Plane Mirror Images

Ann Brandon                    Joliet West High School
                               401 N Larkin Ave
                               Joliet IL 60435
                               (815) 727-6950

Objectives:

The student will know:
    1.  Where a plane mirror image is (behind the mirror, not on the mirror)
    2.  How the light reflects (making equal angles)
    3.  How the "extra" images are formed with multiple mirrors

Materials needed:

Two large (foot square?) mirrors, two sheets of clear plexiglass-also about 1 
foot square, four candles, some matches, large paper (about 2 feet x 2 feet),
Something to hold up the mirrors, scotch tape, some markers or crayons

Strategy:

I do this as a class demo, after the students have experimented with plane 
mirrors. 

Part I:
     Place a large sheet of paper on demo table or desk top.  Put one mirror 
facing the students.  Place a candle in front of the mirror, light it, stand
behind the desk (so the students can see!) and ask where the image is.

     Replace the mirror with a piece of plexiglass.  Turn out the lights for the
best results.  Again, ask where the image is.

     Put a second candle randomly behind the plastic.  Have the students tell 
you where you should move it so that this extra candle is exactly where the 
image is (centered under the image's flame).  When everyone agrees on the 
location it is in the right spot.  You should call their attention to this.  It 
is only right when everyone agrees.  There is only one perfect spot! 

     Mark the location of the mirror and both candles.  Use magic markers or
crayons.

     Now have a student use a meter stick to aim at the image.  Draw this line 
of sight on the paper.  With a different student and a new color, get a line of 
sight from some other direction. 

     Move everything off the paper.  Turn on the lights.  Hang the paper up on
the board.

     Pick one of the lines of sight.  Point out that the light comes off the 
mirror and goes along this line to their eye.  Also point out that we have 
picked out only two of the thousands of possible light rays.  Ask how the light 
got to that point on the mirror.  Point out that it did not come from the candle 
behind the mirror--it was not lit!  They should tell you that it came from the 
front, lit, candle. 

     Draw the path from this candle to the point on the mirror where it 
reflected out the line of sight.  Ask about the angles the light makes when it 
hits the mirror.  They should look equal.  You can measure them with a 
protractor. 

     Repeat this for the other line of sight.  Its angles will also be equal, 
but not necessarily equal to the first set of angles. 

     Ask about the distance of the object and image to the mirror.  They will be
equal.

Part II: 
     With a new piece of paper:  Set up two mirrors at right angles to each 
other, facing the students.  Mark the location of the mirrors.  Place a candle 
in front of the mirrors and light it. 

     Ask how many images they see.  (There should be three, if there are four,
the angle is not quite 90o.  Play with it.)

     Replace the mirrors with plexiglass.  Check the angle.
     
     Work on the image locations, one image at a time.  Do the two "outside"
images first.  The third, central image is the tough one.  After you have all
three, mark the locations of all of the candles.

     Lift one mirror and ask how many images there are now.  Remind them that
images look lit.  Point out that this is a simple image, from one mirror. 

     Replace that mirror and remove the other.  Repeat discussion.  Replace that
mirror.

     Have a student use a meter stick to draw a line of sight to the center
image.  A second student can draw a line of sight to the center image from some
other direction.

     Turn on the lights and hang the paper on the board.

     Follow one of the lines of sight back to the mirror.  Ask how this light 
ray got to that point on the mirror.  If it came from the candle, the angles 
will not be equal.  Draw the light ray so that the angles are equal.  (You can 
do this without measuring.  It will seem to come from the far side image.)  It 
will be seen that this light ray came from the other mirror.  It got to that 
mirror from the object candle, making equal angles at that first mirror, 
reflecting to the second mirror and then out. 

     What can you say about the distances of all the images?  (The mirror is
halfway between the candle and each of its images.  Thus, the object distance
equals the image distance for each image.)

Conclusions:

For Plane Mirrors:  
    Image distance = Object distance
    Light reflects making equal angles. 
       (i.e. Angle of Incidence=Angle of Reflection or ji=jr)

Performance Assessment:

    Ask the question:  How large a mirror do you need to see yourself from top 
to toe?

Rubric:

    This can be solved using either Multicultural Applications:

    The bubonic plague was spread throughout England.  The colleges were closed 
to reduce the chance of spreading the disease through the student population. 
Isaac Newton spent the next 18 months experimenting with light and researching 
his observations in Latin. Before he was 25 he published them in a book called 
"Optics."