Lesson 3
10.2 Properties of Light and
Reflection
P.411-413
Today we will learn:
• About reflection.
• How to predict the location of an object using
rays.
• How to draw a ray diagram for light reflecting off
a smooth surface.
SUCCESS CRITERIA
I will be successful if I can
• Explain what a ray is.
• Draw a ray diagram.
• Explain the laws of reflection.
• Viganella is a small village in Italy.
• Due to the mountains, the town does not
receive direct sunlight from November
11th – February 2nd.
• The villagers love their home and do not
want to move.
Think-Pair-Share:
What can be done so that the
villagers will receive sunlight all
year round?
The Use of Mirrors
• In 2006 they spent
100,000 EUROS on a
5m x 8m mirror that
reflects light down to
the village in the
winter.
• The mirror is
controlled by
computers that
follow the Sun across
the sky.
All light behaves the same, regardless of its source.
Light travels in a straight line as long as it is
moving through the same medium.
REFLECTION: The change in direction of a light ray
when it bounces off a surface.
MEDIUM: The substance through which light
travels.
RAY: A straight line with an arrowhead that shows
the direction in which light waves are travelling.
Shadows
• A shadow occurs when an opaque object
blocks the direct light from a light source
Shadows
Ray diagrams can be used to show how
the size of the shadow depends on the
objects distance from the light source.
• A larger object will produce a larger shadow
• An object closer to the light source will
produce a larger shadow
Shadows
FERMAT’S PRINCIPLE
- Predicts the path light will take after
reflecting from a surface or passing through
more than one medium.
- Light follows the path that will take the least
time.
- i.e. When light reflects from a surface and
remains in one medium, its speed is constant.
RAY DIAGRAM DEMONSTRATING THE
LAW OF REFLECTION
Normal: Dashed line drawn perpendicular to the
mirror at point of reflection.
Incident Ray: The incoming Ray of light from a light
source towards a surface.
Reflected Ray: The
outgoing Ray of light.
Begins at the point
where the incident ray
and normal meet.
Incident
ray
Reflection
ray
Reflecting Surface
RAY DIAGRAM DEMONSTRATING THE
LAW OF REFLECTION
Angle of Incidence: The angle between the
incident ray and the normal. (i)
Angle of Reflection: The angle between the
normal and the reflected ray. (r)
Incident
ray
Reflection
ray
Reflecting Surface
LAWS OF REFLECTION
1. The incident ray, the reflected ray, and the
normal always lie on the same plane.
2. The angle of reflection is equal to the angle
of incidence.
LAW OF REFLECTION
• When light reflects off a surface, the angle of
incidence is always equal to the angle of
reflection.
Using the Law of Reflection
• The angle of incidence and the angle of
reflection are always measured from the
normal and not from the surface of the
object.
• Why? Because some surfaces are curved,
making it difficult to measure an angle from
the surface.
EXAMPLE 1
Consider the diagram.
Which one of the angles (A,
B, C, or D) is the angle of
incidence? ______
B
Which one of the angles is
the angle of reflection?
C
______
EXAMPLE 2
A ray of light is incident towards a plane mirror at an
angle of 30 degrees with the mirror surface.
Determine the angle of reflection.
90 – 30 = 60 degrees
i
r
Law of reflection states:
Angle of incidence = Angle of reflection
⦟i =⦟r
Therefore, the angle of reflection will be 60°
EXAMPLE 3
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?
• CLASSWORK: Measuring Angles Handout
• HOMEWORK: Textbook page 414 # 2-3