EdGE-ucation
Title: Law of Reflection
Subject: Physics/Light & Optics
Authors: Richard Marrese (Pilgrim High School); Erin Bardar, Jodi Asbell-Clarke, Teon Edwards
(EdGE at TERC); John Macuk (Arlington High School); Ed Yoo (Codman Academy)
Grade Level: 9-12
Standards Addressed: TBD
Time Needed: TBD
Objective(s): Discover and demonstrate and understanding of the Law of Reflection
Summary: Students will use slope to empirically discover the Law of Reflection (angle of incidence =
angle of reflection). Students will also demonstrate their understanding of the Law of Reflection by
bouncing laser light off two mirrors and “zapping” an image of a ghost. Students will not be allowed to
turn on their laser pointers until they have placed their mirrors in their final positions.
Materials Needed (per group):
• 2 flat mirrors
• 2 ring stands
• 2 clamps
• protractor
• string
• 1 proton pack (aka, laser pointer)
• 1 ghost image
Teacher page 1
EdGE-ucation
Solutions: Part 1
1.
incident
reflected
rise
run
slope =
rise/run
rise
run
slope =
rise/run
mirror 1
3
3
1
5
5
1
mirror 2
5
5
1
3
3
1
mirror 3
3
3
1
2
2
1
mirror 4
2
2
1
5
5
1
2.
θ = tan-1(slope)
θi
θr
θi
θr
mirror 1
45°
45°
45°
45°
mirror 2
45°
45°
45°
45°
mirror 3
45°
45°
45°
45°
mirror 4
45°
45°
45°
45°
The Law of Reflection states the relationship between the angle of incidence and the
angle of reflection. What is that relationship?
The angle of incidence = the angle of reflection
Teacher page 2
EdGE-ucation
3. The ray diagram below shows that the observer will see the sun reflected in the window at 7 pm,
but not at 1 pm. Based on the Law of Reflection, the angle of incidence = the angle of reflection. In
the evening, the sun is low in the sky and hits the window at a small enough angle (relative to the
normal) that the reflected ray reaches the observer’s eyes. At 1 pm, the sun is high in the sky, so a
ray drawn to the window will reflect and intersect with the ground without ever reaching the observer.
sun at
1 pm
sun at
7 pm
θi
θr
θi
normal
θr
Teacher page 3
EdGE-ucation
Title: Law of Reflection
Part 1: Introduction
The Law of Reflection is a basic principle that describes how light behaves when it reflects off a flat
mirror. Although you experience this phenomenon every single day, you may have never really
stopped to think about the details of how it works. In this activity, you will use simple geometry to
discover the Law of Reflection.
1. Look at the mirror maze below. Starting at the red laser, follow the path of the beam as it reflects
off all four mirrors on its way to the target. Measure the rise and run of each incident and reflected
laser beam segment (in units of grid squares). Record the values in the chart below. Then
calculate the slope for each incident and reflected ray by calculating rise divided by run and fill in
the values on the table.
4
1
run
rise
3
red laser
2
target
incident
rise
run
reflected
slope =
rise/run
rise
run
slope =
rise/run
mirror 1
mirror 2
mirror 3
mirror 4
Student page 1
EdGE-ucation
2. Calculate the angle of incidence (θi) and angle of reflection (θr) for each mirror using the equation:
θ = tan-1(slope)
θi
θr
θi
θr
mirror 1
mirror 2
mirror 3
mirror 4
The Law of Reflection states the relationship between the angle of incidence and the
angle of reflection. What is that relationship?
3. Have you ever seen the sun reflected in the window of a house or other building around sunrise or
sunset? How about in the middle of the day? Use the diagram below, drawing appropriate light
rays, to explain which time of day the observer is most likely to see the sun reflected in the
window of the house.
sun at
1 pm
sun at
7 pm
Student page 2
EdGE-ucation
Part 2: Introduction
Ever wanted to be a Ghostbuster? Of course you have and now is your chance!
Each group has been given a ghost that they must “bust.” In addition to the ghost,
each group has also been given a protractor, string, 2 mirrors, 2 ring stands, 2
clamps, and a proton pack…I mean laser pointer. Your task is to bounce the laser
beam off of the two mirrors and zap the ghost.
Rules
1. The laser must reflect off of BOTH mirrors before hitting the ghost.
2. Mirror #1, mirror #2, and the laser pointer must all be on different tables.
3. The laser may not be turned on until both mirrors are in place.
4. Once your group is confident that your mirrors and laser are lined up, you can turn the laser on.
When the laser is turned on, your group has 30 seconds to adjust the LASER ONLY to try and
bust the ghost.
5. A successful trial is when the red dot of the laser lands anywhere on the ghost’s piece of paper.
Grading
1. If each mirror and laser pointer are on different tables and the laser pointer hits the ghost within 30
seconds on the first trial, each group member will receive a score of 400/400.
2. If the group does not hit the ghost within 30 seconds on the first trial, the ghost will change
location (they do that) and the group must reset their mirrors.
3. For each trial after the first that a group does not bust the ghost, 20 points will be deducted from
the group’s score.
Student page 3