Is Wavelength the Only Measure of Light's Size?

[swerdna]

Does light have a size? In other words, does it have dimensions and occupy space?

ETA - Is measuring the wavelength of light actually measuring light?

 

[Claude Bile]

Yes to all of the above.

Claude.

 

[swerdna]

Yes to all of the above.

Claude.

Thanks - It’s hard to imaging being able to measure the size of something that has no mass and travels at around 300,000 kilometres per second! Can the size of sound also be measured by its wavelength?

 

[Andy Resnick]

Does light have a size? In other words, does it have dimensions and occupy space?

ETA - Is measuring the wavelength of light actually measuring light?

In terms of quantum mechanics, the position operator for modes of the electromagnetic field does not have an eigenvalue. In english, that means that the spatial localization of a single photon is not well-defined.

So, a single photon has no size. Technically, it has an infinite size- a single photon is everywhere becasue the momentum (wavelength) is precisely specified.

Obvously, there is spatial localization to light- we illuminate small parts of space all the time. But that's becasue we are using large numbers of photons and the momentum is no longer so well specified for the group.

As for measuring the wavelength 'actually' measuring light, that's kind of a philosophical question rather than a science question. We can reproducibly make a measurement of *something*- the angle of diffraction off a grating, energy by a photdetector, a frequency (or difference frequency), things that are tied to physical properties of light. Did you have a specific question about a specific result?

 

[Claude Bile]

Can the size of sound also be measured by its wavelength?

Wavelength tells you the distance between successive peaks, it does not tell you anything about the spatial extent of the wave, the intensity of the wave, the direction it is moving in...

You can see where I'm getting at here, wavelength is one piece of the puzzle in terms of characterising a wave, but it is by no means the full story.

Claude.