Some materials will glow when lit with an ultraviolet (UV) light. But when you take the light away, the glowing stops. A phosphorescent material will continue to glow after the light source is removed.
Let’s look at olive oil. The molecules in olive oil absorb the energetic UV light, from, say, a UV pen light. And the molecules get “excited.” When they are “excited,” they release energy often in the form of visible light (heat is released, too). In the case of olive oil, the light is red. As you can see in the video, the light emitted by the canola oil molecules is greenish, and the light emitted by the rock salt molecules is purplish.
Thanks to Randy Attwood for help making the videos.
In a novel I read recently (The Last Widow, by Karin Slaughter), one of the main characters described making a UV light from a cellphone. Will, the character in the book, coloured the light on his cellphone with a blue Sharpie, covered the blue with see-through tape, then coloured over the blue with a purple Sharpie, then covered that with tape. He then turned on the cellphone’s flashlight and used the “UV light” to read a message written with urine. (Urine is fluorescent under a UV light, but it would have been invisible to the bad guys in the story in white light.)
While an exciting application of science as well as a clever addition to the plot, unfortunately, the science does not pan out. It might have worked if Will used filters, such as filters used with a camera, because some light gets through the filter and the rest is absorbed. You can’t make UV light with a material that is coloured by pigment, such as paint, fabrics, and printer ink. The light just gets absorbed. I tried it anyways, by the way. It didn’t work. Some light got through the Sharpie markings but it certainly wasn’t UV. (But the blue colour from the Sharpie came right off the cellphone light; it may be permanent on some materials, but not the glass light.)
There are two models of colour that describe what I wrote above: the additive model of colour and the subtractive model of colour.
According to the additive model, we “add” the three primary colours of light to produce any colour of light, including white. Varying the amounts of each colour of light will give different hues and colours.
With the subtractive model, we are “subtracting” different colours through the processes of absorption and reflection: a red sweater is red because of the red pigment in the dye used to colour it. The sweater absorbs all the colours except red, and the red is reflected to our eyes.