# Thread: Reflection, refraction and scattering

1. I just want to make sure I am understanding each of these concepts properly:

Reflection occurs when an EM (or any?) wave comes into contact with some object and elastically bounces off that object. The angle of reflection is the same as the angle of incidence.

Refraction occurs when an EM wave passes through some object and the speed the wave travels changes, 'bending' the light. The angle of bending follows Snell's law.

Scattering occurs when an EM wave interacts with some molecule, scattering the light in all directions (but intensity depends upon angle of incidence).

The bit I want to clarify most is that in the first two there is no interaction with electronic, rotational or vibrational modes of the molecule?

2.

3. Originally Posted by epimetheus
I just want to make sure I am understanding each of these concepts properly:

Reflection occurs when an EM (or any?) wave comes into contact with some object and elastically bounces off that object. The angle of reflection is the same as the angle of incidence.

Refraction occurs when an EM wave passes through some object and the speed the wave travels changes, 'bending' the light. The angle of bending follows Snell's law.

Scattering occurs when an EM wave interacts with some molecule, scattering the light in all directions (but intensity depends upon angle of incidence).

The bit I want to clarify most is that in the first two there is no interaction with electronic, rotational or vibrational modes of the molecule?
I would not say that exactly. In reflection and refraction the radiation does couple to the molecules or electrons of the medium, inducing an oscillating polarisation in it. I think a better way of describing the distinction is that the radiation is not of the right frequency to change the state of the molecules or electrons to an excited state, i.e. no absorption occurs or if it does it is accompanied by simultaneous re-emission (which I think is how reflection is often treated).

In the case of refraction, the oscillating polarisation induced in the medium results in reducing the phase velocity* of the radiation, hence Snell's Law etc.

*This is a bit tricky. With waves there are several velocities that can be defined: "phase", "group" and "signal". They do not have to be the same.

(P.S. Just waiting now for SS to continue her vendetta by telling me I am talking crap again )

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