Phy 1033
Discovering Physics
Laboratory #2
Reflection and Images
In this laboratory we use a simple mirror and straight pins to test the rules of reflection
and to find the location of images in a plane mirror. The pins are used as “objects” as well
as to mark images and to line up rays of light; for accuracy, when sticking a pin into the
board, keep the pin as vertical as possible. And when lining up three pins make sure that
the bases of the pins are in line.
Put the “setup sheet” on the pin board and carefully position the mirror on the appropriate line. Place a pin at point #1—this pin is the object. Place another pin at point #2a
on line 2—this marks a ray of light.
Look in the mirror and note that if you put your head in just the right spot the two pins
line up in the mirror—in other words a ray of light (following the path marked a) leaves pin
#1, goes past pin #2, hits the mirror, is reflected and goes in your eye.
Stick two more pins in line with the images in the mirror and have one pin on line 2 and
the other on line 3. These last two pins mark the path of the reflected ray of light. Put little
circles around these pins to mark their locations, label the locations with an a and remove
the pins.
Now, move the pin from point #2a over to point #2b and repeat the procedure with
pins on lines 2 and 3 marking the reflected ray of light and labeling the locations with a b.
Remove all of the pins but #1. To find the line which is normal to the surface put a pin
in line 2 which is in line with both pin #1 and the image of pin #1. Mark that point and
draw in the line through pin #1 which is normal to the mirror.
Again, remove all of the pins but #1. Now place a pin behind the mirror at the apparent
position of the image of pin #1. This is a bit tricky to do well. Use depth perception and
move your head from side to side to put the real pin at the same spot as the image of pin
#1. Mark the location of the image with a small circle around the pin, remove the pin and
let all of the laboratory partners have a try at locating the image.
All of the hard work is complete. Draw lines marking the path of light from one pinhole
to another and add an arrow which marks the direction the light ray travels. Put the pins
and mirror away, and extend the rays which mark the path of the light all the way to the
mirror line. Measure the angle of incidence and the angle of reflection of both ray a and ray
b. Put these numbers in a table in the lab book. Are these values in agreement with what
you might have expected based upon your knowledge of reflection?
Extend all of the rays reflected off the mirror back beyond the approximate location of
the image. Do these three lines intersect? Measure the distance from the object to the
mirror along the normal, and measure the distance from the image to the mirror along the
normal. Do these distances agree with your expectations? How good were your estimates of
the image location?
When you are finished, staple the “setup sheet” into your lab book.