The Ray Model of Light

1
The Ray Model of Light

• Since light seems to move in straight lines, why
  not follow a light wave along a straight line
  path and simply draw the line to represent how
  the light will behave
• This line will be perpendicular to the wave front
  of the light wave
• Well see how this model helps understand many
  characteristics of light behavior

2
The Ray Model of Light

• Light from an object either results because the
  object is emitting light or light is reflecting
  from the surface of the object

3
Reflection from a Plane Mirror
The angle of incidence equals the angle of
reflection. This assumes the surface is
perfectly smooth.
4
Diffuse Reflection
When the surface is rough, the surface at any
point makes some angle w.r.t. the horizontal.
The angle of incidence still equals the angle of
reflection.
5
Plane Mirrors
In the left hand picture with a rough surface,
you can place your eye anywhere and you will see
some reflected rays. On the right hand side, you
eye has to be in the correct position to see the
reflected light. This is called specular
reflection.
6
Plane Mirrors
A plane mirror provides the opportunity to fool
you by making your eye and brain perceive an
image.
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Plane Mirrors
The image appears to be the same distance behind
the mirror as the object is in front of the
mirror.
8
Plane Mirrors
The image is called a virtual image because if
you placed a piece of paper at the image
location, you wouldnt see any light.
9
How Big a Mirror?
10
Spherical Mirrors
Again, we use the angle of incidence equals the
angle of reflection. It is convenient to trace
what happens to parallel light rays hitting the
mirrors. Remember the definition of convex and
concave!!
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Spherical Mirrors
Precisely parallel rays do NOT meet at the same
point after reflection from the surface of the
mirror. Of course, precisely parallel rays only
come from objects at huge distances away. To
avoid this problem and to form real images, we
need to restrict ourselves to just a very small
central region of the mirror.
12
Spherical Mirrors
In this limited region of the surface, the rays
do intersect at the focus.
This picture defines the principal axis, the
focal point and the focal length of the mirror.
The line CB is a radius of the spherical
surface. The focal point is at 1/2 the radius
length.
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Images in Spherical Mirrors

• Any ray parallel to the principal axis will pass
  through the focal point!
• Now we need to look at more rays leaving the same
  point on the object

14
Images in Spherical Mirrors

• Any ray from the object passing through the focal
  point will emerge parallel to the principal axis!!

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Images in Spherical Mirrors

• Any ray striking the mirror at right angles will
  reflect straight back and will pass through the
  center of curvature!!

16
Images in Spherical Mirrors

• Now we want to derive an equation that will
  express the observations we have just made

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Images in Spherical Mirrors

• The image and object distances are di and do
• The image and object heights are hi and ho

18
Images in Spherical Mirrors

• The right triangles IAI and OAO are similar
  (angles the same)

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Images in Spherical Mirrors
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Images in Spherical Mirrors

• Triangles OFO and AFB are similar
• AB hi and FA f (focal length)

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Images in Spherical Mirrors
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Images in Spherical Mirrors
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Images in Spherical Mirrors
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Images in Spherical Mirrors

• Image is upright and virtual!
• Makeup Mirror

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Images in Spherical Mirrors

• Passenger side outside car mirror
• Image is virtual and upright