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figure 87.

For hundreds of years, scientists believed that light energy was made up of tiny part i c l e s

which they called “corpuscles.” In the 1600’s, re s e a rchers observed that light energy also

had many characteristics of waves. Modern scientists know that all energy is both part i c l e s—

which they call p h o t o n s—and waves.

P h o t o n sa re e l e c t ro m a g n e t i cwaves. These waves oscillate at diff e rent f re q u e n c i e s, but

all travel at the speed of light. The e l e c t romagnetic spectru mis the range of wave fre-

quencies from low frequencies (below visible light) to high frequencies (above visible light).

The radio wavec a t e g o ry includes radio and television waves, and cordless and mobile

telephone waves. These low-energy waves bounce off many materials including the ionos-

p h e re, a characteristic that enables radar applications. Radio waves are received and re t r a n s-

mitted by satellites for long-distance communication.

M i c ro w a v e spass through some materials but are absorbed by others. In a micro w a v e

oven, the energy passes through the glass and is absorbed by the moisture in the food. The

food cooks, but the glass container is not aff e c t e d.

I n f r a re dor heat waves are more readily absorbed by some materials than by others. Dark

materials absorb infrared waves while light materials reflect them.

Visible lightwaves are the very smallest part of the spectrum and the only frequencies to

which the human eye is sensitive. Colors are diff e rent frequencies within this category.

The atmosphere protects Earth from dangerous u l t r a v i o l e trays from the Sun. Ultraviolet

and extreme ultraviolet radiation are absorbed in Eart h ’s atmosphere, although some longer

ultraviolet wavelengths (UVB) penetrate to the ground. UVB can cause sunburn and is

linked to most cases of skin cancer.

X - r a y scan penetrate muscle and tissue, making medical and dental X-ray photographs possible.

Gamma-ray waves, the highest frequency waves, are more powerful than X-rays and are

used to kill cancerous cells.

Humans’ limited senses are extended by technology. Technology that utilizes the full electro-

magnetic spectrum enable the most comprehensive investigation of Eart h.

TH E EL E C T R O M A G N E T I C SP E C T R U M

Radio

Microwave

Infrared

Visible

Ultraviolet X-ray Gamma ray