# B Question about a beam of light

1. May 21, 2017

### Mosio

I can't understand this term. Specially a beam of white light. Is it a combination of photons or electromagnetic waves that moving in space with different wavelengths?
Since output of a prism is analogous transition of colors that makes me think there are infinite number of wavelengths in visible light in contrast to a laser beam. Much like infinite numbers between two numbers in math but how's the physical translation? Cuz numbers don't exist but light do. What am I missing here?
Thanks a lot

2. May 21, 2017

### phinds

yes
yes
huh? The physical translation is exactly what you just said ... light can have an infinite range of frequencies.
so?
You don't seem to be missing anything other than confidence that you understand it.

3. May 21, 2017

### Mosio

So this means infinite amount of waves, photons and thus energy. how can we simply measure it and say e.g. it's n lux?
And how will the waveform look like?

4. May 21, 2017

### Staff: Mentor

It is an electromagnetic wave moving through space. Photons don't come into the picture until the light beam hits something and we need to do detailed calculations about exactly how the electromagnetic waves exchange energy and momentum with whatever they've hit.
All waves - not just electromagnetic, but also sound waves, water waves in the ocean, the waves in a violin string, vibrations in a tuning fork, whatever - behave according to particular differential equation called the "wave equation". The wave equation is linear, meaning that it has the very nice property that if A and B are solutions of the wave equation, then A+B is also a solution. Furthermore, a pure wave with a specific wavelength is always a solution so any waveform, no matter how complex, can be treated as a bunch of different pure single-wavelength waves all moving through the same point in space at the same time, adding in ways that produce the actual physical waveform that we're working with. And it turns out that a beam of ordinary white ligtht is produced by summing waves at all possible wavelengths, so yes.... There really are an infinite number of wavelengths in a beam of ordinary white light, and that's why prisms produce a continuous spectrum.