# Heating question

1. Jan 6, 2008

why do visible light have less heating affect than infra red radiations ,in spite of the fact that photon of visible light have more energy....

2. Jan 6, 2008

### Staff: Mentor

It doesn't. Visible light is more energetic, which means for the same number of photons, you get more heat when you absorb it.

3. Jan 6, 2008

### pixel01

OK, visible light have more energy than IR. But the percentage of total energy to heat the absober may be less .

4. Jan 6, 2008

and what determines that percentage......

5. Jan 6, 2008

ok then why does a fluorescent tube is less hot then a bulb after sane time
certainly glass absorbs far more infra red radiations than visible light , but the thing is same with other substances , otherwise why dont we use a bulb to warm the room instead of heater of same power ..?

6. Jan 6, 2008

### Staff: Mentor

Think about conservation of energy, where does the energy from the light go? What happens if you shine visible light on a black surface? What happens if you shine the same amount of visible light on a white surface? Are most surfaces "black" or "white" to infra red?

7. Jan 6, 2008

### Staff: Mentor

Fluorescent lamps produce mostly UV light. When the UV strikes the coating on the lamp, it causes the coating to fluoresce, emitting visible light.

It is important to remember that certain devices don't necessarily emit a uniform or continuous spectrum. And even for ones that do, like light bulbs, they may emit more in the IR range than in the visible light range. The black body distribution of frequencies isn't consistent, it is a bell curve. See halfway down the page: http://csep10.phys.utk.edu/astr162/lect/sun/spectrum.html

Last edited: Jan 6, 2008
8. Jan 6, 2008

well i never said that IR is more energetic

9. Jan 6, 2008

ok then you mean to say that visible light have same heating affect like IR provided they are absorbed by the substances ........
and a heater and bulb of same power will heat a room equally

thanks now i get it...........

10. Jan 6, 2008

### Staff: Mentor

A heater and a light bulb of the same wattage will heat a room equally, but the spectrums of light that they create to do that will be different. The light bulb will heat the room with about 1/4 visible light and the heater almost all infrared.

11. Jan 6, 2008

### Staff: Mentor

No one said you did. But it is a logical contradiction that you were operating under, to believe something that was more energetic could produce less heat.

12. Jan 6, 2008

### Staff: Mentor

Exactly. Assuming that they are all completely absorbed 1 W of IR heats the same as 1 W of UV which heats the same as 1 W of visible light which heats the same as 1 W of radio ...