Concepts

Concave mirrors, infrared radiation, heat transfer by radiation

Overview

A pair of parabolic reflectors focus heat from a filament onto a match, and the match ignites.

Details

Equipment

• [2] Concave mirror
  
• [2] Lab stand
  
• [1] Heating coil
  
• [1] Variac
• [3] Spare fuse
• [1] Optical rail
  
• [1] Match holder
• [1] Match
• [2] Saddle
• [1] Plastic ruler
  
• [1] Piece of paper
• [1] Extension cord
• [1] Fire extinguisher

Classroom Assembly

1. Place the optical rail perpendicular to the class.
   
2. Mount the coil and match holder in saddles and secure them to opposite ends of the optical rail. The match holder should farthest from the class.
   
3. Place the mirrors on either side of the optical rail.
4. Carefully align the mirrors so that the match and coil are each about 9.5 cm from the centre of the closest mirror.
5. Make sure the variac is at 0 V and off. Plug the coil into the variac and the variac into a power outlet.
6. Remove the match and quickly run the experiment with a piece of paper at the match-side focus. It should brown and possibly catch fire in a few seconds. If not, re-align the mirrors and try again.
   
7. Set the variac to 0 V and turn it off.
8. Place the match back in the holder.

Important Notes

• The filament voltage is potentially lethal!
• The filament and focussed beam are extremely hot! Do not run this experiment near anything highly flammable. Refresh yourself with the fire extinguisher controls, if necessary.
• Turn the voltage up from 0 AFTER you have turned on the variac. If you increase the voltage too quickly, you will blow a variac fuse.
• Do not run the heating coil for more than 5 minutes at a time. The power cord can get very hot.

Script

1. Turn on the variac and, with moderate speed, turn it up to 60 V.
2. Wait for the coil to redden a bit, then turn the variac to 120 V with moderate speed.
3. The match should start to smoke in a second and ignite within 10 seconds.
4. Blow out the match.
5. Turn down the variac to 0 V and turn it off.

Additional Resources