Optical Images

Submitted by:Jan Mader, Great Falls High School, Great Falls, MT
Endorsed by: These lesson plans are the result of the work of the teachers who have attended the Columbia Education Center's Summer Workshop. CEC is a consortium of teacher from 14 western states dedicated to improving the quality of education in the rural, western, United States, and particularly the quality of math and science Education. CEC uses Big Sky Telegraph as the hub of their telecommunications network that allows the participating teachers to stay in contact with their trainers and peers that they have met at the Workshops.
Date: May 1994

GRADE LEVEL/SUBJECT: Appropriate for physical science or physics classes

OVERVIEW: Students beginning the study of optics will recite the law of reflection, but are unable to apply that concept to ray diagrams when viewing images. Using a laser students can aim at the image and conceptualize the law of reflection.

This lesson is written in a learning cycle format.

PURPOSE: How does the reflected light from an object travel to your eyes?

RESOURCES/MATERIALS: Two plane mirrors(K Mart mirror tiles work well), supports for the mirrors, a golf tube cut in half, a laser, chalk dust or a vaporizer.

ACTIVITIES AND PROCEDURES:
Set one plane mirror upright on a sheet of paper. Place a half of a golf tube in front of the mirror. Locate the image formed by the mirror and place the second half of the golf tube behind the mirror where the image of the first appears to be. (HINT: If you have located the image correctly, the image of the first tube and the second tube will remain together as you move your head from side to side.)
Draw the path you think the light takes from the first tube to your eye as you observe the image.
Draw a dotted line to where the image appears to be located or seen by the observing eye.
Check your diagrams by utilizing a laser and chalk dust or a vaporizer. Aim the laser at the image in the mirror. Use chalk dust or the vaporizer to make the beam visible.

Summing Up:
1. How did the distance from the first golf tube to the mirror compare to the distance of the mirror to the image (Location of the second golf tube)?
2. Did your laser beams line up with the lines that you had drawn for the path that the light takes from the first tube to your eye as you observe the image?
3. If you were to draw a line perpendicular to the surface of the mirror where the laser beam struck the mirror and then measure the angles formed between this line and the incoming and exiting beam what did you discover?

For many students, the typical ray diagram will be a line from the object straight to the mirror (usually perpendicular to the mirror) and reflecting off at an angle to the location of their eye when they were looking at the object's image. Have the students use a laser and shine it at a mirror, water's surface or a glassy surface at different angles. Have them measure the incoming or incident ray from the perpendicular, the exiting or reflected ray from the perpendicular and compare them. Have them try several different angles. From the lab, the students are now able to state for you the law of reflection. The angle of incidence is equal to the angle of reflection for any reflecting surface and any incident angle.

The students may apply this concept by predicting the length of a mirror that they must use to see their entire image.

PURPOSE: What is the minimum length of a plane mirror necessary to see the full image of yourself?
How does the distance of the person from the mirror affect the length of mirror needed to see the entire image?
How is this related to the law of reflection?

MATERIALS: Full length mirror, meter stick, construction paper, masking tape

ACTIVITIES AND PROCEDURES:
Have a student stand at arms length from a full length mirror.
Slowly move a piece of construction paper from the top of the mirror, stop when the student says that he/she can just see the top of his/her head.
Repeat the process from the bottom of the mirror, stopping when the feet are just visible.
Measure this distance and have the student compare it to his/her original height.
Double and quadruple the distance from the mirror, repeating the above procedure.
Organize the data in a table and answer the following questions.

Summing Up:
1. How did the length of the mirror required to see the entire image compare to your actual height?
2. Explain the reason for the minimum mirror length that you found.
3. How did the distance that you stood from the mirror affect the size of the mirror that was required to see the entire image?

Teacher's notes.
If the students still have difficulty recognizing that they only need a mirror that is half of their height, allow them to use the laser and aim at the image of their feet. They will see that the laser beam aimed at their feet will reflect from the midpoint of the length of mirror that is available and strike their feet.