Lesson 3 10.2 Properties of Light and Reflection P.411-413

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Lesson 310.2 Properties of Light and
ReflectionP.411-413
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• 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.

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Think-Pair-ShareWhat can be done so that the
villagers will receive sunlight all year round?

• 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.

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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.

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• 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.

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Shadows

• A shadow occurs when an opaque object blocks the
  direct light from a light source

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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

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Shadows
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• FERMATS 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.

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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.
Reflection ray
Incident ray
Reflecting Surface
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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)

Reflection ray
Incident ray
Reflecting Surface
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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.

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LAW OF REFLECTION

• When light reflects off a surface, the angle of
  incidence is always equal to the angle of
  reflection.

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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.

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EXAMPLE 1

• Consider the diagram.
• Which one of the angles (A, B, C, or D) is the
  angle of incidence? ______
• Which one of the angles is the angle of
  reflection? ______

B
C
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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.

r
90 30 60 degrees
i
Law of reflection states Angle of incidence
Angle of reflection ? i ? r Therefore, the
angle of reflection will be 60
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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?

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• CLASSWORK Measuring Angles Handout
• HOMEWORK Textbook page 414 2-3