SNC 2D
Name:__________________
11.5 Reflecting Light Off a Plane Mirror
Purpose: to compare the angle of incidence with the angle of reflection in a plane mirror
Hypothesis: It is hypothesized that when hitting a plane mirror with an incident ray at a specific angle from
the normal, that you would get a reflected ray that would reflect with the identical angle as the incident
angle. Meaning, the angle of incidence will equal the angle of reflection no matter what angle you aim the
incident ray at.
Equipment and Materials:
- ray box
- plane mirror
- pencil and ruler
- protractor
- sheet of paper
Com:
T/I:
Procedure:
- see page 482-483
1. Draw a horizontal dashed line across the center of a sheet of paper. Label it mirror
2. Draw a perpendicular solid line coming down from the center of the dashed line. This will form a T on
the bottom half of your page. Label this line the “normal”.
3. Place a mirror, facing you, on the dashed line with the reflective backing on the line, not the glass.
4. Set a ray box with a single slit on the table in front of you but to one side of the normal. Point the ray on
an angle but so that the beam of light hits the intersection of the “normal” and the mirror (see photo).
5. Place several dots along the incident ray. Then draw several dots along the reflected ray. Label the
incident ray I1 and the reflected ray R1
6. Remove the ray box and mirror. Using your ruler, draw a line through the dots.
7. Use your protractor to measure the angle of incidence and angle of reflection. Remember to measure
with respect to the normal (the angle between the normal and the ray). Record your results.
8. Repeat steps 3-7 for an additional 3 trials using different angles on incidence. Labelling each ray
appropriately (I2, R2, I3, R3).
9. Do a fifth trial but this time aim the incident ray directly along the normal.
Observations:
Trial number
Angle of incidence
Angle of reflection
1
Whatever angles you used for the incident
rays, they should be very close to being
equal to the reflected angle
It was ok to have angles
that were not exactly
equal as it would show
that these labs are hard
to get perfect results
due to human error
2
3
4
5
SNC 2D
Name:__________________
Analysis:
1. How did the angle of incidence compare with the angle of reflection?
The angle of incidence was equal (or at least very close to being equal) to the angle of refraction. This is
because you are hitting a flat surface and the fact that light travels in straight lines, therefore reflecting off
in a symmetrical direction.
2. In trial #5, you aimed the incident ray directly along the normal. Describe the path of the incident
and reflected rays for this special case.
The incident ray was aimed along the normal and the reflected ray reflected right back along the normal.
This is because you are hitting the mirror at a 90 degree angle to the mirror which causes it to reflect right
back. It’s hitting the surface head on so there is not angle to make the light reflect in a different direction.
3. Where might errors occur in this activity?
There are many places for errors to occur in this activity.
-The mirror not being set exactly on the horizontal line drawn, causing it to be on an angle with respect to
the normal (no longer perpendicular to each other).
-The mirror could be tilted too much, causing the reflected angle to be slightly off.
-When tracing out the reflected ray, you might not be accurate enough.
-If you didn’t aim the incident ray exactly at the vertex.
-If you made errors when measuring the angles with the protractor (not measuring accurately)
4. How would these errors affect your conclusion?
All of the errors listed above could cause your measured angles to be slightly off, making it hard to
conclude that the incident angle should equal the reflected angle.
5. Billiards is a game that makes use of reflection. How could the results of this activity help you in
such a game?
When playing Billiards (pool), one often uses the sides of the table to bounce the ball off to either hit
another ball or to sink a ball. If the player understands that the angle of incidence equals the angle of
reflection, then they will know where they must aim along the sides in order to get that symmetrical
reflection and hit the desired target.
6. What other sports or activities make use of the reflection rule that you discovered in this activity?
Hockey uses this reflection rule when one uses the boards to pass the puck to a teammate. Basketball uses
this rule when bounce passing the ball to a teammate, or when using the back boards when getting a
basket. Squash uses this rule when players hit the squash ball at the wall to the opposing challenger. There
are many other examples, but overall, it would be any sport that requires you to use a wall to bounce a ball
off of.
Conclusion: What can you conclude about incident and reflected ray of a plane mirror?
One could conclude from this lab that observations support the law of reflection, in that the angle of
incidence is equal to the angle of reflection. The observations support the hypothesis as all of the angles of
reflection were measured to be the same or close to the same as the angles of incidence. The values
measured in the lab were very accurate and helpful in proving the law of reflection.