ELI5, if black is the absence of light, how do black lights work
In: Technology
I’ll keep it simple but it’s a bit long. First, some background.
Light is just electromagnetic waves. (We’ll ignore the dual nature of light and the fact that it sometimes acts like a particle).
Waves of any kind can be described by amplitude and frequency.
Amplitude is how big the waves are, frequency is how many waves are in given chunk. What about wavelength? Don’t worry, wavelength is just 1/frequency and it’s not really important for this answer anyway. Just remember that at the same amplitude higher frequencies have more energy. Think about wiggling a slinky, it takes more energy to make big swings but if you keep the swing size the same it takes more energy to make lots of swings quickly than it takes to make a few swings slowly.
Our eyes are able to detect a small band of electromagnetic waves. We call that band the “visible spectrum”. Different wavelengths look like different colors to us. The lowest frequency we can see is red; anything less than that we call “infra red”. The highest frequency we can see is violet; anything more than that we call “ultra violet”. All the other colors are the frequencies between red and violet (that’s why the rainbow has the color order ROYGBIV).
Most substances absorb some light and turn it into heat and reflect some light. The reflected light is what we think of as the color of that object.
Some substances do a weird trick called “florescence”. They mostly absorb light like normal but they’ll take a few very specific wavelengths, absorb them, turn some of the energy into heat and emit the rest at a slightly lower frequency.
So back to your question. Black is the absence of detectable frequencies. You can be in a completely dark room but there might be x-rays streaming through that you just can’t see. They’re dangerous so don’t do that.
Black light is ultraviolet light. When it hits most substances you don’t see them because reflected ultraviolet light is still ultraviolet. But if you have a florescent substance it will emit some of that ultraviolet light in the visible spectrum. That’s what gives you the weird effect in a club.
Black is the absence of visible light. Black lights emit Ultraviolet light (UV).
Some materials when exposed to UV absorb it and re emit the energy by glowing with visible light. This process is called Fluorescence.
Black lights are just the nickname for ultraviolet lightbulbs. Ultraviolet is a color past purple on the spectrum that we can’t see. You’ll seem some of the purple light that is also being emitted but the main point of the light is to emit invisible uv light which causes the blacklight posters to glow.
“Black light” is just a marketing term for ultraviolet lights. Ultraviolet is outside of the visible spectrum, and while some violet tends to be produced as well, you can’t see most of the light produced by a black light, hence the name.
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