Physics: Principles and Problems


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Can You See in the Dark?
April 2003

Technology has given us the capability to do things we otherwise could not do. Night vision devices are an example of technology that has enhanced our ability to see in the dark. Night vision devices were invented during World War II for use by American, British, and Soviet military personnel to detect enemy targets at night. Today, technology has advanced to the point that military pilots can fly at night to locate targets. Soldiers can see in the dark by detecting heat differences which can indicate the possible presence of enemies.

Night-vision equipment is separated into two categories-image enhancement and thermal devices. In order to understand how these devices operate, it is important to understand the fundamentals of light.

A light wave is composed of energy in the form of electric and magnetic fields. The amount of energy in a wave is related to its wavelength, which is the distance between any two corresponding points on successive waves. Light waves come in a variety of sizes, frequencies, and energies. Shorter wavelengths have higher energy. Light that you see is made up of one or more photons traveling through space as electromagnetic waves.

Image enhancement works by collecting small quantities of light and magnifying it so that the human eye can see the image. A familiar example of an image enhancement device is night vision goggles. Images viewed with image enhancement devices, such as night vision goggles, have a greenish hue. The reason for this is so the human eye is still able to see when the goggles are removed. The pupils will still be dilated so the ability to see in low light remains. If a color other than green is used, the pupils will constrict when the night vision goggles are removed.

The other device used for night vision is thermal imaging. This device captures the upper portion of the infrared light spectrum, which is emitted as heat instead of reflected light. The majority of thermal imaging devices have a temperature range from –20°C to 2000°C. They can detect changes in temperature of about 0.2°C. Thermal imaging equipment is best used to detect people or to see in near-absolute darkness.

In addition to the military, night vision devices also are used for hunting, law enforcement, and entertainment. Law enforcement officers use thermal imaging devices to uncover objects, such as money and drugs, which have been buried or hidden by criminals. Campers and hunters use night vision devices to maneuver through the woods at night.

Activity
Many types of night vision devices exist. What type of device would you use if you wanted to explore the dark recesses of a cave? Research the different types of devices that are available to find out.

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Physics: Principles and Problems