# Effect of heat on light radiation

1. Feb 23, 2005

### DREAMs

I'm a biologist by profession, but I have a basic question about the effect of heat on light wave/particle, that could be monitor('able). If there's a slight change in the temperature of the medium, through which light is passing, due to a thermodynamic heat changes occuring in the medium of the order of 0.1 microcals, would that have any detectable effect on any of the properties of the light? If so what would be the relationship with respect to change in temperature? I'd be grateful if anyone could respond to this question! Thanks in advance.

Dreams.

2. Feb 23, 2005

### godzilla7

Knee jerk reaction, no, light is not affected by heat in the way you describe, I'll think on it though, certain tissues are changed by heat, which would indirectly affect light, if they expand then light would be affected, light energy changes when its medium is affected, so erm yes I supose so, not that well read, but yeah if light heats up a body as it is want to do, then indirectly, it will change it's absorption rates when propogating through the subject matter.

3. Feb 23, 2005

### Edgardo

Welcome to physicsforums Dreams,

if you heat for example air, you'll notice a flickering. You can observe this
when you look at the hot air above a grill. This flickering occurs due to refraction. Refraction can be explained by a change of the speed of light.
(see here: http://www.geom.uiuc.edu/education/calc-init/rainbow/refraction.html).
So if you heat the medium, the speed of light changes.

Here's a quote from the website above:
"The speed of light in air depends on the temperature and pressure of the air, and similarly for water and other substances".

-Edgardo

4. Feb 23, 2005

### Claude Bile

Heat does not directly affect the light, but it does indirectly interact with light in two ways.

1. Heat causes a slight change in the refractive index of the medium.
2. Heat causes the medium to expand slight, causing an increase in the optical path length through the medium.

Both these effects result in a net phase shift of the light, which can be measured relative to some reference beam using interferomentry. Sensitive heat detectors using Mach-Zender (fibre) interferometers work on exactly this principle. These sensors are VERY sensitive, certainly capable of detecting 0.1 microcals.

Most textbooks on optoelectronics will have the relevant equations contained within them, or even a quick Google may yield some results.

Good Luck,
Claude.

5. Mar 7, 2005

### DREAMs

Thanks a lot guys! I think I got my answer. The reason I was interested in it was to think of an alternative method to Isothermal titration calorimetry (ITC) used to monitor protein-ligand interactions. Since ITC requires large amounts of protein sample which is not always possible, I wished to know if changes in properties of light could be correlated with thermodynamic heat changes. Turns out that it does and interferometry and calorimetry could be coupled to increase the sensitivity and thus in decreasing sample requirements. Thanks a lot for your replies. But alas! someone else thought of the idea much earlier- Kudos! to brilliant minds that are ahead of their times!!

http://patft.uspto.gov/netacgi/nph-...0&s1=6381025.WKU.&OS=PN/6381025&RS=PN/6381025