The visible spectrum comprises a relatively small portion of the spectrum, ranging from about 4 x 10-7 m to 7 x 10-7 m in wavelength. The higher wavelengths correspond to the red end of the visible spectrum, with still higher wavelengths up to about 10-5 m outside the visible spectrum corresponding to the infrared spectrum. This is the type of radiation associated with heat thrown off by objects at temperatures of the order of room temperature. The lower wavelengths of the visible spectrum correspond to the violet end, with still lower wavelengths down to about 10-8 m being termed the ultraviolet region.
Generally as one goes down in wavelength, or up in frequency, the type of electromagnetic radiation encountered becomes more and more dangerous. We will see in the next chapter why this is so.