Visible light can be thought of as a wave, but it's more than that. It's electromagnetic radiation whose wavelength and frequency can vary. Visible light is just one form of this wave ... the part we can see. Visible light is made up of those waves which our eyes can detect; their frequencies range from about 4 x 1014 to 7.5 x 1014 Hz. But this is only a narrow band in the total spectrum of all frequencies of electromagnetic radiation.

Below is a chart showing the band of visible radiation as a segment of the entire electromagnetic spectrum. Have a look at the other forms of radiation which are also part of this spectrum:




Radio Waves
These waves are extremely long, anywhere from about 1 metre to as big as 10 kilometres (10,000 m) in length. These waves include AM and FM radio signals, and TV broadcasting signals. Radio waves can also be created naturally by lightning, and by atoms releasing energy in space. Much of the static you hear between stations on your radio, for example, is a collection of radio waves released by gases in the atmosphere of planet Jupiter. Here on Earth we generate these waves from antennas and use them to send information by varying their amplitude or frequency.


Radar and Microwaves
These waves are considerably shorter and have a higher frequency, and so can't be created with a simple antenna; they need a special device to make them called a resonant cavity. These waves are about the size of water molecules, so when they hit matter with water in it, they cause the water molecules to gain energy ... this causes the matter the water is in to get hot.

Infrared
Infrared radiation can't be seen with your eyes, but it can be felt ... it's the heat that radiates off hot objects, like a stove burner or a fire, as the molecules in those objects vibrate. The frequency of these waves (anywhere from 1011 to 1014 Hz) matches the vibration frequency of most molecules, so most matter will gain energy (heat up) when exposed to infrared rays. As their name suggests, these waves are just 'beyond red' on the spectrum. You can not only detect heat with your body, but you can also detect these waves with special camera film or electronic devices. Some snakes have organs that will detect infrared rays; they use them to find their warm-blooded prey.

Visible Light
These are the electromagnetic waves that eyes are able to detect. All living organisms have evolved eyes that will register the energy from these waves, which are anywhere from 4 x 10-7 to 7.5 x 10-7 m long. In order for an organism's eyes to detect a wave, the cells in the eye must be approximately the same size as the waves. For this reason, no creature can detect long radio waves, for example, because their eyes (and the cells inside) are too small for these huge waves. Similarly, the cells in living tissue are too large to capture tiny gamma rays or x-rays ... although these waves can hit the much tinier atoms within the cells and destroy them. When visible light strikes atoms of matter, the result is often a chemical change; photosynthesis in plants is an example of this.

Ultraviolet
These waves are just 'beyond violet'. They have more energy than visible light, and so more energy is delivered when they strike atoms of matter. One common reaction occurs in the upper atmosphere, where UV rays hit oxygen molecules and cause them to form ozone. Ultraviolet light is also responsible for breaking chemical bonds in certain cells of your skin, leading to sunburn, wrinkles, and cancer.

X-rays
X-rays have a much higher frequency and contain a lot more energy than ultraviolet waves. They are more dangerous because they can hit molecules within your body and disrupt them; these damaged molecules can become cancerous. If X-rays happen to hit cells when they are dividing (this happens most frequently in the bone marrow, the skin, and the lining of the intestine), or if you have prolonged exposure to X-rays, this risk increases. Nevertheless X-rays are useful (in small quantities) because they are so energetic; they can penetrate the soft tissues of your body, and photographic images of the more solid parts can be made.

Gamma Rays
These electromagnetic waves are the most energetic of all. They are emitted by unstable nuclei of radioactive substances, and their high energy makes them very dangerous to living organisms.



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