Law of Reflection and
Mirrors
How Light can be
Redirected
The Law of Reflection
• When an object is smooth, the light that
reflects off of the surface of that object will
form an image
– An image is a reproduction of an object
produced by an optical device
• An optical device is anything that uses light
• Light rays bounce off of a mirror in a similar
way that a pool ball bounces off the
bumpers of a pool table
Law of Reflection (cont’d)
• When an incoming ray of light, called the
incident ray, hits a mirror at a specific
angle, called the angle of incidence, it will
reflect that ray off at precisely the same
angle, called angle of reflection.
• The diagram on the next slide will give you
a better idea of what this looks like
Law of
Reflection
(cont’d)
Notice how the
angle of incident,
labelled i, and the
angle of reflection,
labelled r , are
measured from a
line drawn
perpendicular to the
plane of the mirror,
called the normal.
i
r
The Law of Reflection
• The law of reflection can be written using
the Greek letter theta, θ.
– This symbol is commonly used to represent an
angle
• Putting subscripts next to theta helps to
identify what angle the symbol represents
θi = θr
• Once again remember that θ is measured
from the normal line.
Plane Mirrors
• The law of reflection applies to both flat mirrors and
curved mirrors.
• Any mirror that has a flat reflective surface is called a
plane mirror.
• Images in a plane mirror appear as far behind the mirror
as the object being imaged is in front of the mirror
• The image appears to be behind the mirror, but it would
not be possible to catch this image on a piece of paper if
it were put behind the mirror because no light form the
object reaches this point.
• When no light comes from where an image appears to
be, we call this a virtual image.
Mirror
Plane Mirror (cont’d)
Object
do
di
Reflected Virtual
Image
For a plane mirror, the distance of the object from the mirror is equal to the
distance the image appears to be in the mirror. In other words, do = di.
Plane Mirrors Exercise
• Take a Post-It note and put the initial of your first name
on in dots on the far left, as shown:
• Then draw a line down the middle using a ruler as shown
• Finally measure the distances the dots are from the line
and make a dot an equal distance from line on the other
side of the line as seen above
Plane Mirrors
Exercise(cont’d)
• What did you notice about your letters?
• Were the two letters exactly the same?
• If they were exactly the same try the same
exercise with the letter R.
• This shows why letters appear backwards in
images in mirrors.
• You’ll see examples of how we try to correct for
such problems on the front of ambulances (see
above).
Questions
• Do Plane Mirror Reflection Quick Lab on
page 428 of the textbook.
– Answer questions 7 and 8 a & b for the lab.
• Do questions 1, 2 and 9 from page 433 in
the textbook. Put your answers to these
questions in the title page document that
coincides with this PowerPoint
presentation.