# Does Light Really Have a Wavelength?

1. Aug 31, 2011

### peter.ell

I'm curious as to how we know the wavelength of light.

Correct me if I'm wrong, but my understanding of light is that it is travelling variations/vibrations in the electromagnetic field, as such it has a speed and a frequency, and therefore a wavelength...but since light isn't really a little squiggly line like many drawings depict it, then light can't truly have a wavelength in the way that most people think of it.

So my question is: does light really have a wavelength, and if so, how does it manifest, what's the best way to conceptualize of it? Sure light waves aren't little moving lines of energy with the wavelength being the space between peaks, right?

Oh yeah, and how do we know the wavelength of light for various frequencies? Is it just by measuring the frequency and speed and doing the math, or have we actually measured the wavelength directly? If so, how?

Thank you so much!

2. Aug 31, 2011

### Staff: Mentor

Diffraction and scattering are both based on wavelength and are commonly observed with light. It truly has a wavelength.

3. Aug 31, 2011

### xeryx35

True dat. However, I think the question that the OP was really trying to ask was, "How can we reconcile the particle and wave sides of light?" There has to be some sort of way to reconcile the two, after all, they are one and the same.

4. Sep 1, 2011

### sophiecentaur

The OP could be confusing Rays and waves. The line joining a light source and detector can be treated as a Ray in many cases but there are still Waves, spreading out from the source and being intercepted by the detector.
Early theories about light ASSUMED that it travelled as Rays of particles and ideas theories apply to most everyday situations (shadow formation etc.). But there are many instances where light can be diffracted and these show that it has to have a wavelike nature. The Ray approach is actually an approximation. Quantum theory tells is that light (and all other electromagnetic energy) has a dual nature - which brings on the pains until you just accept it.
Personally, I find it easier not to insist that photons are 'really' particles at all. The quantised nature of energy, to me, can be just regarded as what happens at each end of the journey from source to detector. It gets over the serious problem of what is the actual 'extent' of a photon.

5. Sep 1, 2011

### Staff: Mentor

That is what QM does. There is no wave-particle duality in QED, all aspects of light are treated with one consistent formalism.

6. Sep 8, 2011

### Claude Bile

But light is a "little squiggly line". Or rather, to put it in a correct technical sense, optical fields do indeed exhibit phase variations with respect to time (i.e. frequency) and space (i.e. wavelength).

Photons only become spatially localised when they interact with something (such as an atom).

We measure the wavelength of light through spatial interference. Similarly, we can measure frequency through temporal interference.

Claude.

7. Sep 8, 2011

### LJW

Yes it does- light energy has a dual nature, and the wavelength of light can be evaluated with the double slit experiment.

8. Sep 8, 2011

### sophiecentaur

I like all of that except the "squiggly line" bit. I guess that was a bit tongue in cheek.

Young's slits only show a localized interaction and not, necessarily a 'particle' nature. I think that too much is made of the particle thing. But I have a feeling that is because it't easier to deal with.

But, of course, Young's Slits show the wave nature - hardly worth pointing out.

Last edited: Sep 8, 2011
9. Sep 8, 2011

### Mordred

I won't post much on this as I am just coming into my own understanding of it and others on this site can explain it far better than I can. Various experiments have shown a phenonemen referred to as the "wave-particle duality" To clarify Niels Bohr proposed the principality of complementary. It states that to understand any given experiment , we must use either the wave or the photon theory, but not both. Yet we mustg be aware of both the wave or particle aspects of light if we are to understand all the aspects of light. Therefore the two aspects of light compliment each other.
It is not possible to vilualize this duality. We cannot picture a combination of wave or particle. Instead we must recongnize that the two aspects of light are different faces that show to experimentors.

At that point I'll let someone more familiar with Quantum theory explain that further aqnd how they arrived at that conclusion ( I'm still learning it so by no means an expert)

10. Sep 9, 2011

### johnbbahm

My mind has always fit the wave model better. "if it quacks like a duck"
An excited atom's electron drops so many ev, producing a photon oscillating at frequency
X, C/X= wavelength. If the wavelength is in the visible spectrum, we see it as a color.

11. Sep 9, 2011

### sophiecentaur

Except it may not be like that. It could be a molecule rotating. Again, you there is the danger of using the electron to account for everything, when things are fundamentally more complicated.

Last edited: Sep 9, 2011
12. Sep 9, 2011

### BruceW

I think he was talking about how the amplitude of the EM wave is given by the amplitude of the electric field (or magnetic field in the correct units).

In the most basic examples of a wave, the amplitude is height of a sea wave or displacement of a string, but in EM, the wave amplitude is not related to some displacement in a spatial dimension.

So an EM wave doesn't make a wave-like pattern in space. You must imagine the electric and magnetic field as extra dimensions, then the EM wave does look like an actual wave.

13. Sep 9, 2011

### Naty1

"..how we know the wavelength of light.,,"

Wait till you find out EVERYTHING has a wavelength!!! See DeBroglie wavelength...in Wiki or elsewhere....which led us to discover that wavelength is inversely proportional to the momentum of a particle!!!!

One good way, see the illustration here:
http://en.wikipedia.org/wiki/Light#Electromagnetic_theory

and note that the E and M fields are in phase, but at right angles to each other.

But don't mistakeningly think you REALLY understand it, because:

http://en.wikipedia.org/wiki/Special_theory_of_relativity

which to me is one of the craziest, and on the surface, absurd, but accurate and experementally verified, concepts in all of physics.

14. Sep 9, 2011

### chrisbaird

You can measure the wavelength of electromagnetic waves directly. We do it all the time in the student labs. Set a radio wave source on a table and pointed into a two-mirror setup so that a standing wave is produced. Using a handheld detector (like a little antenna) that reports field strength in real-time, mark on the table the locations where the field reaches a maximum. Measure the distance between two marks and voila, you have directly measured the wavelength (over 2).

15. Sep 9, 2011

### sophiecentaur

You can also measure the wavelength of the electrons travelling through a vacuum. When you pass them through a thin piece of graphite they form a diffraction pattern of concentric rings. The ring spacing actually tells you their wavelength.
(It's different for different electron speeds, of course).