asphalt:

Wet/Dry Asphalt at Night After it rains, however, light will bounce off the

wet roadway and cause specular reflection.