Physics Van 3-site Navigational Menu

Physics Van Navigational Menu

Q & A: light spectrum

Learn more physics!

Most recent answer: 10/22/2007
Q:
I have a question about the electrmagnetic spectrum. 1.Does the red photon make heat? And does the blue photon make cold? 2. If infrared comes before the visible spectrum and it burns you, then how come ultra violet which is after the visible spectrum still burns you?
- Socrates (age 13)
SFS (Seoul Foreign school), South Korea
A:
1. Any color photon carries some energy. If it's absorbed in some material it will generally heat the material up.

2. Since all the colors carry energy, they can all burn your skin if enough light is absorbed. That applies to visible light as well as infrared and ultraviolet. However, you can see visible light so you're not likely to be surprised by whatever damage it does. The ultraviolet not only dumps heat into your skin but also triggers some specific chemical changes that can be dangerous, especially by breaking DNA molecules. Cancer is one common result of ultraviolet skin damage. "Sunburn' usually refers to some aspects of this chemical damage, not to the simple heating produced by lower-frequency light.

Mike W.

While all colors of light carry energy, people who design lighting for movies, television, interior decorating, cars, etc. do speak of "warm" and "cold" colors. These names have absolutely nothing to do with the amount of energy carried by the photons or their ability to burn skin (or eyes). They have something to do with our perception of what warm and cold objects look like when we encounter them in our environment. Objects that are hot will glow red. The hotter they get, the more white or even blue they will glow, because blue light carries more energy per photon than red light. But most of us have rarely, if ever, seen anything that's so hot it glows blue (except for some stars), so most of us think that red things are hot. Blue and white are called "cold" colors by interior decorators and by the people who arrange the lights on movie sets because those are the dominant colors we see when we go out on a clear, snowy day. Blue is also the color people's lips turn when they get cold (and whole faces if they are really really cold).

Tom

(published on 10/22/2007)

Follow-up on this answer.