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Hot Spot

Science Snack
Hot Spot
Focus the invisible light from an electric heater.
Hot Spot
Focus the invisible light from an electric heater.

Infrared radiation from an electric heater is just another “color” of light. Although 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. Try this Snack and you'll also learn how parabolic shapes concentrate energy.

Note: While Snacks were originally conceived to require only low-cost materials, a few Snacks—such as this one—require materials that are costly (but there is a reasonable chance they might be found in a middle- or high-school classroom). If your school has such items, then this Snack may be appropriate. If not, its usefulness may depend on your ability to obtain or improvise suitable substitutes.

Tools and Materials
  • A small electric heater
  • A large concave mirror, about 16 inches (40 cm) in diameter (this type of mirror is sometimes called a spherical or demonstration mirror—you can also use the bottom mirror from a commercially made Mirage Maker or Mind Boggling Optic Mirage, available from various online vendors)

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.)

To Do and Notice

Move your hand around in front of the mirror until you can feel the hot spot. This spot will be close to a point in space called the mirror’s focal point. Look into the mirror and find the visible image of the heater: This image of the heater will also be near the focal point.

Now look into the mirror and move around, observing your reflection. Move toward and away from the mirror, and see how your image and the images of the objects around you change.

Next, move the heater off the axis of the mirror or closer to the mirror. Search around, using the back of one hand as an infrared-radiation 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 when the heater is off-axis, 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 ear. When this happens, the mirror is making a sound image of your voice at the position of your ear.

What's Going On?

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.

Going Further

Satellite dishes for TV reception operate on this same principle, except 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: 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, and so on) 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) takes advantage of this effect to locate its prey. These snakes have two or more sensory pits, which they use like pinhole cameras to image infrared light. The snake can 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.