Why do photographers use a red light in the dark room when developing film?

Photographers use this red light called a safelight because the paper onto which the photograph gets exposed is not sensitive to the frequencies emitted by the 'red' light bulb. This is for black and white Photography. Keep in mind, that although the bulb seems red, it has other filters, as opposed to a Christmas red light, or a red LED. These filters, usually block frequencies under (estimate) 590-650nm (nanometer). So anything above those frequencies the paper doesn't see, but we can, up until it becomes invisible light to us (350-700 nm is more or less what we see).