Luminescent Light Bulb Changing
When you wake up in the middle of the night, not sure where you are, there’s nothing more reassuring than the luminous dial of a watch. You don’t need to find a light: just glance at your wrist and you know exactly what time it is. Watches like this glow all day long—we just don’t notice their ghostly shine in the daytime. What makes them glow at night, long after all the other sources of light are dim?
What is luminescence?
Luminous simply means giving off light; most things in our world produce light because they have energy that originally came from the Sun, which is the biggest, most luminous thing we can see. Strictly speaking, although the Moon appears to give off light, it’s not actually luminous because it’s simply reflecting light from the Sun like a giant mirror made of rock. Luminous is quite a vague word really. Arguably, even a flashlight bulb is luminous, because it turns electricity (electrical energy) into the light and shines it toward us. But bulbs like this are incandescent and make light by making heat. Luminescent things, by contrast, make light when their atoms become excited in a process that needs little or no heat to make it happen.