TITLE: Optical Images
AUTHOR: Jan Mader, Great Falls High School,
Great Falls, MT
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.
Mirror Images
Exploration
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?
Concept Development for the Lab - Mirror Images
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.
Mirror Mirror on the Wall, Who is that Dashing Physics
Student?
Application
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.
Data Table
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.
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John Kurilecjmk@ofcn.org