Electronic Guidebook for High-Speed Flash Photography, revised 12-00

Activity 10.  Capturing vertical jumps with a photogate

Equipment needed:

• Vivitar 283 flash unit
• 4 AA batteries or SB-4 AC adapter
• Cord to connect flash unit to photogate
• Flash clamp
• Photogate trigger (transistor type)
• Bimetallic discs

Background: This activity requires a bimetallic disc. For such discs,  the top and bottom of the disc are actually made of thin pieces of two different metals such as brass and iron. These metals expand (or contract) at different rates when heated (or cooled). This effect makes it possible for a disc to jump off a table on its own. The disc has two states which can be obtained by snapping the center of the disc in or out. The disc naturally snaps into its low-energy state when left on its own. In order to make the disc work, snap it into its high-energy state and hold it there while heating it in your hand. Then place the disc, convex upward, on a cool surface. As the disc cools, the metal on top contracts faster than the metal on the bottom. This soon causes the disc to snap back to its low-energy state. When it does so, the middle part pushes off of the table, causing the disc to jump. While the disc used in this activity is a novelty, bimetallic strips are extremely important as temperature sensors in heating systems.

1. In Activity 9, you used an interrupter as the photo- gate. In this activity, you’ll set up a photogate using a separate LED and photodetector (PD). Put together the equipment shown in the diagram. You’ll need a cool surface to make the disc jump quickly. One way to cool down a surface rapidly is to fill a cup with ice and then fill it with water to the very top. Then place a metal plate on top of the cup. The plate will cool down very fast. Tape the LED and PD into place a few inches above the center of the plate, using blocks or other risers to position the photogate at the right height. Aim the LED and PD at each other several inches apart. Check to see that the flash discharges when you pass your hand through the photogate. If the flash doesn’t discharge, check the alignment of the LED and PD. Also try adjusting the sensitivity control. If this doesn’t work, bring the LED and PD close together, about an inch apart. Once you get the photogate working, then you can move the two parts away from each other gradually, checking to make sure that the photogate continues to work. When you have the LED and PD positioned where you want them, adjust the sensitivity for a maximum as described in Activity 9.

2. Prepare a disc for jumping as described in the Background. Then place it in the center of the metal plate. When it jumps and passes through the photogate, the flash unit will discharge and you’ll be able to see the disc. Does the disc flip as it jumps? You can try placing the photogate at different heights in order to see the disc in different parts of its jump. You can also try putting several discs on the plate at once. They won’t all jump at the same time, so you can see them one after the other.

A related activity--Capturing the burst of a popcorn kernel: If you can capture a jumping disc with a photogate, you can also capture a jumping popcorn kernel as it pops. This is easiest to do by heating one kernel at a time on a hot plate. You’ll need to put your LED and PD far apart and just above the surface of the hot plate. This will require some careful adjusting. Do this before turning on the hot plate so that you don’t burn yourself.

A good way to heat a single kernel is to place it inside a small iron washer and then put a few drops of cooking oil inside the washer. Position the washer so that the kernel will pass through the photogate as it’s jumping off the hot plate. Most kernels open very rapidly, so you’ll probably see the kernel bursting open.

You may find that the spray of oil from the burst can break the infrared beam prematurely. If so, decrease the sensitivity of the detector so that tiny oil globules will not trigger the flash unit.

One problem with taking photos of bursting popcorn kernels is that you’re never sure when they’re going to pop. You may have to hold the shutter of your camera open for a long time in the dark room, waiting for the kernel to pop. After a while, you may learn to anticipate when a kernel is going to pop so that you can open the shutter just before that. Some variables that you’ll learn to optimize if you work long enough are the temperature of the griddle and the amount of oil that you drop in the washer. You need to be patient and be willing to make many trials. You may want to try placing the photogate at different heights above the hot plate so that you can capture a kernel later in its burst. Be careful not get cooking oil spray on your camera and, more importantly, in your eyes. It may be a good idea to wear glasses or goggles.