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Comment by mpolichette | Hacker News Reader

I've always wondered about this... if an EM field is composed of photons which have higher energy than light, how come I've never seen image/video captured where the device is 'glowing' or something as a photon source? Is there something I'm missing that prevents us from detecting them similar to the way we detect visible light?

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Nope, it is definetly possible to capture images in the non-visible spectrum. We simply need to convert them into the visible spectrum using some transform (usually just translation) so we can see them.

You probably have seen images like this, you just might not have realized. Many of the most beautiful astronomy photographs are actually captured in different spectrums. If you've ever seen a "thermal image" that is also non-visible photons.

Some examples:

https://asd.gsfc.nasa.gov/blueshift/wp-content/uploads/2013/...

https://smd-prod.s3.amazonaws.com/science-red/s3fs-public/th...

http://www.greenbuildingadvisor.com/sites/default/files/Ther...

> how come I've never seen image

Of course you have, it's called an x-ray.

In addition to other replies, you can easily "visualize" IR light from say a remote control. Just point your phone's camera to the bulb on the end of a remote that you point at the device it controls. You'll see a flash on the display because the IR flash from the remote saturates the CCD in your phone.

> if an EM field is composed of photons which have higher energy than light

That's not right - visible light is sort of a "midrange" photon energy that's exceeded by ultraviolet, x-rays and gamma rays and preceded by infrared, microwaves and radio waves. Everyday devices emit at low frequencies, with energies significantly below that of visible light. The higher energies (ultraviolet, xray, gamma) absolutely do have images:

- Ultraviolet [https://en.wikipedia.org/wiki/Ultraviolet_photography#/media...]

- Xray https://en.wikipedia.org/wiki/X-ray#/media/File:Radiograf%C3...

- Gamma https://en.wikipedia.org/wiki/Gamma_ray#/media/File:VACIS_Ga...

Now, the photons with lower energies than visible light form an interesting case. First off there's infrared, which is fairly familiar:

- Infrared https://en.wikipedia.org/wiki/Infrared_photography#/media/Fi...

Now, as the photon energies get lower the wavelengths get longer. This introduces a problem for lower-energy images: when the wavelength is around the size of the aperture, wave-like things will happen involving the aperture. This tends to blur the images, meaning that for longer and longer wavelengths larger and larger cameras are required to achieve the same level of detail. However, these images still exist - but they are rarely taken of everyday things. Note that the imaging you are talking about (to capture the glowing of everyday electronic devices in low frequencies) would have to happen in the microwave and radio bands - because that's around the frequency at which electronics operate. The result is that a reasonably-sized camera would not be able to take a meaningfully sharp image of a router's "glow."

- Microwave https://en.wikipedia.org/wiki/Microwave_imaging#/media/File:...

- Radio Waves http://www.gb.nrao.edu/epo/PageMill_Resources/galaxy.jpg

The radio wave picture depicts a galaxy. Radio imaging setups are usually very large, like this one: http://mstecker.com/pages/astroVLAa15vlad-2c1.htm

So does this mean infrared wavelengths are around the lower limit for taking images with a reasonable size camera? I've played around with the thermal IR camera attachements for smartphones (i.e https://www.amazon.com/FLIR-ONE-Thermal-Imager-iOS/dp/B00VIL...) and they're pretty small.

The law for diffraction-limited systems is that the blurred spot size increases linearly with wavelength. (Double the wavelength, all else being equal, means twice as blurry.) Engineering concerns (for example, how big can you manufacture a CCD before it becomes prohibitively expensive?) determine what constitutes an "excessively large camera." Although, because infrared covers a 1000-fold range between visible and micro, I feel like it would be safe to say that cellphone-sized cameras might never be made for microwaves.

[0]https://en.wikipedia.org/wiki/Diffraction-limited_system

Light is an EM vibration, but the fields here are essentially stationary potentials (similar to a bar magnet, or static electricity.) So it is not light, and not composed of phitons (that last part might be a simplification.)

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