# Do all visible radiations have heat?

1. Jun 28, 2012

### Karimspencer

For example when you make a laser , does heat come with it?

2. Jun 28, 2012

### Staff: Mentor

That is oddly worded, but all EM radiation carries energy, so yes.

3. Jun 28, 2012

### Karimspencer

Well i don't mean radiant energy , i mean do all light have thermal energy?

4. Jun 28, 2012

### DragonPetter

Are you asking all light, rather than just infra red light that we commonly call as thermal radiation?

I think the answer is yes, because I asked a similar question about a month ago and people helped clarify.

5. Jun 28, 2012

### Khashishi

Your questions indicate that you are confused about what heat is. Heat is basically energy, but the exact definition depends on the physical context. If you get into the nitty gritty, heat is a pretty difficult subject.

Heat is energy that is more or less randomly distributed across all the degrees of freedom within a system. In typical terrestrial situations, that means most of the energy is in random kinetic motion of the molecules, as well as various vibrational and rotational modes of the molecules. For typical terrestrial situations, very little heat energy is in the form of light.

On the other hand, in a vacuum, most of the heat energy could be in the form of light, because without any other particles, photons are the main degrees of freedom of the system.

If you shine a light on something on Earth, then the light is not considered heat because it doesn't randomly occupy all the degrees of freedom of the air that it is traveling through. But, after the light is absorbed by various materials, it is converted to heat.

6. Jun 28, 2012

### DragonPetter

Heat is the flow of energy I thought. As in energy transfer. Temperature, as opposed to heat, is the way to quantitatively describe energy level increase in particles after heat has been transferred, so I don't think you would say light is converted to heat, but rather converted to temperature increase. Heat would be the process of light energy converted to thermal energy, or in other words heat is the amount of energy that has been transferred to thermal energy.

Last edited: Jun 28, 2012
7. Jun 28, 2012

### Karimspencer

So lets say you get this laser and concentrate on a small object , will the object be heated up?

8. Jun 28, 2012

### Khashishi

Well, yes, heat is used to describe the flow of energy. But not just any energy. It describes a sort of randomly distributed, high entropy energy. Structured energy could be considered as "work" when talking about the flow of energy.

9. Jun 28, 2012

### DragonPetter

So your point is that light does not carry heat at all? I think I would agree with that; its electromagnetic energy can only be converted to thermal energy by electrical interaction with charged particles, and the amount of energy converted would be considered heat transfer.

10. Jun 28, 2012

### Khashishi

Karimspencer: yes.

DragonPetter: It depends on the context. Like a lot of physics problems, it depends on where you put the boundaries of your system. I don't know if everyone agrees on this. The way I see it, blackbody radiation can be considered heat depending on how it is used. Sunlight within the Sun is heat energy, and it travels to the Earth and heats the Earth. Now, sunlight within the Earth is not in thermal equilibrium with the Earth, so, if you just consider a system including the sunlight and the Earth, then the sunlight isn't heat. But if you consider a system including the Sun, sunlight, and the Earth, then sunlight can be considered heat. Yeah, it's complicated...

The Earth emits a certain amount of infrared emission because it is warm. It also reflects a certain amount of sunlight. Viewed from afar, the infrared emission is "heat" emission from the Earth, and the reflected sunlight is not.

Last edited: Jun 28, 2012
11. Jun 28, 2012

### Karimspencer

I think i would disagree , a lot of visible light rays carry heat for example microwaves, the sun ..but what i want to know :"Do all light carry heat(thermal) energy?"

12. Jun 28, 2012

### Karimspencer

I agree with you Khashishi

13. Jun 28, 2012

### Staff: Mentor

I would say that if it is emitted and absorbed via black body radiation, then it transferred heat. If the intermediate form was electromagnetic rather than thermal, I'm not sure that matters - I'd want to know more about the context of the question.

Heat is the transfer of thermal energy and thermal energy is molecular vibration. Light is not molecular vibration.

14. Jun 28, 2012

### Karimspencer

Okay lets say you concentrate any kind of light on an object would lt heat up russ

15. Jun 28, 2012

### Khashishi

Karim, have you ever been in a spotlight?

16. Jun 28, 2012

### Staff: Mentor

Not necessarily and even if it does, it wouldn't necessarily heat up in an amount equal to the light's energy. A solar panel, for example, converts some light directly into electrical energy.

17. Jun 28, 2012

### Staff: Mentor

When you put a dark object in sunlight, its temperature and thermal energy increase. Thermodynamically the energy transfer into the object is called "work" not "heat". The alternating electric field of the light waves causes electrical forces on electrons near the surface of the material. The electrons move in response, so work is done by the electrical forces.

18. Jun 28, 2012

### Karimspencer

Well what are the difference between the properties of light that carries heat and one that doesn't.