Is Wavelength the Only Measure of Light's Size?

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SUMMARY

The discussion centers on the concept of light's size, specifically addressing whether light has dimensions or occupies space. It is established that a single photon does not possess a well-defined size, as its spatial localization is not precisely defined in quantum mechanics. The measurement of light's wavelength is discussed as a method to characterize light, but it does not provide complete information about the wave's properties. The conversation also touches on the analogy of measuring sound through its wavelength, emphasizing that wavelength is only one aspect of wave characterization.

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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?
 
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Yes to all of the above.

Claude.
 
Claude Bile said:
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?
 
swerdna said:
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?
 
swerdna said:
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.
 

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