Experiment: Investigating the Law of Reflection

Experiment Investigating

A fundamental law of light is that the angle of incidence equals the angle of reflection when light strikes a reflecting medium. The angles are commonly measured normal (perpendicular) to the reflecting surface.

This law can be studied quantitatively by students using only seven littleBits, three mounting boards, a small mirror, and commonly available materials.

Duration: 1 45-minute class

Middle School (ages 11-13)
High School (ages 14-17)



long led (1)
light sensor (1)
power (2)
mounting boards (2)
battery + cable (2)
number. (1)
wire (2)


STEP 1 : Make the Large Protractor

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Using half of a 22″ x 28″ poster board and a protractor, create a large protractor as shown below. Space the lines 5 degrees apart and marked in degrees measured from either side of the center line.

STEP 2 : Set up the Light Source

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This step requires only two littleBit modules–a power source and a long led. We want the light beam to be as narrow as possible. This can be accomplished by inserting the long led into a soda straw. The battery can be attached to the mounting board by Velcro. Notice that the straw has been carefully lined up with one of the lines on the large protractor. The end of the straw where the light emerges is about two inches from the mirror to keep the light beam as narrow as possible before it hits the mirror.

STEP 3 : Set up the Light Sensor

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The light sensor circuit consists of five modules in this order: power>wire>light sensor>wire>number. The circuit is on two back-to-back mounting boards as shown in the picture below. The power/battery and number module are on the mounting board facing the experimenter, and the light sensor is on the mounting board facing the mirror. You can use Velcro and rubber bands to hold the two mounting boards together. Velcro was also used to hold the battery to the mounting board, and the bottom of the battery is flush with the bottom of the mounting boards to provide support to keep the boards from falling over. The number module is set to values so that it will read from 00 to 99. The light sensor is set to “light” and the sensitivity is adjusted so that readings do not exceed 100. It is best to have the room where the experiment is done darkened, so that the light sensor is sensing mostly light from the reflected beam.

STEP 4 : Set up the Mirror

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The mirror can be small, about 2″ x 3″ is fine. It can be attached to a block of wood using Velcro to keep it upright. It should be centered at the origin point of all of the protractor lines, and perpendicular to the table holding the large protractor.

STEP 5 : Begin Data Collection

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The figure below shows how to measure the angle of incidence and angle of refraction. As you can see, they are measured from a line normal (perpendicular) to the surface of the mirror.

It is suggested that the student start with an angle of incidence of 40 degrees by carefully placing the soda straw with the light beam so that it is aligned with the protractor grid line marked 40 degrees on the right side of the large protractor. Then move the light sensor board on the left side of the protractor, keeping it perpendicular to the grid lines as you move it. The number module should reach a peak value when the light sensor is at the brightest part of the reflected beam. The reading will decrease if you increase or decrease the angle where the sensor gives this maximum number reading. For your final value for the angle of reflection, select the angle of the sensor that is at about the center of the range for which the number reading stays at a maximum. Students should interpolate to the nearest degree if possible by noting carefully the location of the light sensor along the degree grid.This process can be repeated for 50, 60, 70, 80, 30, 20, and 10 degrees as angles of incidence. Students can record their data on the attached sheet called Light_Reflection_Experiment.pdf. Finally, the data can be recorded on the graph included in this pdf file.

If you have several groups of students in the class doing the experiment, all of the data from the groups combined could be plotted on the same graph.

STEP 6 : Data Analysis

Ask students if their data seems to verify the law of reflection–angle of incidence equals angle of reflection–within reasonable limits of error.

Ask students what characteristic of the graph validates the law of reflection?

Ask students to suggest possible sources of error in the experimental results.