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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.
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