Hot Spot
You can focus the invisible light from an electric heater.
Infrared radiation from an electric heater is just another "color" of light. Though you can't
see this light with your eyes, you can focus it with a mirror or a lens and feel the warmth
it produces on the back of your hand. From this Snack, you can also learn how parabolic
shapes concentrate energy.
A large concave mirror, 16 inches (40 cm) in diameter (available for about $40 from
Sargent-Welch, P.O. Box 1026, Skokie, IL 60076-8026).
A small electric heater.
Adult help.
(15 minutes or less)
Place the heater many focal lengths away from the mirror. (The focal length for the
recommended mirror is about 1 foot [30 cm], so place the heater about 10 feet [3 m]
away.)
(15 minutes or more)
Move your hand around in front of the
mirror until you can feel the hot spot.
This spot will be close to the mirror's
focal point.
Look into the mirror and find the
visible image of the heater: This image
of the heater is also near the focal
point.
Look into the mirror and move around,
observing your reflection. Move
forward and backward in front of the
mirror, and see how your image and
the images of the objects around you change.
Move the heater off the axis of the mirror or closer to the mirror. Search around, using
the back of one hand as an infraredradiation detector. Find where the infrared radiation
from the heater is concentrated. This will be the position of the infrared image of the
heater. Locate the visible-light image of the heater with your eyes. Notice that the
infrared energy from the heater comes back together at the visible-light image position.
Notice also that the image point is not the focal point.
Place your face close to the mirror and talk into the mirror. Keep talking as you move
away from the mirror. At one point, your voice will sound much louder. At this point, the
sound waves radiating from your mouth bounce off the mirror and are concentrated at
your ears. When this happens, the mirror is making a sound image of your voice at the
position of your ears.
Every parabolic mirror has a focal point, a place where all parallel light waves, sound
waves, or any other form of radiation directed at the mirror along its axis will be
concentrated. Infrared radiation is simply light with a wavelength too long for your eyes
to see. But your skin can feel infrared radiation as heat. The skin of your cheek and on the
back of each of your hands is particularly sensitive to warming by infrared radiation. The
parabolic mirror concentrates the infrared radiation coming from a distant heater at its
focal point. That's why you feel a hot spot when you put your hand at the mirror's focal
point.
When the heater is moved closer to the mirror, the point where its radiation is
concentrated, the infrared image of the heater, moves away from the focal point.
Satellite dishes for TV reception operate on this same principle, except that they focus
radio waves instead of light waves. The surface of such a reflector doesn't need to be
polished like the parabolic mirror because radio waves are much longer than light waves.
To a radio wave, the surface of the dish looks very smooth: Its small irregularities don't
affect the long radio waves. The very short light waves, on the other hand, are bounced
off in all directions by the radio dish's surface roughness, so light waves are not focused.
For reasons of economy and weight, the large dishes used to focus the even longer radio
waves from astronomical objects (stars, quasars, etc.) sometimes are constructed out of a
rather large mesh screen, which appears smooth to the very long waves.
A family of snakes called pit vipers, which includes rattlesnakes, has two or more sensory
"pits," which they use like pinhole cameras to image infrared light. The snake can then
locate a warm-blooded creature, such as a mouse, by imaging the infrared heat radiated
by the mouse. With two pits, the snake even has some depth perception in the infrared.