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How Does Light Travel Like a Wave?

-- | December 15, 2013

Question: I have a question regarding the speed of light from a reader on my website. I wonder if you could help answer it?

So here’s a thought that just occurred to me. Light moves at C. In a year, light moves a C * 1 year. Great. Got it.

My question is: Since light is partially a wave (and yes, partially photons), and our understanding of a wave is a sort of side-to-side vibration, does the distance actually travelled by photons in a light year (or light-second, week, month, Sunday afternoon or whatever) take into consideration the side-to-side variances in the nature of a wave?
In other words, a car (or a photon) traveling a mile down the road at 60 MPH in a straight line travels a mile in 1 minute. But if the car swerves left and right as it travels (in other words, travels in a wave) and still makes the mile in one minute, hasn’t it a.) moved more than a mile, accounting for the additional distance in the left/right motion and b.) gone faster than 60 MPH to get a mile down the road in one minute plus the additional distance for the left/right maneuvers? — Jon

Answer: When one thinks of the wave properties of light and how it travels it is best to think of how a point on the wave would travel. It is the individual crests and troughs of the wave that travel at the speed of light in the medium through which it is travelling. This is referred to as the “phase velocity” of light. So, with the car analogy above you can think of the car sitting on a crest of the light wave rather than travelling along the wave. You can see a nice visualization of this property of light in the “Propagation of Light” section of the Wikipedia page on the Speed of Light”.

Jeff Mangum

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