Hot mirrors are coated with a special substance that allows them to reflect infrared rays of light while allowing only visible light to pass through them. Visible light includes the colors the human eye can see while infrared light and ultraviolet light, IR and UV for short, fall outside the visible spectrum that the human eye can pick up. The hot mirror is a type of dielectric mirror. Dielectric mirrors are coated with several thin layers to give them special reflective properties that control which types of light can pass through and which is reflected back. In the case of the hot mirror, this property allows it to reflect infrared light so that only visible light wavelengths can pass through the mirror.
Reflecting infrared light holds many advantages. Scientists who want to study a particular type of light, created by different wavelengths, can use the hot mirror to separate out the infrared wavelengths they want to study. They can also use the mirror on objects that would be damaged by the infrared light if it passed through. Fiber-optic cable, for example, benefits when only visible light can get through because infrared and ultraviolet light can cause radiation damage to it.
Though the name suggests the mirror is reflecting heat, this is not entirely the case. The mirror does create a good deal of heat and the object it's reflecting on may heat up in temperature, but this is due to the infrared and sometimes ultraviolet light being bounced back creating the heat. A hot mirror is designed mainly to reflect IR wavelengths, but it may also reflect some ultraviolet light as well depending on the angle and materials the mirror is made up of.
Typically, hot mirrors are coated yellow for best results, but sometimes this can interfere with an experiment. The scientist may need a pure white light and to achieve this he needs a neutral-colored hot mirror. This type is also available, though not as common as the yellow-tinted mirrors.
The hot mirror is not alone in its function of separating light. Another popular dielectric mirror is the cold mirror. Instead of reflecting infrared and ultraviolet light, the cold mirror reflects visible light while transmitting the IR and UV light. It is used just as the hot mirror to help separate light either to protect an object or to view certain wavelengths. Combining the two mirrors at different angles can create a wide variety of results in reflecting and transmitting different wavelengths of light.