Today’s Topic:
The Law of Reflection
Learning Goal:
Students will be able to draw lines normal
to a surface and determine if an object is
emitting diffuse or specular reflections.
 Please take out your notes from last time – as
we are going to continue working on them.
 Define the following terms:
Normal, Incident Ray, Angle of Reflection
Homework
 Complete The Law of Reflection packet
(Due Thursday 5/21)
 Complete the Instrument Design
Worksheet (Four Days Late)
 Complete the Doppler Effect Worksheet
(Four Days Late)
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
“normal”.
Mirror
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.
Incident ray
Angle of
incidence
Normal
Mirror
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? They’re equal!
Incident ray
Reflected ray
Angle of
incidence
Mirror
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.
Incident ray
Reflected ray
Angle of
incidence
Mirror
Angle of
reflection
The Law of Reflection
Angle of Incidence = Angle of Reflection
θi = θr
Incident ray
Reflected ray
Angle of
incidence
Mirror
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
 Many common object in your house diffuse
light by design.
 Lampshades, for example, diffuse light to light
up a room.
 Why would specular
reflection not be ideal for
lighting up a room?
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.
Law of Reflection Worksheet
 Please take out your Law of Reflection
worksheet.
 I am giving you all another sheet to
attach to the worksheet, but I would like
you give you all time to complete it in
class.
 Let’s complete problem two together.