What is the infrared light The rays of infrared light are invisible and are also known as heat rays. Find out what's infrared light and how it works. Invisible light Sunlight consists of different light wavelengths. Infrared light has a longer wavelength than visible light and therefore the rays are invisible. Heat rays Infrared rays only have effect if they are absorbed. When infrared rays are absorbed, they are converted into heat and that's why they are also known as heat rays.