Your Father's Nose

Sometimes, you're your partner's splitting image.

In a normal mirror, you see your own face looking back at you. But what if you looked into a mirror and saw a face that was partly yours and partly another person’s? This Snack—a mirror with horizontal gaps in it—will allow you to have this odd and amusing experience.

Grade Bands:

Subject:

Light, Color & Seeing

Physics

Light

Keywords:

mirror

friend

reflections

exhibit-based

NGSS and EP&Cs:

Tools and Materials

A 69-inch (1.75-meter) length of 1/2-inch PVC pipe, Schedule 40, cut into two 18-in (46-cm) pieces, one 9-in (23-cm) piece, and four 6-in (16 cm) pieces
Two 1/2-inch PVC 90° elbows
Two 1/2-inch PVC T-joints
Four 1/2-inch PVC couplings or PVC caps
12 brass fasteners (roundhead fasteners), each 1 1/2 inches long
12 acrylic (plastic) mirrors, each measuring 1 inch x 12 inches (3 cm x 30 cm), available from a plastics supply store
PVC cutter or a hacksaw
Electric drill or drill press
3/16-inch drill bit
Clamps to hold material in place when drilling (not shown; if you've never drilled PVC pipe or plastic mirror, ask for advice or help)
Pencil
Ruler
A partner

Assembly

Using the PVC cutter or hacksaw, cut the PVC pipe into the following lengths:
- Two 18-in (46-cm) pieces
- One 9-in (23-cm) piece
- Four 6-in (15-cm) pieces
Draw a straight line along the entire length of each of the 18-inch (46-cm) pipes.
At a point on the line 2 inches (5 cm) from one end, use the 3/16-in drill bit to drill a hole completely through both walls of one of the 18-in (46-cm) pipes. Drill five more similar holes along the line, each 2 in (5 cm) from the last one. Be sure the holes go straight through the pipe. Drill an identical set of six holes in the other 18-in (46-cm) piece of pipe.
Assemble the PVC frame as follows (see the main photo above). The two 18-in (46-cm) pieces are the vertical legs, the 9-in (23-cm) piece is the horizontal top, and the four 6-in (15-cm) pieces form the horizontal feet of the base. Orient the holes in the 18-in (46-cm) pipes so that a nail stuck through a pair of holes would be parallel to the feet of the frame.
Measure the distance across the frame between the highest holes on each pipe.
Place two of the mirror strips back to back, and drill holes through them the same distance apart as the distance between the highest holes on the frame.
Repeat step 6 for the other five pairs of mirror strips.
Use the brass fasteners to attach the back-to-back pairs of mirror strips to the PVC frame as shown.
If there is any bowing of the mirror strips, try twisting the vertical PVC pipes to adjust the mirrors. If the bowing persists, try drilling slightly larger holes in the mirror strips that bowed.

To Do and Notice

Sit down at a small table with another person directly across from you and put the assembled frame on the table between you.

With your face about 12 inches (30 cm) from the mirror strips, move your head up or down until you can see your eyes in the second mirror strip from the top. Have the person on the other side hold his or her head the same distance away from the mirror strips on the other side, and move his or her head up or down until he or she can see his or her eyes in the other side of the same mirror strip that you are looking into.

You should now see a “composite” face, made up of some of your features and some of the other person’s. To get the best composite face, you may need to move closer to or farther away from the mirror while your partner remains stationary. You may also need to move your head up or down or sideways. Experiment and see what happens.

Move your head downward until your eyes are looking through the space directly below the mirror strip you were looking in previously. Have the other person move his or her head the same way. How does this composite face compare with the previous one? Move your heads back and forth between the two positions to compare the two composite faces a few times.

Try it again with another person.

What’s Going On?

When you see your eyes in the mirror, you will probably see the other person’s mouth through the gap. At the same time, the other person will be seeing his or her own eyes, but your mouth. You see a “composite” face made up of alternating horizontal bands of your own features and the other person’s features. The other person has a similar experience.

Your image in the mirror is the same distance “behind” the mirror as your actual face is in front of the mirror. To get your facial features to blend with those of the other person’s, therefore, that person must be just as far away from the mirror as you are, so that his or her face is in the same place as the virtual image of your face.

It may seem odd that your reflection appears to be behind the surface of a plane (flat) mirror rather than at the surface of the mirror. Why is this so?

Light bounces off the tip of your nose as you look into a mirror. That light reflects off the mirror and enters your eyes. Your eyes and brain work together to make a picture of the world. This eye-brain system assumes that the light has traveled in straight lines to reach your eyes. In order for the light to travel in a straight line to your eyes, your head would have to be behind the mirror.

The image below (click to enlarge) shows how, in a plane mirror, the image of a face is the same distance behind the mirror as the face is in front of the mirror.