Transcript Ray Diagrams

Ray Diagrams
J.M. Gabrielse
Quiz
1. Describe the difference between
specular and diffuse reflection.
2. Define transparent. Give an example of
a transparent object.
3. Define translucent. Give an example of a
translucent object.
4. Define opaque. Give an example of a
opaque object.
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A ray of light is an extremely narrow
beam of light.
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All visible objects emit or reflect
light rays in all directions.
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Our eyes detect light rays.
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We see images when
light rays
converge in our eyes.
converge: come together
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How Light Travels
• Rectilinear Propagation: Light travels in a straight line.
• When light strikes an opaque object, directly behind the
object a shadow is created.
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• The part of a shadow where no light hits
(directly behind the object) is called the
umbra.
• Part of a shadow that receives some light
is called the penumbra.
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Mirrors
It is possible to see
images in mirrors.
This is a plane
mirror, such as
you would see in a
bathroom.
image
object
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Mirrors reflect light rays.
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Interactive mirror activity
Students can manipulate mirrors to reflect
light in different directions:
http://www.bbc.co.uk/schools/ks2bitesize/sci
ence/activities/see_things.shtml
Now try this one:
http://www.bbc.co.uk/schools/scienceclips/a
ges/10_11/see_things_fs.shtml
J.M. Gabrielse
Reflection
(bouncing light)
normal
Reflection is when light
changes direction by
bouncing off a surface.
When light is reflected off
a mirror, it hits the mirror
at the same angle (the
incidence angle, i) as it
reflects off the mirror (the
reflection angle, r).
The normal is an
imaginary line which lies
at right angles to the
mirror where the ray hits it.
reflected
ray
incident
ray
r
i
Mirror
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The Ray Model of Light
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• Incident Ray: Ray of light that travels
towards a reflecting surface.
• Angle of Incidence (i): The angle
between the incident ray and the normal.
• Normal: The line drawn from the point of
incidence at 90° to the surface of the
optical device.
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• Point of Incidence: The spot where the
incident ray strikes the reflecting surface.
• Angle of Reflection (r) : The angle
between the reflected ray and the normal.
• Reflected Ray: A ray of light that
bounces off a reflecting surface.
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Virtual Images
• When an object's image lies behind the
mirror, the image is called a virtual
image.
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Plane Mirrors
(flat mirrors)
How do we see images in
mirrors?
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Plane Mirrors
(flat mirrors)
object
image
How do we see images in
mirrors?
Light reflected off the mirror converges to form an image in the eye.
J.M. Gabrielse
Plane Mirrors
(flat mirrors)
object
image
How do we see images in
mirrors?
Light reflected off the mirror converges to form an image in the eye.
The eye perceives light rays as if they came through the mirror.
Imaginary light rays extended behind mirrors are called sight lines.
J.M. Gabrielse
Plane Mirrors
(flat mirrors)
object
image
How do we see images in
mirrors?
Light reflected off the mirror converges to form an image in the eye.
The eye perceives light rays as if they came through the mirror.
Imaginary light rays extended behind mirrors are called sight lines.
The image is virtual since it is formed by imaginary sight lines, not real light rays.
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• Images in a plane mirror are the same size and
distance away from the mirror.
• For Example: A dog of 50 cm tall is 4 m in front
of a plane mirror.
– What is the height of its image?
– If the dog moves 2 m away from the mirror, how will
the size of its image be affected?
• Hint: Size of the image = size of the object
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• Figure 1r shows how the image formed by a plane mirror is
constructed.
• Note that the image I must be:
– the same size as the object, (2m tall)
– as far behind the mirror as the object is in front, (3m away from the
mirror)
– Have the same orientation (upright).
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Drawing an Image formed by a
Plane Mirror
• Construct the image of the object.
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• Draw a cone of rays from I1 to the eye.
Use dotted lines for rays behind the mirror
since they are not the actual path.
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• Draw two rays from O1 to the points where
the rays from I1 meet the mirror.
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• Repeat steps ii and iii for I2 and O2.
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Characteristics of Images in Plane
Mirrors
•S(size) Image size is equal to object size.
•P(position) Image distance is equal to object distance.
•O(orientation) The image is upright. (Its orientation is
the same as that of the object.)
•T(type) The image is virtual (object's image lies behind
the mirror).
• Another interesting feature is that all images in a mirror are
reversed, right to left and left to right, compared to the objects
being reflected.
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