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In a separate question I'm struggling to figure out the nature of EM waves. But it's a vast topic and I'm trying to narrow it down to small specific questions.

It turns out that all electromagnetic waves are spatial wavefronts and neither waves are rays nor they consist of longitudinal rays. There is no such thing as rays in nature. Rays in optics are merely a mathematical approximation.

Is that true?

What about emitting individual photons? Do they travel in straight line trajectories?

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lolmaus - Andrey Mikhaylov
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    Rays in optics aren't a "thought experiment", they are a mathematical approximation which, as described on the wikipedia page, "are valid as long as the light waves propagate through and around objects whose dimensions are much greater than the light's wavelength". Physics is full of approximations which are known to work well in some appropriate limit. – Hypnosifl Jul 05 '15 at 04:51
  • Thank you Hypnosifl, I've changed "thought experiment" to "mathematical approximation". – lolmaus - Andrey Mikhaylov Jul 05 '15 at 04:53
  • Ray optics is a special case of wave optics for small wavelengths. It is not straightforward to write down the formalism. You can read "Fundamentals of Photonics" 2nd Ed., section 2.3. – Rol Jul 05 '15 at 06:01
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    Photons are not "traveling things", they are the result of a measurement, i.e., a click. The (probability) wave is traveling, which might be localized to some extent. – Rol Jul 05 '15 at 06:04
  • Not that they are light, but we have cosmic rays as well. Point is, misnomers exist and you shouldn't much stock in a thing's name. – Kyle Kanos Jul 05 '15 at 11:28

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Individual photons are not considered rays. Because of the wave and particle nature of photons, they are much more complicated than what they are generally thought of: a projectile of light. In fact, they do not have an exact measurable position, but do travel in straight line trajectories. What we consider rays are lines perpendicular to the wave front of light, which is basically its trajectory. Therefore, light can be represented as rays, but is not actually made up of rays.

P.S.: Light = EM waves.

Jon Snow
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    "but do travel in straight line trajectories. " Really?but Feynman said,(in his QED book) ,that photons take all the possible path,so what do you mean? – Paul Jul 05 '15 at 13:26
  • When we observe photons, they travel in straight line trajectories. They do take all possible paths, but when observed, they take the path of least energy. – Jon Snow Jul 05 '15 at 17:09
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    No they don't. When you observe a photon, you can't say anything about where it's been. You just catch it in some particular location, and that's all. It's the collection of huge number of photons which does propagate in some sense in straight path — you can at least measure the path probabilistically. – Ruslan Jul 05 '15 at 17:28
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A "ray" in geometric optics is a locus of continuous propagation of light. Think of it as mapping where the energy is going in space. In principle there are an arbitrarily large number of them, but we draw a manageable number for visualization purposes.

The various [letter]-rays were so named when people didn't know what they were beyond being things that carried energy. X- and gamma-rays are high-frequency light. At those frequencies it is generally easier to use the photon picture to get a comprehensive understanding of how they work but they are still light. (Alpha-, beta- and delta-rays are sub-atomic particles, not light.)

You seem to be very hung up on getting some kind of concrete analogy for the nature of light. That's going to be a problem because light isn't like the things that you are used to at human scale. It acts like light.

Physicists use several different model to talk about light depending on the circumstance.

  • Ray are used in geometric optics.
  • Photons are used where quantum effects are important.
  • Maxwell's equations and the resulting wave equation are used in lot of cases, and comes in several approximated forms (near fields, far fields, ...).

All of these tools capture some features of the behavior of light and are shaky on others. Each of them gives a partial understanding. By picking the right tool you can get "close enough" for a particular situation. Physic is all about "close enough".

Learning how to uses these models and how the correspond to one another is a big job and you aren't going to get it right unless you are able to look beyond analogies.

dmckee --- ex-moderator kitten
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