The substances which will glow in the dark after being exposed to sunlight are called phosphorescent materials. Phosphorescence is the ability of a material to absorb energy from a light source and then emit it as visible light over a period of time. This process is different from fluorescence, which is the emission of light immediately after the material has been exposed to light. Phosphorescent materials are often used in glow-in-the-dark products, such as toys, signs, and clothing. Some common phosphorescent materials include zinc sulfide, strontium aluminate, and calcium sulfide.