SPH3UW
Notes
Unit 7.2 Reflection
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Physics Tool box
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Law of Reflection –On flat surfaces, the angle of incidence equals the angle
of reflection.
Diffuse Reflection – when light is incident on a rough surface, it is reflected
in many surfaces.
Image – the name of the reflection you see in a mirror.
Virtual Image – If light rays do not pass through the image, a piece of
paper placed at the image would not detect it.
Real Image – If light rays do pass through the image, a piece of paper
placed at the image location would have the image appearing on it.
When light strikes the surface of an object, some of the light is reflected, the remainder is either
absorbed by the object (transformed to thermal energy), or transmitted through (like glass of water).
When a narrow beam of light strikes a flat surface, we define the angle of incidence,  i , to be the angle
an incident ray makes with the normal to the surface, and the angle of reflection,  r , to be the angle
the reflected ray makes with the normal.
For flat surfaces (specular reflection), we have the law of reflection:
The angle of incidence,  i , equals the angle of reflection,  r , and the incident light ray, the reflected
light ray, and the normal to the surface all lie in the same plane.
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Unit 7.2 Reflection
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Scattered reflection from a rough surface is called diffuse reflection.
Image formation by a Plane Mirror
To determine the precise location of the virtual image P’ that a plane mirror forms of a real object
located at point P.
We draw two lines from P (at a distance s to the left of the mirror. This distance, s, is known as the
object distance. The ray PV is perpendicular to the mirror surface and returns along its original path.
The ray PB makes an angle  with PV. This ray strikes the mirror at an angle of incidence  and is
reflected at an equal angle with the normal.
When we extend the two reflected rays backwards, we discover that they intersect at a point P’. P’ is at
a distance s’ behind the mirror. The distance s’ is called the image distance.
The line connecting P to P’ is perpendicular to the mirror, and the two created triangles PVB and P’VB
are congruent and thus the distance s and s’ are equal.
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Unit 7.2 Reflection
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How to Draw images on a Plane Mirror
Description
Reference Line
Draw a normal line from the base
of the object through the mirror to
the other side
Light Ray 1
Draw a ray from the top of the
object to the mirror parallel to the
reference line.
Light Ray 2
Draw a line from the top of the
object to the point where the
reference line meets the mirror.
Draw the reflected ray according
to the law of reflection.
Extended Rays
Extend both rays behind the
mirror, using dashed lines, until
they intersect
Illustration
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Unit 7.2 Reflection
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Drawing Examples
For flat mirrors the Law of Plane Mirrors states that "the image is always the same distance behind the
mirror as the object is in front of the mirror." The image, I, and the object, O, always line up along the
same normal. The image is upright, but left-right reversed.
Notice that the rays entering the eye are diverged from the mirror's surface. Since the eye has to "dot
back" the rays to form an image, this image is virtual - a cool image, trapped within the mirror.
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Unit 7.2 Reflection
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As mentioned earlier, the Law of Plane Mirrors states that di = -do. This reflection, like ALL reflections,
obeys the Law of Reflection: the angle of incidence equals the angle of reflection. Note that these
angles are measured from the normal to either the incident or reflected rays.
The following diagram illustrates that the minimum length of a plane mirror required for someone to
view their entire image equals half their height. Note that the top of the mirror should be placed at eye
level.
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Example 1
Sketch a ray diagram that shows how light will travel from the object to the eye by reflecting from the
mirror in Illustration. Identify the position of the image.
Solution:
SPH3UW
Unit 7.2 Reflection
Extra Notes and Comments
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