# Light interference with light and other EM waves [duplicate]

Lets assume a room with one or two sources of light (normal light bulbs or fluorescent lamps). So if we look at lamp from any direction we see it. We also see different objects in the room because of light reflected from it from the source. So I would like to visualize that at any point in space inside the room there is a EM wave with a particular E and H component. Also the room might be filled with Radio waves, Microwaves and other EM waves. So at any point in space the Electric and Magnetic field components are the sum of all these individual wave components at that point. Also if we specifically assume light then it will be sum of light directly coming from the source and other reflected components of that light. If that being true how are we able to see clearly, meaning we are able to see individual objects in a room distinctly without any interference from these reflected component. Also how these Radio waves, microwaves etc do not impact the images we see in any way. They are just Electric and magnetic fields oscillating at different frequencies but still they will have an impact on the the E and H components of visible light at all points in space inside the room.

## marked as duplicate by John Rennie, user36790, Kyle Kanos, Bill N, GertDec 1 '15 at 3:23

Interference requires very precise phase matching of the light sources, and room light being made up of an incoherent source over a large frequency span, the phase matching does not occur unless the path length difference of the light is very short. Making an interference pattern is possible for a white light source with a Michaelson Interferometer, but this is a precision instrument that can be used to match the path lengths very well. The path lengths difference required to cause interference is given by the coherence length:

$$L = \frac{c}{n\Delta f}$$

where $c$ is the speed of light, $n$ is the index of refraction and $\Delta f$ is the spread in frequency of the light source. The coherence length for visible light is on the order of a few hundred nanometers, so you shouldn't expect any interference from bounces off of anything in the room because the phases are too random for this to occur.

Now, things could destructively interfere for an instant in time, but what you are seeing is an average intensity of the light, because your eyes do not respond fast enough to see the individual oscillations of the fields (if they did, things would be very weird).

For radio waves, our eyes are not sensitive to those either and are tuned to pick up the visible spectrum. Also, the electric field oscillations for the visible spectrum are so much faster than that of the radio waves, there is not really a way for them to interfere with each other.

• Can reflected light rays from two different objects interact with each other and create a distorted image ? – Keerthi Varman Nov 30 '15 at 20:53
• @KeerthiVarman That depends on what you mean by a distorted image. They won't have interference because they would be incoherent and the phases of the light would be too random to produce any interference (for white light, a single laser source may produce interference after being reflected off of two mirrors, because it is a coherent source). If you're trying to produce a clear image using two different sources, then it could be distorted because things might not be focused at the same location. – tmwilson26 Nov 30 '15 at 20:57

The easiest way for me is to forget about the waves and remember the individual photons that do not mix as they propagate. Our eyes only register the visible photons and seem sensitive enough to distinguish the individual frequencies.

Nice conclusions from wrong assumptions. From infrared to ultraviolet the radiation comes from electron excitations. Electrons emit and absorb photons and this photons don't interact which each other at this energies. Due to our everyday experience we know that double interactions from two or more photons on one electron do not blur the images we see or didn't happen often at usual for us intensities.

Another thing are radio waves. This waves are modulated radiations with a lot of polarized photons. Our senses are not sensitive enough to feel them (exception: near a high power antenna we could feel the heat from the rod and perhaps we are in danger to get exposure from X-rays). But a radio receiver is under the influence of radio waves from different sources or reflections from the walls from the same source. The energies of this radio waves reinforce or sometime cancel each other out.

The interference of radiowaves happens very often in the past. This was one reason to switch from longwaves to very high frequency radio waves. VHF radio waves get not reflected from the higher atmosphere, what long waves are do and so different sources of long waves interfere each other.