The electromagnetic spectrum
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The Sun emits light. Sunlight appears white, but it consists of different colors, as can be seen in the rainbow. Every color has its own wavelength: red waves of lights are longer than violet waves of light.
But there’s more than the visible colors of the rainbow. Scientists have discovered many forms of invisible light. For instance, infrared light (‘heat radiation’) cannot be seen by people, but it can be seen by snakes. Ultraviolet light (which can cause sunburn) is invisible to us, but not to bees.
Taken together, all the existing forms of light (or radiation) make up the so-called electromagnetic spectrum. It stretches all the way from extremely long radio waves to ultra-short gamma rays. What we call visible light is just a very small part of all there is.
In the image, the electromagnetic spectrum is depicted in the middle. On the top you can see the corresponding wavelengths; on the bottom are examples of objects and instruments that emit the various types of radiation.