# I Is light really a wave?

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1. Sep 21, 2015

### Heretoignore

A pond lies flat when there is no wind, a river runs flat unless obstructed , a wave is not a wave without interaction. So is light before it hits something not a wave and a straight line instead?

2. Sep 21, 2015

### axmls

Light is not made up of water molecules, there's the first flaw.

Light is both a particle and a wave, and exhibits the behaviors of both. In some situations, treating light as a wave yields very accurate results, and in some, it's better to treat it as a particle.

3. Sep 21, 2015

### mrnike992

I'm gonna throw my answer out here and get it corrected by someone who knows what they're talking about, but light's also a particle, and I'm pretty sure it's just the innate property of light that it is a wave, or a disturbance in the electromagnetic field. So I don't believe it's ever actually a straight line

4. Sep 21, 2015

### Staff: Mentor

Well, if I observe a wave in one location, and then I observe what I think is the same wave in another location, why would I think that it wasn't a wave between point A and point B? It certainly acts as if it is a wave, given that it appears to diffract around objects and do all sorts of wave-like things while in route between emission and absorption. In other words, light acts exactly as our models, which describe it as a wave, predict it will. Therefor we say that light is a wave, regardless of whether or not it is interacting with something or not. (note that I'm strictly speaking of classical light, not of photons)

5. Sep 21, 2015

### jbriggs444

You seem to be thinking of light as a transverse wave, depicted as a wavy line through space. That depiction is certainly inaccurate. Light does not move back and forth from side to side as it propagates from point A to point B.

Nonetheless, it has wave properties such as refraction and diffraction. As to what it "really" is -- that ends up depending on exactly what you mean by "real".

6. Sep 21, 2015

### Heretoignore

Light has no net charge in space, light only has a net charge when interacting with matter, light can only have a frequency when it touches something.

7. Sep 21, 2015

### phinds

You need to be careful about proposing unsupported personal theories. Have you read the forum rules?

8. Sep 21, 2015

### blue_leaf77

In classical way, light is described as a wave because observation shows that it does behave like a wave, that is, it oscillates periodically with certain wavelength and propagates. WIth the invent of quantum mechanics it turned out that light is not perfectly sinusoidal in all cases, in particular when the number of the so called light quanta photons is small, it's randomness (which is a manifestation of the Heisenberg uncertainty principle between amplitude and phase) in phase at a given instant of time begins to show itself. When the number of quanta is sufficiently small, it's not even relevant anymore to talk about oscillation period as the time interval a crest repeats itself as the oscillation becomes significantly random.
Light is composed of photons, and a photon is defined as a quantum having energy of $hf$ with $f$ the frequency. So, by definition, light always has frequency.

9. Sep 21, 2015

### Dr. Manoj

Light behaves as a particle as well as a wave. You must have studied about photoelectric effect, where light shows the nature of a particle. It even shows diffraction which is a property of a wave. So, light has dual nature.

10. Sep 21, 2015

### nasu

The wave in water is not produced by "hitting something". First you need to have a source producing the wave, like wind or a boat or some object moving in the water.
Once you excite the wave, the wave propagates and may hit something and be reflected, for example.
Before the wave is produced you have no wave, the water surface is in equilibrium.

Same with light. Before something (sun, bulb, etc) producing the electromagnetic wave we call light we have what we call darkness.

The quantum nature of light is completely irrelevant for the question in OP. Don't bother with this yet.

11. Sep 21, 2015

### Richmonder

You can't thoroughly describe a quantum phenomenon (light) using classical analogies (wave, particle). You can say that in some situations light behaves as a wave and in other situations it behaves as a particle. But light is richer in properties - light is light and if you want to describe it precisely you need to use the language of quantum mechanics.

12. Sep 21, 2015

### phinds

Excellent first post. Welcome to the forum.

13. Sep 21, 2015

### Heretoignore

Between point A and point B there is nothing to propagate EMR.

14. Sep 21, 2015

### jbriggs444

So you imagine that to be a "wave", something has to be a patterned mechanical motion of some underlying substrate, e.g. of the luminiferous ether?

15. Sep 21, 2015

### Staff: Mentor

There is such a thing. It is an observed fact that electrical and magnetic fields can and do exist in a vacuum, and electromagnetic radiation is waves in these fields.

16. Sep 21, 2015

### Heretoignore

Would you be referring to CBR?

c also remains a constant in a vacuum , there is no propagation?

Last edited: Sep 21, 2015
17. Sep 21, 2015

### Staff: Mentor

No. It's easy to produce hard vacuums in laboratories on earth, and it's easy to create and observe electrical and magnetic fields in these vacuums. For example, you can evacuate the space between the plates of a parallel-plate capacitor and then charge the capacitor.... and there are many many more examples. Stuff like this was routine even back in the 19th century.

[WARNING: The description below relies on an analogy. It will help you form an intuitive picture of what's going on, but if you want more than an intuitive "OK, I see how that could work" picture, you have to write down and solve the differential equations involved. If you try building on the analogy without checking it against the math, you'll probably be misled]

Let's go back to your very first post in this thread, the one in which you said that a pond lies flat if there is no wind. That's not quite right - even if there is no wind, you can toss a small pebble in the pond and ripples will propagate away from the point where the pebble splashed in. These ripples move horizontally across the surface even though the water itself is not moving horizontally (a cork floating on the surface will bob up and then down as the ripple passes by, but it won't be pushed sideways). Thus, those ripples have a propagation speed; it's the speed that they move across the surface of the water.

Now, if I have electrical and magnetic fields in a vacuum and I do something to disturb them, there will be ripples in these fields just as the disturbing the surface of the water with the tossed pebble created ripples. Just as the ripples in the surface of the water spread out from the point of disturbance at some speed, the ripples in the electrical and magnetic fields spread out from the point of disturbance at some speed... and that speed happens to be $c$. The one difference that you have to keep in mind is that a horizontally moving ripple in water makes the vertical height of the water increase and decrease (the cork bobs up and down) as it passes by, while a ripple in the electrical and magnetic fields makes the field strength increase and decrease as it passes by.

18. Sep 21, 2015

### Heretoignore

Thank you for the informative post, however this does not explain something travelling at a linear direction at the speed of c without any obstruction . Is it not true that light slows down in a medium such as a cloud, a propagation of light?
Is it not true that on the surface of matter light propagates to reveal spectral content?
Is it not true that air has a low refractive index and is transparent to light, light does not propagate in air ?
Is it not true that we do not see any spectral content (frequency) in air?
Is it not true that light passing through air is not seen?

19. Sep 21, 2015

### Staff: Mentor

The full explanation would require an understanding of Maxwell's equations. Both how they work and how they were developed. Also, remember that science is all about observations. We observe that all EM waves behave as if Maxwell's equations are a correct description. Since these equations describe EM waves as always being waves, we accept that light, and all EM radiation, is indeed a wave at all times, regardless of whether it is interacting with matter or not.

Yes, but I don't see how that's related to your question.

I'm not sure what this means. Are you referring to how reflected light gives us colors and such?

Of course light propagates through air. If it didn't, you wouldn't be able to see anything.

No, it's not true. A sensitive enough spectrograph could see the absorption and emission lines of air.

Light that passes through air, or any medium, without scattering off of the molecules cannot be seen unless you are in the original path of the light. However, not all light makes it through the air without scattering. The sky is blue because sunlight is scattered off of air molecules and some of it eventually reaches your eye.

20. Sep 21, 2015

### Staff: Mentor

Yes, it is true. The behavior of the electrical and magnetic fields are influenced by interactions with nearby charged particles including the ones electrons and protons that make up the medium, and this can affect the speed with which the ripples propagate. If we're going to keep pushing the analogy (but please do remember the warning in my earlier post)... water waves change their speed if we pour oil on the surface of the water, and electromagnetic waves change their speed if we introduce something other than vacuum.
Some matter does, some doesn't. Clear air doesn't reflect much of anything, a mirror reflects just about everything at every frequency, and many other things fall in between. None of this has much to do with the behavior of light in a vacuum (although you might want to to consider that all matter consists of particles with vacuum in between).
It is true that air has a low refractive index, but that means that it doesn't interfere much with light propagating through it. It's obvious that light propagates through air - otherwise you wouldn't be able to see anything because no light would ever make it to your eyes.
Not true. Have you ever seen a sunset? For that matter, the sky is blue because different wavelengths interact differently with air. The effect is subtle enough that you don't notice it in a room with artificial lighting, or even in a ship looking at a lighthouse fifty kilometers away, but it's there.
If it hits your eyes you will see it, which is why drivers are supposed to dim their high beams for oncoming traffic. If the light isn't aimed directly at your eyes, you'll only see what's scattered by the air so that it reaches your eyes and this may be a lot or little depending on the conditions.

But none of this has much to do with the original question, "Is light really a wave"? The answer to that question is "yes". All of the phenomena that we're talking about now and many more can be directly calculated from the wave equation that describes light as ripples in the electrical and magnetic fields, whether in vacuum or in a medium.