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Mirror, Mirror on the Wall?

Project by Kathy Wellborn

Teacher Pages

In this activity the students will be looking at the phenomenon of reflection. It is very common to check ourselves out in a mirror, but what about a pane of glass? How many times have you walked past a window and seen your reflection? With the use of a CBL and a light sensor probe we will measure the intensity of light off a pane of glass.

Materials Needed:

1CBL with graphing calculator

1Light sensor probe for the CBL

1Quilting or sewing template with grids

1light source with a parallel beam. A ray box or mag light that will focus into a parallel beam.

1 — Flat glass with a frame. Any picture frame will work well.

1- Set of corner braces

The light and sensor have to be on the same plane if you need to adjust the height of the light you may need a book or pieces of scrap wood.

Construction and Set-up:


Before the activity attach the corner braces to the back of the frame evenly so that when you stand the frame up it is at a 90-degree angle to the table.

Corner brace at both back corners to stand the glass at a

90-degree angle.

Once you have the corner braces on you may use a weight on each brace to keep the glass from falling over or you may attach it to a larger board to stabilize it.


Using the template sheet lay it flat on the table. Choose the line which will represent the 90-degree angle. Using a protractor place the center of the protractor on the bottom line and line it to the 90-degree line. It helps to tape the template to keep it from sliding. When you begin to measure the angles remember that you are measuring away from the normal and not towards it. It is easiest to measure the large angles starting with 45-

degrees. Draw a line from the center of the protractor out to the edge of the paper. It makes an easy reference to place the degrees of the angle away form the 90-degree line at the top of the angle and the degree of the angle away from the 0-degree at the bottom of the line.


When drawing the angles a permanent marker works nicely.


When the set up is ready you will need to have the students program the

CBL for an event with entry with their light probe.


A small piece of tape or glow in the dark paint may be placed on the frame or a small amount on the glass to help the students keep in focus the center of the glass.


Now the equipment is ready. The set up below is just a suggestion you may find a better way to set it up depending on individual classrooms.

Mirror, Mirror on the Wall?

Objective: To observe the light intensity of reflection off of a pane of glass using a light probe and a CBL.

Materials needed:

1 - CBL and graphing calculator

1 — ray box, or a parallel light source

1 — pane of glass (covered on one side with black paper) in a frame with corner braces attached to the back

1 — light probe for the CBL

1 — template with angles of 44, 60 and 75 already drawn on them

Procedure :


Place the pane of glass on a table top with a flat smooth surface. Check to make sure that the glass is at a 90-degree angle to the table.


Place your light source on the table to make sure that the light will strike the glass with out any interference from the frame. If you need to raise the light source place an object on the table large enough for the light box to be moved.


Once you have the light source working and at the correct height set up your



Turn on your CBL and place the light probe in the CH 1 slot.


Set up the CBL to have and event with entry mode for the light probe.


Darken the lights in your working area. Place the template in front of the glass so that the center of the radius is at the base of the mirror.


Place the light source on the pre-drawn 45-degree line and line up the light so that it hits directly center of the template.


Carefully align the probe to catch the reflection. The probe should be at 45degrees and there should be a line across the front of the probe.


Collect the data for the 45, 60, and 75 degree angles.


Graph the information and answer the following questions.


Was the intensity of the light increasing or decreasing with the increasing of the angles?

Why do you think this happened this way?

Why would it be almost impossible to measure the intensity at a 0 degree angle? 90degree angle?

Will there be any two angles which will have the same light intensity?

Now take the same glass and with the lights on move the glass in front of you so that you see your reflection. At what point was your reflection clearest? At what point could you not see your reflection?

How does looking in the glass at your reflection compare to the intensity of the light being reflected?

Mirror, Mirror on the Wall ?

Resource Page

Before beginning this activity the students should have some information on light. The following is a brief summary of some of the information that will be helpful in doing the activity.

Reflection- returning or bouncing of a wave off of a surface.

Normal — Line perpendicular to the surface is imagined at the point of reflection.

Incident Ray — Ray of light striking the surface.

Incident Ray — Ray that leaves the surface.

Angle of Incidence — angle between the incident ray and the normal.

Angle of Reflection — angle between the reflected ray and the normal.

Scientific Law of Reflection

Oi = Or Angle of incidence = Angle of reflection.

Picture goes here

Helpful Web Pages: http://mind.net/~zona/mstm/physics/light/rayOptics/reflection/reflection1.html

http://library .thinkquest.org/28160/English/reflection/reflec.html


this site will give you more information on CBLs

angle of incidence of light

(degrees) intensityof light

(arbitrary units)

45 0.007967

60 0.009911

70 0.011854

80 0.038088

Intensity of light reflected off of a pane of glass










0 20 40 60 80

Angle of incidence of light(degrees) measured from the normal