# Why doesn't light interfere in a room?

1. Jun 28, 2013

### johnqwertyful

I was asked this question and couldn't think of a good answer. In a room there's infinite different light waves going in every possible direction. How can we possible see anything at all? If I put a clock in a room, light waves bounce off it different than a wall. Why doesn't that interfere with the light given off by the couch? How can anyone possibly see anything ever?

2. Jun 28, 2013

### ModusPwnd

Generally, and classically, light does not interact with itself. EM waves pass right through each other.

3. Jun 29, 2013

### johnqwertyful

I seem to remember doing quite a few problems in both optics and EM about interference.

4. Jun 29, 2013

### cmcraes

My guess is that they do but the chance of it being even noticable is nearly nothing because of how fast they are going and how Short their periods are. so even if there was destructive interference it would only last for a few nanoseconds if even that.

5. Jun 29, 2013

### SteamKing

Staff Emeritus
6. Jun 29, 2013

### ModusPwnd

Yes, you can get interference but not interaction. Two EM waves can approach each other, superimpose and thus interfere. Then they continue on their way towards your eyeball, etc. There are some exceptions to this, but I think by and large this is the general rule. Light does not interact with itself, when two EM waves occupy the same space they superimpose briefly then continue on their way.

7. Jun 29, 2013

### technician

Interference is a property of waves not conditions.
OBSERVABLE interference requires special conditions to be met.
Natural and most artificial sources do show interference effects in the form of moire fringes etc.
Some one may argue that these are not interference effects then we will have a discussion about interference and diffraction.....both of which are properties of waves.
Also.... The colours seen in soap bubbles and oil films on water are interference effects from natural and artificial sources.

Last edited: Jun 29, 2013
8. Jun 29, 2013

### johnqwertyful

Thanks everyone. I think I get it. My understanding now is that waves can be detected as superimposed, but the waves themselves don't "mix together". Just what you DETECT is the sum of the two. So yes, the waves do cross paths and if we were to observe it there we would see the sum, but then they go on to mind their own business unaffected, right?

9. Jun 29, 2013

### cmcraes

More or less yeah. So really the only time YOU would see interferece would be when two waves are are super imposing on eachother ad both going in the same direction. And as you may have guessed that is sometimes what causes colours

EDIT: Im not entirely sure that im correct, if im not please do correct in a respectful manner, Thanks!

Last edited: Jun 29, 2013
10. Jul 1, 2013

### mrspeedybob

Saying that light waves "interfere" with each other may be a misleading way of saying it. Classily EM waves do not ever, under any circumstances, directly interfere with each other. The way EV waves are detected is observing the way that they push and pull on charged particles. If 1 wave is pushing on a particular particle and another wave is pulling on it with the same force then the particle will not move and therefore no EM wave will be detected. Nothing actually happened to the waves, they each continue on their way completely unaffected.

11. Jul 1, 2013

### Cthugha

Well, technically speaking all the em waves in the room do interfere. However, light from the sun or from light bulbs has a coherence time in the femtosecond range. This means that you will only get a stable phase relationship (and some certain interference pattern) over a range of femtoseconds. Considering spatial coherence, too, makes for even worse visibility. If our eyes were able to take images with a frame rate in the petahertz range, we might be able to actually see that. However, our eyes are slow and what we see is an integration over all possible interference patterns which corresponds to no pattern at all.

12. Jul 1, 2013

### technician

I have seen colours in soap bubbles and oil films that last for more than a few femtoseconds.
The integration over all possible interference patterns for my eyes show as beautiful clear coloured fringes in soap bubbles and oil films.
What is a petahertz?????

13. Jul 1, 2013

### Cthugha

But this is thin-film interference. The oil film acts like an antireflection coating or Fabry Perot cavity. The thin film interference condition gives you different bright and dark areas depending on the film thickness for every single wavelength present. However, you enforce a fixed phase relationship by having a thin film which acts like a filter increasing coherence. This is not what is happening in free space and it is not integration over all possible interference patterns. It is integration over all interference patterns compatible with the boundary conditions given by geometry. In free space you have no narrow cavity-like thing fixing your phase, so you do not only have to integrate ovaer all wavelengths, but also over all relative phases.

The fact that you can get interference by filtering is trivial, no?

The inverse of a femtosecond (10^15 Hz).

14. Jul 1, 2013

### technician

Thank you for making everything so clear

Last edited: Jul 1, 2013
15. Jul 1, 2013

### mrspeedybob

I detect a hint of sarcasm...

Cthugha's answer requires a little background in QED in order to understand. Feynman presented a series of 4 lectures which are available on youtube which will give you this background. Here is a link to #1

Last edited by a moderator: Sep 25, 2014
16. Jul 1, 2013

### Cthugha

Hmm, using QED might be a bit of overkill. It is in fact not too different from what happens in a double slit or in diffraction from a crystal. If one has a look on the wikipedia page on thin-film interference (http://en.wikipedia.org/wiki/Thin-film_interference), the second image gives a good impression of what is happening.

The reflection gives you a path difference between the reflected beam at the two surfaces and if that difference is the equivalent of one wavelength, you get constructive interference. Obviously, the difference needed for constructive interference will vary for each wavelength in the visible spectrum giving you a different needed angle for each wavelength to get constructive interference. This will give you the colorful ring pattern seen. However, it is the presence of this thin film which enforces the appearance of the interference on every single incoming beam. You get interference between the transmitted and reflected part of each light beam arriving, where each of these light beams finds similar conditions for constructive interference. In other words, only the relative phase between the transmitted and the reflected beam matters, but not the phase relative to all the other light beams arriving simultaneously.

In free space you need to consider the interference between different light beams, which is a very different story.

17. Jul 1, 2013

### johnqwertyful

Hahaha, I love physics. I asked a question about an observation a friend had. Within 10 posts, QED comes up.

Thanks everyone, this has been a great/amusing read. I've learned a bit and understand now.

18. Jul 1, 2013

### Staff: Mentor

Here's a simple answer: the light ray from your couch and the light ray from the table next to it don't interfere with each other because when traveling toward your eye, they never intersect!

19. Jul 2, 2013

### Khashishi

Light interferes with itself, but the wavelength is small compared to the typical object size in your room and there are so many colors that the effects all wash out. If your room was much smaller, and filled with just a single wavelength of light, then there would be interference effects.

20. Jul 2, 2013

### technician

Now I detect a hint of sarcasm!
Chuga is right on the money. I thank him again for making it so clear. I try to confine my comments to physics.

Last edited by a moderator: Sep 25, 2014