Why does it make everything glow without making the room brighter??
In: Physics
So first, black lights don’t make everything glow, they only make some substances glow.
Typically we see things because photos bounce off of it. The light comes in, and the light bounces back.
If it bounces back all or mostly in the same direction you get a reflection, like a mirror or a pool of still water. If the light is more scattered when it bounces back you get non-reflective surfaces like most other objects.
But not all the light that hits an object necessarily bounces back. Some of it can be absorbed. When it is absorbed it gains energy. That energy typical does one of three things.
The first, which is most typical is it raises the temperature of the object. Much of the time this effect is too small to notice, but not always. If you’ve ever worn black clothes on a sunny day you’ve experienced this effect.
The second, which is what happens in the scenario you are asking about is called fluorescence. Rather than heating up the object, in a fluorescing material some of the energy from the incoming light causes a photon to be sent back out. But unlike reflected light it’s not the same photon, it’s a new photon of lower energy. UV light is outside our ability to view, but the lower energy photon that it triggers is visible.
Think of it like this, normal light is like playing catch with a softball, you throw it to me, I throw it back to you.
Fluorescening light is like you throwing a baseball to me and then I throw a softball back to you. The baseball can go faster than the softball, much like the incoming photon is higher energy than the returned photon.
Since fluorescing materials only absorb the energy for a short time, when you remove the high energy source (UV bulb) they stop emitting particles and thus the objects stop glowing.
And since not all materials fluoresce not all objects that the black light hits glow.
The third scenario is called phosphorescence. It’s similar to fluorescence in that the material absorbs energy from incoming light and emits light in return but it’s different in that it can store some of the incoming energy and release photons over time as a result. Thus a phosphorescent material can continue glowing even after the original light source has been removed. Glow in the dark materials, such as stickers, work that way.
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