I'm a newbie with physics but I'm wondering how a ray of light can essentially be represented. I have always known that a ray of light proceeds in a straight line until it encounters another object (or material) that refracts/reflects it. But a light ray should be part of an electro-magnetic wave, is this correct? If it is so, it should obey to the wave equation and this doesn't seem to me to describe a straight line ray.
I'm having problems visualizing how light is emitted and how it relates with the wave equation. Can someone with a clear understanding of the problem explain it to me in simple terms?
Answer
I'm sure Emilio Pisanty's answer is fine, +1, but it goes a little over my head. It also appeals to specific properties of electromagnetic waves, whereas the ray approximation is much more general than that. Here's a simpler plausiblity argument that may be more at the level that the OP can understand.
If you diffract a wave through a slit of width $w$, you get a diffraction pattern with an angular width $\theta$ of order $\lambda/w$ (in radians). When $\lambda$ is small compared to $w$, $\theta$ gets small. In the limit where $\lambda/w\rightarrow0$, $\theta\rightarrow0$, and you have a ray coming through the slit. Different types of waves (water waves, sound waves, light waves, ...) will have different details relating to things like polarization, but none of that affects the above argument.
If it is so, it should obey to the wave equation and this doesn't seem to me to describe a straight line ray.
Right. A perfectly collimated, parallel wave train can never be a solution of the wave equation. However, a diffraction pattern with a very small angular width can be a solution, and if the width is small enough, it's indistinguishable from a parallel wave train.
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