PLEASE DO NOT SHINE ANY FLASHLIGHT, LASER POINTER OR OTHER STRONG LIGHT SOURCE INTO THESE MIRRORS.
Look into the corner. In the reflection, find your right eye. Without moving your head, use your right hand to touch your right cheek.
Look into the corner. Close one eye and try to move your head so that your nose is in the intersection of the three mirrors. You will probably find that you can't put your nose exactly in the corner.
Look into the corner while holding a finger on your right cheek (for reference). Close your right eye and observe the image in the corner. Open your right eye and close your left eye and observe the image in the corner.
Look into the corner and then move your head a few inches to one side or the other.
The three mirrors are at right angles to each other. Light striking one of the mirrors close to the corner will be reflected back parallel to and very close to the incoming light.
Some bicycle reflectors, automobile tail lamps and highway reflectors have corner reflectors to make them more noticeable to traffic.
One of the Apollo lunar missions left a corner reflector on the surface of the moon so that scientists on earth could easily measure the time needed for a beam of light to travel to the moon and back.
An inexpensive corner reflector can be made using masking tape and three small square mirrors. For safety, ask a glass shop to make the mirrors using plastic rather than glass.
Without touching any of the mirrors, extend your right hand close to the corner with your thumb pointing up. How many images of your hand do you see? Why are the left and right images of your hand different?
There are a total of seven reflections of your hand. The left side reflection is showing the left side of your hand, and the right side reflection is showing the right side of your hand.
Without touching any of the mirrors, hold your fingers very close to one of the mirrors. You might notice a very faint ghost image of your fingertip and fingernail alongside the obvious reflection. What is causing the ghost image? Similarly, what is causing the thick lines between the mirrors?
The actual reflecting surface is on the back side of the mirror. Most of the light from your fingertip passes through the front surface and is reflected by the mirrored back. A small fraction of the light however is reflected by the front surface. If the room is dim, this ghost image may not be visible.
The mirrors overlap at the corners. The edge of one mirror is facing the other. When you look into the corner, you are seeing both the edge and its reflection.
Look at one of the seams between two mirrors. After a few moments you may see two images of the seam. Blink your eyes and you probably see one seam again. Explain.
Both of your eyes are receiving an image of the seam. Initially, your brain ignores the image from one of the eyes, but after a few moments it accepts both images. After blinking, the process repeats.
Hold a finger on your right cheek and look into the corner. Alternate closing first one eye and then the other. What happens to the image of your eye in the corner? Why?
Light rays which travel exactly into the corner are reflected Your eyes are several inches apart on your face. Light from your left eye strikes the corner and is reflected back to your left eye. Light from your right eye strikes the corner and is reflected back to your right eye. When you close your left eye, your right eye sees your face with your right eye centered on the corner. When you close your right eye, your left eye sees your face but now with your left eye centered on the corner. As you do this, the corner also seems to move left and right a few inches because of the distance between your eyes.
Look into the corner. Blink one eye for an instant, wait about a second, and then blink the other eye for an instant. Repeat this several times. First one eye and then the other is centered in the corner, even when both eyes are open. Why?
Your brain is always receiving a slightly different image from each eye. The brain treats one eye as dominant so that you are not seeing double. When you alternate blinking, you are briefly alternating this dominant status back and forth between your eyes.
For the corner exhibit to be most effective, the three mirrors should be at exact right angles to each other. Why?
When a beam of light strikes a mirror, the angle of incidence equals the angle of reflectance. If it strikes the mirror at x degrees, then it must reflect at x degrees. When the beam hits one mirror and reflects into another mirror which is at right angles to the first, the section of the beam between the two mirrors forms the hypotenuse of a right triangle. The sum of the angles of a triangle equals 90 degrees, so if it strikes the first mirror at an angle of x degrees, it strikes the second at an angle of (90-x) degrees. Because angle of incidence equals angle of reflectance, the beam reflects from the second mirror at (90-x) degrees. Extend the lines representing the first mirror and the beam leaving the second mirror to form another right triangle. Opposite angles at the intersection of two lines are equal, so the upper angle of this second triangle must be (90-x) degrees. The third angle in this triangle is (180-(90-x)) degrees, which simplifies to x degrees, so that the two beams of light are parallel.
challenge question 7 needed
challenge question 7 answer
Geometry, reflectance, first surface mirror, second surface mirror, vision
further information listing needed
This exhibit is described in the Exploratorium Cookbook series.
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