Photographing the Collisions

The photogate that was used to trigger the flash unit is shown here. The air rifle points to the right. The photogate was placed just beyond the end of the barrel. The light from an infrared LED (pointing up) was aimed at a photodetector (PT). When the BB passed through the infrared beam, a brief flash of light was triggered.

The BB appears as a bright, blurry spot just to the right of the photogate. The streak that extends in front of the BB is called afterglow. It results from the gradual falloff in light intensity from the flash unit.

The duration of the flash of light can be estimated from the length of the streak and the known speed of the BB. The length of the streak is about the same as the diameter of the BB, 5 mm. Dividing by the speed of 150 m/s, the flash duration is about 1/30,000 second.

Of course, one wants the flash unit to discharge when the BB is colliding with the elastic strip. The strip was hanging far enough in front of the gun that the puff of air from the end of the barrel wouldn't push the strip appreciably. An electronic timing circuit was used to delay the discharge of the flash unit the necessary amount. Follow the links for more information on the photogate and delay circuits.