14-2 Flat mirrors
1. Distinguish between specular and diffuse
reflection of light.
2. Apply the law of reflection for flat mirrors
3. Describe the nature of image formed by flat
mirrors.
Reflection of Light Waves – the law of
reflection
• The ray of light approaching the mirror is known as the incident ray (I).
The ray of light which leaves the mirror is known as the reflected ray (R).
• At the point of incidence where the ray strikes the mirror, a line can be
drawn perpendicular to the surface of the mirror. This line is known as a
normal line (N).
• The angle between the incident ray and the normal is known as the angle of
incidence (θi). The angle between the reflected ray and the normal is known
as the angle of reflection (θr). The law of reflection states that when a ray
of light reflects off a surface, the angle of incidence is equal to the angle
of reflection.
θi = θr
Example #1
• The diagram above shows two rays of light striking a plane
mirror. Which diagram below best represents the reflected
rays?
1.
2.
3.
4.
Example #2
•
1.
2.
3.
4.
A ray of light strikes a mirror at an angle of
incidence of 60°. What is the angle of
reflection?
0°
30°
60°
90°
Example #3
•
The diagram represents a light ray being reflected from a
plane mirror. The angle between the incident ray and the
reflected ray is 70.°. What is the angle of incidence for this
ray?
1.
2.
3.
4.
20.°
35°
55°
70.°
• Once a normal to the surface at the point of
incidence is drawn, the angle of incidence can then
be determined. The light ray will then reflect in such
a manner that the angle of incidence is equal to the
angle of reflection.
The Law of Reflection is Always Observed
(regardless of the orientation of the surface)
Specular vs. Diffuse Reflection
• light reflects off surfaces in accordance with the law of
reflection. The Law of Reflection is Always Observed
(regardless of the orientation of the surface)
• Specular reflection: Reflection off of SMOOTH
SURFACES such as mirrors or a calm body of water.
• Diffuse reflection: Reflection off of ROUGH SURFACES
such as clothing, paper, and the asphalt roadway.
• Each individual ray obeys the laws of reflection.
Why Does a Rough Surface Diffuses A Beam of
Light?
• For each type of reflection, each individual ray follows the law
of reflection. However, the roughness of the material means
that each individual ray meets a surface which has a different
orientation. The normal line at the point of incidence is
different for different rays. Subsequently, when the individual
rays reflect off the rough surface according to the law of
reflection, they SCATTER in different directions. The result is
that the rays of light are incident upon the surface in a
concentrated bundle and are diffused upon reflection.
Class work – today’s date
1. Identify which angle is angle of incidence and
which angle is angle of reflection.
• Incident angle
is ___
• Reflected angle
is _____
A
B
C
D
2. A ray of light is incident towards a plane mirror at an angle of
30-degrees with the mirror surface. What will be the angle
of reflection?
3. A ray of light is approaching a set of
three mirrors as shown in the
diagram. The light ray is approaching
the first mirror at an angle of 45degrees with the mirror surface.
Trace the path of the light ray as it
bounces off the mirror. Continue
tracing the ray until it finally exits
from the mirror system. How many
times will the ray reflect before it
finally exits?
4. A ray is incident on a plane mirror at 25 degrees
(incident angle). Draw a diagram to indicate the
a. The surface of the mirror
b. The line of normal
c. The incident ray
d. The reflected ray
Image Formation in Plane Mirrors
• An image is formed in a plane mirror because light gives off
from an object in a variety of directions. Some of this light
(rays) reaches the mirror and reflects off the mirror according to
the law of reflection. Each one of these rays of light can be
extended backwards behind the mirror where they will all
intersect at a point (the image point). Any person who is
positioned along the line of one of these reflected rays can sight
along the line and view the image - a representation of the
object.
Image characteristics in a plane mirror
1. An image has the same size as the object.
2. The image is as far behind the mirror as the
object is in front of the mirror.
3. The image has the same orientation as the object.
4. The image is laterally inverted. (left and right
reversal)
5. The image is virtual, no actual light meet at the
image position. Virtual image can not be captured
on a screen.
Ray diagrams
1. Draw the image of the object.
2. Pick one extreme on the image of
the object and draw the reflected
ray that will travel to the eye as it
sights at this point.
3. Draw the incident ray for light traveling from the corresponding extreme
on the object to the mirror.
4. Repeat steps 2 and 3 for another extreme on the object.
Practice – draw ray diagram
What Portion of a Mirror is Required?
• Ray diagrams can be used to determine what portion of
a plane mirror must be used in order to view an image.
•In order to view his image, the man
must look as low as his feet, and as
high as the tip of his head. The man
only needs the portion of mirror
extending between points X and Y in
order to view his entire image. All
other portions of the mirror are
useless to the task of this man
viewing his own image.
to view an image of yourself in a plane mirror, you
will need an amount of mirror equal to one-half of
your height.
Class work – today’s date
1. If Suzie stands 3 feet in front of a plane mirror, how far from
the person will her image be located?
2. If a toddler crawls towards a mirror at a rate of 0.25 m/s, then
at what speed will the toddler and the toddler's image approach
each other?
3. A ray is incident on a plane mirror at 60 degrees (incident
angle). Draw a diagram to indicate the
a. The surface of the mirror
b. The line of normal
c. The incident ray with direction
d. The reflected ray with direction
4.
5.
In the diagram, a light ray leaves a light source and reflects
from a plane mirror. At which point does the image of the
source appear to be located?
a. A
b. B
c. C
d. D
When a ray of light strikes a mirror perpendicular to its surface, the angle of
reflection is
a.
0°
b.
45°
c.
60°
d.
90°
6.
Parallel light rays are incident on the surface of a plane
mirror. Upon reflection from the mirror, the light rays will
a. converge
b. diverge
c. be parallel
d. be scattered
7.
A plane mirror produces an image of an object. Compared to the object, the
image appears
a.
inverted and the same size
b.
reversed and the same size
c.
inverted and larger
d.
reversed and larger
8. When a student looks into a plane mirror, she sees
a virtual image of herself. However, when she looks
into a sheet of paper, no such image forms. Which
light phenomenon occurs at the surface of the
paper?
a. regular reflection
b. diffuse reflection
c. polarization
d. resonance
9.
In the diagram, a light ray, R, strikes the boundary of air and
water. Look at the graph and estimate the approximate
degree of the angle of incidence.
a.
b.
c.
d.
31°
61°
119°
149°
10. A ray of light strikes a plane mirror at an angle of incidence
equal to 45°. What is the angle between the incident ray and
the reflected ray?
11. Use a protractor and ruler to measure and draw the reflected
rays in each of the following pictures.
a.
b.
Lab 35 – law of reflection
Purpose: Verify the law of reflection: the angle of incidence and the angle of
reflection are equal.
Material: Plane mirror, Pins, Plane paper, Cardboard, Straightedge, protractor,
pencil
Procedur:
– Briefly describe how the lab is going to be done. Someone who was not
present during the lab should be able to understand how the experiment
was perforem and be able to reporduce the results by reading your
procedure.
Data section:
– The Data Section should include a data table with labeled column
headings (and units)
Conclusion/discussion of results:
1. What is the relationship between the incident and reflected angles?
2. Does the light reflect off the front surface or back surface of the mirror? Is
there any evidence of this from your experiment?
Procedure:
1. Place the mirror on a piece of paper. Draw a line to indicate
the front of the mirror.
2. Stick two pins to indicate the path of the ray going from the
pin to the mirror; stick two more pins – make sure they line
up with the images of previous two pins – this line indicate
the reflected rays. Now you should have all four pins in front
of the mirror.
3. Remove the pins and carefully trace the path of the incident
beam and the reflected beam on the paper.
4. Using a protractor, draw a normal line for the place where the
incident ray from the pins hit the mirror.
5. Now measure both the incident and reflected angles.
• Each person in your lab team must do this for themselves.
Combine your data on your data chart.
Data table
name
angle of
incidence (o)
angle of
reflection (o)