To measure light's frequency, do we interact with each crest + trough?

[syfry]

TL;DR Summary

We must interact with light to detect it, so to detect its frequency, do we interact with every crest and trough?

How are we interacting with light to measure its frequency? And how'd we learn the distance between its crests and troughs? What sort of interactions are giving us such info?

 

[Baluncore]

Photon energy is directly proportional to the frequency of the EM radiation.
There are many ways that depend on measuring the wavelength, rather than the frequency, of the light.

We could split the light into two paths of different lengths, then recombine the light from both paths to measure phase difference by interference.

We might sort the light by wavelength with a prism, then measure the energy of individual photons in the part of the spectrum that interests us.

 

[syfry]

So we indirectly measure the frequency of light by directly measuring the light's energy?

Also, seems like frequency and wavelength are identical by count even with their inverse values. (higher frequency means we count a greater number of crests which means a greater number of wavelengths in that count)

Is so, then makes sense to recombine the light and measure the phase difference for its wavelength, which we then equate to frequency.

But is that how devices that measure frequency do their thing?

(my knowledge is a bit spotty in the particulars of this subject)

 

[Mister T]

And how'd we learn the distance between its crests and troughs?

That's simply the strength of the electromagnetic field. Light is an oscillating electromagnetic field.

 

[syfry]

That's simply the strength of the electromagnetic field. Light is an oscillating electromagnetic field.

But the wavelengths have actual precise lengths. What type of interactions had revealed that info? Baluncore may have answered that: was revealed by creating a phase difference. If I understood correctly.

I'm imagining there's probably some way to display the light onto a surface at a size we can view and then we measure that and calculate what the actual size is. (or I'm faintly recollecting that from memory)

 

[Baluncore]

But is that how devices that measure frequency do their thing?

Yes. If you introduce a wedge of known angle into the optical path, you can count the interference lines, to get a good estimate of wavelength.

And how'd we learn the distance between its crests and troughs?

That is a confusing question.
The 'height' difference between a crest and a trough is twice the amplitude. The separation in travel distance, between two adjacent cycles, is the wavelength. The separation in time between cycles is the period. The frequency is the reciprocal of the period.

The crests and troughs I believe you refer to are actually the positive and negative extremes of either the electric or magnetic field. Those extremes are actually half a wavelength apart. The wave repeats once in every wavelength.

 

[syfry]

That is a confusing question.

Yeah, I worded that bad, should've said only crests, as in from crest to crest (or, trough to trough). Wasn't referring to the height.

But, glad I did (accidentally) ask in a confusing way, because got to learn about positive and negative extremes existing in those fields!

 

[DaveE]

But is that how devices that measure frequency do their thing?

Probably the most common method in spectroscopy is to use a diffraction grating and then measure the intensity (with a photodiode) versus angle of diffraction.

https://www.ossila.com/en-us/pages/spectrometer-working-principles

Another method is a scanning Fabry-Perot interferometer, which is often used in optical spectrum analyzers.

 

[syfry]

Probably the most common method in spectroscopy is to use a diffraction grating and then measure the intensity (with a photodiode) versus angle of diffraction.

https://www.ossila.com/en-us/pages/spectrometer-working-principles

This thread has upended my lifelong perception of a machine that counts the frequency of light individually by each wavelength as it passes through and triggers the counter.

It's so helpful to see the actual method (in action is even better)

 

[Baluncore]

Yeah, I worded that bad, should've said only crests, as in from crest to crest (or, trough to trough).

When we measure the period of a sinewave, we measure between two positive, or two negative, zero crossings. That is because the greatest slope of a sinewave is at the zero crossings, so zeros can be identified in position, more accurately than the extremes.

When we measure interference fringes, we measure the distance between the nulls. That is because we can identify the position of a null accurately, as it is deep and very narrow.

The null occurs when there is close to total cancellation, that is, when there is a half-wavelength difference in phase.

When we count interference fringes by eye, we count the wide peaks, which occur where the waves are in phase.

 

[DaveE]

This thread has upended my lifelong perception of a machine that counts the frequency of light individually by each wavelength as it passes through and triggers the counter.

It's so helpful to see the actual method (in action is even better)

You can literally see it in action yourself if you look at a light reflecting off of a CD, DVD, etc. Your eye can be the detector, your hand can control the angle.

 

[Baluncore]

This thread has upended my lifelong perception of a machine that counts the frequency of light individually by each wavelength as it passes through and triggers the counter.

Such instruments are now assembled in some laboratories. By mixing light of different frequencies (colours) in a non-linear optical device, sum and difference frequencies are generated. The difference frequency can be in the IR or RF microwave band. That may be down-converted further to a frequency that can be measured with a digital electronic counter. It is necessary to know the frequency of the laser that is used as the local reference frequency.

Atomic clocks, based for example on hydrogen or caesium, mix emission lines from the spectrum of a plasma, to get a microwave difference frequency. That is selected to become the master reference frequency.

 

[tech99]

Light frequency is too high to measure electronically, so the usual method is to find the wavelength and then calculate the frequency from knowledge of the speed of light. Wavelength is most easily found by creating standing waves using mirrors or by measuring the interference pattern created by one of the experiments such as Young's Slits or Newton's Rings.

 

[Drakkith]

Note that we can easily measure the amplitude of EM waves of lower frequencies, and thus we can get excellent measurements of wavelengths and frequencies. Radio astronomy takes heavy advantage of this fact to do things like aperture synthesis, which wouldn't be possible without knowing the amplitude and phase of the EM waves.

 

[Baluncore]

Radio astronomy takes heavy advantage of this fact to do things like aperture synthesis, which wouldn't be possible without knowing the amplitude and phase of the EM waves.

When I was playing that game, wideband RA signals were digitised with one or two voltage comparators. It was the polarity that was correlated to create the image. Most of the signal was noise, so more than two bits was excessive.

 

[Mister T]

But the wavelengths have actual precise lengths. What type of interactions had revealed that info?

Oh, sorry, I thought you were talking about the distance from trough to crest. The wavelength is simply the distance between a repetition of the electromagnetic field. In other words, how far do have to travel before the wave repeats itself.

You would need an experimental setup that measures the value of the electromagnetic field, and some way to measure how far you have to travel along the direction of propagation to get that same value.

 

[berkeman]

@syfry -- You might find this technology interesting. A number of years ago Hewlett Packard introduced laser interferometers for measuring distance displacements very accurately. They counted bright and dark transitions in the interferometer to count the number of light wavelengths in the displacement. It was a pretty neat technology, but because it was limited to relative movement and displacements (instead of absolute distance), it had limited uses and was eventually discontinued as a product line.

https://www.vaisala.com/sites/default/files/documents/VN151_HP_Laser_Interferometers.pdf