Properties of Light and Reflection

1
Properties of Light and Reflection
2
Reflection

• Is the change in direction of a wave when it
  reaches a surface and bounces off that surface.

3
Rays of Light

• Light travels in s straight line as long as it is
  moving through a medium.
• Medium is the substance through which light
  travels.
• You can use a technique called ray tracing to
  make ray diagrams.
• A ray is a straight line with an arrowhead that
  shows the direction in which light waves are
  traveling.

4
Fermats Principle

• Predicts the path that light will take after
  reflecting from a surface or passing through more
  than one medium.
• According to the principle, light follows the
  path that will take the least time.
• When light reflects from a surface and stays in
  the same medium, its speed is constant.

5
Laws of Reflection

• A ray of light coming toward a surface is called
  an incident ray.
• The angle of incidence is measured between the
  incident ray and a perpendicular line drawn from
  the point of contact of the incident ray at the
  surface.
• Perpendicular line is called the normal.
• The reflected ray begins at the point of contact.
  
• The angle of reflection is measured between the
  reflected and the normal.

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• The angle of reflection and the angle of
  incidence are the same.
• The reflected ray always lies on the plane that
  is defined by the incident ray and the normal.
• These laws of reflection apply to light and all
  other forms of waves (sound waves).

7
Drawing Ray Diagrams

• Step 1 Draw the incident ray using a ruler
• Step 2 At the contact point, draw a normal at
  90?with a protractor
• Step 3 measure the angle of incidence (i)
  between incident ray and the normal.
• Step 4 Draw the reflected ray
• Step 5 label the reflected ray, the incident
  ray, the angle of incidence (i), the angle of
  refraction (r) and the normal.

8
normal
Incident ray
Reflected ray
i
r
Reflecting surface
9
Images in Plane Mirrors

• If you apply the laws of reflection to rays going
  from an object, you can predict where the image
  will be and what the image will look like.
• A plane mirror is a mirror with a flat,
  reflective surface.
• The brain assumes light travels in a straight
  line, so to find out where the eye sees the
  image, extend the rays that reach the eye
  backwards until the meet at a point beyond the
  mirror.

10
Image in Plane Mirror
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Image in Plane Mirror
12
Virtual Images

• An image formed by rays that appear to be coming
  from a certain position, but are not actually
  coming from this position image does not form a
  visible projection on a screen.
• If light rays hit a screen and form an image, the
  image is real and not virtual.
• If the image is behind a mirror, there is no way
  that light rays could get there, and it would be
  virtual.

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Ray Diagrams and Plane Mirrors

• An image has four characteristics
• Its location (closer than, farter than, same
  distance as the object to the mirrow)
• Orientation (upright or inverted)
• Size (same size, larger, smaller than the object)
• Type (real image or virtual image).

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• You can predict characteristics by drawing ray
  diagrams to locate the image of an object.
• The four characteristics of an image in a plane
  mirror are always
• Same size
• Same distance
• Same orientation
• Virtual image

15
You Try It!

• On page 416
• Follow the instructions in table 10.1 in order to
  draw a pencil.
• P. 418 1-4, 6-8.
• Section 10.2 Review Handout