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Old 15th Feb 2019, 10:48 pm   #1
Tyso_Bl
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Default The electromagnetic spectrum

Don't know where else to ask, google isn't helping tonight.

Last nights insomnia problem, the EM spectrum, as the frequency gets higher it is perceivable as light, we can see it via chemical changes in our retinas. So good so far.

Light is also photons, astronomers can with a telescope build up an image on a photographic plate or CCD sensor, one photon at a time, a time exposure, from
light that has traveled so far and is so spread out that it arrives as individual photons.

Physicists seem to be able to manipulate light photons on an individual basis too.

So the question that has been bothering me is at what point (as the frequency goes up) does an EM wave become photons? Or are radio waves just different sorts of photons?

I may be misunderstanding something basic, but I can't work out what it may be.

T
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Old 16th Feb 2019, 12:20 am   #2
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Default Re: The electromagnetic spectrum

Don't photons specify the visual spectrum? Albeit from far IR to UV this is still, theoretically, the 'visual' spectrum.

I know that the borders of MF, HF, VHF etc are defined therefore photons are just the same 'specification' for a range of frequencies.

My 3 ha'peth worth...... happy to be proven wrong!
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Old 16th Feb 2019, 12:39 am   #3
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Default Re: The electromagnetic spectrum

Hello,

All electromagnetic radiation consists of particles (photons).

The energy and therefore the mass of the photons increases with the frequency.

(The Energy of the photon equals Planck's Constant multiplied by the frequency and its mass is its energy divided by the velocity of light squared.)

So radio waves are low energy, low mass photons whilst gamma rays are high energy high mass photons and light is somewhere in between etc..

Yours Richard

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Old 16th Feb 2019, 12:59 am   #4
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Default Re: The electromagnetic spectrum

Simplified a little, through E=mc2, energy and mass can be converted one to the other but don’t exist at the same time. So photons do not have mass. Light (an EM wave) is bent by gravity because gravity bends space-time, not because photons have mass.
As said above, all EM waves consist of photons.
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Old 16th Feb 2019, 1:04 am   #5
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Default Re: The electromagnetic spectrum

Electromagnetic radiation exists. We can use it, we can make it do tricks. We know some things about it but we don't know everything.

Photons and waves are two rather different mental models, ways of imagining the workings of electromagnetic radiation. Neither is entirely right. Neither alone can tell the whole story.
We can make electromagnetic radiation do things particles can do and waves cannot. We can make it do things waves can do and particles cannot. Some people interpret this as showing that it is both a particle AND a wave. But really, it shows that it is neither. It is a something-else. A something that can sometimes show particle-like behaviour and sometimes wave-like.

The universe is not only weirder than we imagine, it may be weirder than we can imagine.

For day-to-day use, we can flip between wave and particle views to suit whatever we're doing and it works OK so long as we pick the right one. This leaves an uneasy feeling, waves and particles seem mutually exclusive. How can it be both at once? But if you see it as something else, and that waves and particles are just convenient approximations then the uneasiness is settled.

It goes on further. E=MCsquared is well enough known, so you can say the mass of an electron is worth an amount of energy, then add in its charge-related energy and kinetic and potential energy and you have a little parcel of energy. There's another equation E=hf h is Planck's constant and is the energy versus frequency of a photon. So we can calculate the frequency of a photon equivalent to our electron, or its wavelength. Quantum mechanics!

Now we rely on this rather heavily. It holds everything together for us. From chemistry, we know that electrons fill up shells and orbitals... well it seems that those stable orbits have circumferences which are related to integer numbers of wavelengths if you think of the electrons as photons. The outermost electron shells determine chemical properties and we have quantum-mechanical basis for the periodic table.

All quite mind-blasting, eh?

Staggers me!

David
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Old 16th Feb 2019, 1:07 am   #6
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Default Re: The electromagnetic spectrum

But as far as insomnia is concerned (?) isn't 'current thinking' that the brain is mostly affected by blue light, heading towards the edge of the visible spectrum, which is supposed to reduce the production of the hormone melatonin? Of course, in the past, physicians thought that a good bleed was the cure for most everything, so you have to take what they say with a pinch of salt.

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Old 16th Feb 2019, 4:13 am   #7
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Default Re: The electromagnetic spectrum

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Originally Posted by Bazz4CQJ View Post
But as far as insomnia is concerned (?) isn't 'current thinking' that the brain is mostly affected by blue light, heading towards the edge of the visible spectrum,
It always seems easy to demonstrate that light or other EM radiation acts like a wave with various interference experiments. But it is always cool to watch the paddle wheel spin around in a glass bulb with a light shining on it. Since E=hf and e=mC^2, then the momentum of a photon, mC is hf/C, simply directly proportional to its frequency and it gives radiation a pressure and it transfers momentum to mechanical objects just like you were throwing little balls at them.

RW sums it up nicely, neither particle or wave theory is 100% correct but both are needed to describe the observations.

As for the blue light, some Psychiatrists were advocating blue light therapy to treat depression and also noted that the natural lens in the eye turns yellow with age (blocking blue light). The lens can get quite yellow-orange looking (like an amber bead on a necklace). When they are cloudy as well, they are called cataracts. Artificial lenses implanted at the time of cataract surgery are generally crystal clear. Some Psychiatrists advocated early cataract extractions and lens implants, to let more blue spectrum light into the eye, to treat depression & insomnia ! It never went mainstream.

After cataract operations, people report a change in color temperature of their vision, blues & whites look brighter. After Claude Monet had his cataracts removed, he was unhappy with the apparent colder color temperature of some of his works and over painted them. The art restorers found the changes in the colors in the paint layers.

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Old 16th Feb 2019, 4:54 am   #8
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Default Re: The electromagnetic spectrum

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Originally Posted by Argus25 View Post
After cataract operations, people report a change in color temperature of their vision, blues & whites look brighter. After Claude Monet had his cataracts removed, he was unhappy with the apparent colder color temperature of some of his works and over painted them. The art restorers found the changes in the colors in the paint layers.
There is a Professor based at the John Radcliffe Hospital in Oxford who has published one or more papers about cataracts, corneal implants and sleep patterns. She reported that she found evidence of poor sleep patterns in people with cataracts, which improved when the cataracts were removed. Concerns about the possible adverse effects of the implants (light absorption by the polymer) on sleep were not observed.

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Old 16th Feb 2019, 8:11 am   #9
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Default Re: The electromagnetic spectrum

The ocular media, cornea, aqueous and vitreous are very poor at transmitting wavelengths shorter than 400nm. Blue is about 400 to 470nm. UVA & UVB are in the 280 to 400nm range. Obviously it is not good to let UVB into the eye (or onto skin) as it is ionizing radiation and can eject electrons from DNA and break it up causing mutations.This could damage retinal cells.

I have attached a transmission curve of the natural lens, it is a pretty sharp BPF. As we age it starts to block blue light too, which is why it starts to look yellow.

Somebody once decided that blue light might have accelerated macula degeneration, so came up with the notion that it is better that the natural lens in the eye yellows as we age. Some manufacturers started to make yellow (blue blocking) lens implants for use in cataract surgery. I thought it was nonsense because in a normal eye under the age of 40 the lens is not yellow. So if you are replacing a lens, why put in a lens that is like a 70 year old one, better it looks clear like a 20 year old lens. Still, for a while, yellow lens implants had some popularity. It was like the idea of replacing a hip or a knee joint with one that was partially arthritic or worn out.
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Old 16th Feb 2019, 9:06 am   #10
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Default Re: The electromagnetic spectrum

The nature of light is a strange thing - you can view a large ensemble of photons as a wave, and it exhibits wave-like behaviour. Diffraction, the diffraction grating, the prism, apparent depth etc can all be viewed as wave-like behaviour.

But if you turn the light intensity down to the level of individual photons being in the experiment at any particular time - the effect still happens. One example is Young's slits, which produce a pattern of ripples by the light from the two slits interfering with each other. But turning the light down to single photons - in due course a pattern of ripples eventually emerges. The single photon cannot pass through both slits simultaneously and "interfere" with itself - so what is going on? The answer is that there is no conventional analogy that helps at all. Quantum theory is needed to get to grips with why. Richard Feynman said "If anyone says that they understand quantum effects they are lying - no one understands this stuff. All we have is predictive theory"

Although you can't do an E=mc^2 because the photon does not have a rest mass, light does produce a pressure - because it has momentum. The pressure is the Poynting Vector divided by the speed of light, which is the same as I/c where I is the intensity in watts per square meter and c is the speed of light.

For solar radiation outside the atmosphere I~3000W/m^2, so the radiation pressure is 10^-5 Pascals (10uPa) or about 10^-10 of an atmosphere. That is enough for using a large area "sail" for propulsion in space, actually now demonstrated.

This light pressure phenomenon is used for "optical tweezers" where focused lasers are used to manipulate tiny objects. With 30mW lasers you can generate 100Pa at the focus of two lasers.

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Old 16th Feb 2019, 9:36 am   #11
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Default Re: The electromagnetic spectrum

Quote:
Originally Posted by Mr Moose View Post
Hello,

All electromagnetic radiation consists of particles (photons).

The energy and therefore the mass of the photons increases with the frequency.

(The Energy of the photon equals Planck's Constant multiplied by the frequency and its mass is its energy divided by the velocity of light squared.)

So radio waves are low energy, low mass photons

Yours Richard
I have no desire nor data to argue, but how can this be? I priduce radio waves in my little TX by connecting an electron source to the wires and pushing them through special devices till they pop out of my aerial. I never invoke photons.

Edit...answer is follows... I had misunderstood...

An electron is a specimen of matter. That is, it is a particle with mass. It has an electric charge of -1.602 * 10^-19 coulombs. It is a fermion, that is, it has a half integer spin and can therefore not occupy the same quantum state in the same space as another electron. To occupy the same space, it would have to have a different energy or an oppositely aligned spin. An electron is part of a group of subatomic particles called leptons. Some leptons are electrically charged and therefore sensitive to the electromagnetic force, and all leptons are sensitive to the weak nuclear force. Electrons, via the electromagnetic force and the rules of quantum mechanics, form bound states with atomic nuclei which we call atoms and molecules.

A photon is the force carrier of the electromagnetic force. All electrically charged particles like electrons and protons attract and repel each other via an exchange of photons. A photon has no mass and can be thought of as pure energy. It is a boson, that is, it has integer spin and therefore, unlike the electron, can share the same space with other photons in the same quantum state. A photon in a vacuum always moves at the speed of light. There is no frame of reference in which the photon can be defined as being at rest.

In an atom, if an electron absorbs a photon that has an energy that just happens to correspond to an energy difference between two different orbital levels in the atom, the electron will be elevated to that higher energy level.
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Old 16th Feb 2019, 10:40 am   #12
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Default Re: The electromagnetic spectrum

As someone who had a cataract operation on my right eye five weeks ago, and is awaiting the operation on my left eye, I can certainly vouch for the fact that the image seen in my right eye is astonishingly bluer and brighter than the murky yellow image in my left eye!
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Old 16th Feb 2019, 12:29 pm   #13
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Default Re: The electromagnetic spectrum

I had elective laser surgery on both corneas in around 2005, for myopia and astigmatism. (So they're removing material for the former and creating a smooth surface for the latter)
For those with a strong stomach, the cornea is in 3 layers, the outer layer is temporarily cut and peeled back and the laser etching occurs on the middle layer.

For some unknown reason the procedure reduced myopia from 6 to 1.25, and it had to be repeated at a second consultation to get to the bottom line of the chart (well, one eye is on the second line, but it's still impressive)

I too experienced a boost in blue. Everything in blue (particularly light blue) and also lilac or purple, was suddenly very strident.

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Old 16th Feb 2019, 12:35 pm   #14
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Default Re: The electromagnetic spectrum

I've not had an eye operation but from as long ago as I remember I've had a difference between my eyes. One eye sees a much bluer hue than the other, the other seeing a much browner hue in everything. I,ve always considered it odd but rather fun to switch between them.
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Old 16th Feb 2019, 12:48 pm   #15
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Default Re: The electromagnetic spectrum

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Originally Posted by Tyso_Bl View Post
So the question that has been bothering me is at what point (as the frequency goes up) does an EM wave become photons? Or are radio waves just different sorts of photons?
Light puzzled scientists for a long time. There were two theories - first, that it was a wave phenomenon; the other, that it was streams of particles. Some well-respected authorities - including Isaac Newton - favoured particles; others - Christian Huygens - said waves.

As phenomena were discovered, it lent weight to one theory at the expense of the other. Polarizing of light couldn't be explained by particles, but it could by waves. Interference and diffraction, similarly. Postulating light as particles predicted that in a refracting medium, speed must increase to explain the bending of a ray, whereas waves, the bending had to be explained by a decrease in velocity. When it became possible to measure the speed of light in a dense medium, it was found to be slower than in free space. This sounded the death knell for particles. It seemed Newton had been backing the wrong horse.

And then along came Einstein, fooling around with the photoelectric effect with a potassium photocathode in a vacuum cell. He found that blue light caused a photocurrent, but red light - no matter how intense the beam - didn't. Reviving the particle theory, he said that perhaps each particle of red light just didn't have the energy to liberate an electron, so it didn't matter how many you threw at the potassium surface, but blue light particles did. His experiments investigated the effect, he measured the energy of the emitted electrons for different light colours, and mopped up a Nobel prize.

Today, we regard light as a wave/particle duality, as others have said.

So, what about radio waves?

Well, they can be considered as streams of very low-energy photons, too. It's just that each photon is so incredibly low-energy, that it is so far impossible to detect them except en masse. It's a bit like trying to come up with some means of detecting individual water molecules in the stream coming out of a hosepipe. So far, experimental techniques haven't been refined to that extent. But, the photon nature of the Radio 4 emissions on 198kHz are there, waiting to be discovered.
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Old 16th Feb 2019, 12:56 pm   #16
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Default Re: The electromagnetic spectrum

Well it's the first time I have seen Quantum Physics implicity creep into cateracts, but the eye being light a cloud chamber, in a distant way, may see the odd flash lol.

I must admit to being very confused at first when they spoke about photons in radiotherapy. I suppose we are far more used to the particle theory of light than with radiowaves, but with microwaves things do start to appear to come together, even though they apply thoughout the spectrum.
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Old 16th Feb 2019, 1:42 pm   #17
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Default Re: The electromagnetic spectrum

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...

So, what about radio waves?

Well, they can be considered as streams of very low-energy photons, too. It's just that each photon is so incredibly low-energy, that it is so far impossible to detect them except en masse. It's a bit like trying to come up with some means of detecting individual water molecules in the stream coming out of a hosepipe. So far, experimental techniques haven't been refined to that extent. But, the photon nature of the Radio 4 emissions on 198kHz are there, waiting to be discovered.
I haven't done the sums on this but I wonder whether photon statistics are starting to become significant in the radio signal returning from Voyager 1 ? I also wonder whether radio astronomers looking at very distant objects have to worry about them ? Perhaps the cosmic microwave background, which must be the ultimate noise source, is always sufficiently 'loud' that it swamps radio signals before they get weak enough for their photon nature to start to matter ?

Cheers,

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Old 16th Feb 2019, 2:00 pm   #18
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Default Re: The electromagnetic spectrum

A 1MHz photon has energy 6.6 x 10^(-28) J. Energy scales with frequency.

Thermal energy at 2.7K (microwave background) is 3.7 x 10^(-23) J. Looking at it another way, the power from a 50R antenna in 1Hz bandwidth at 2.7K noise temperature is 8.6 x 10^(-11) J/s. There will be lots of photons!

I am going to hazard a guess that if you want to see photon effects in radio waves then you have to look at terahertz frequencies.
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Old 16th Feb 2019, 2:43 pm   #19
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Default Re: The electromagnetic spectrum

It's sorting out the signal photons from the random noise photons that's the fun.

Eb/No as they say

Energy per bit over noise spectral density in watts per Hertz.

With very low signal powers and an unavoidable noise density floor of kT, then you have to have plenty of time per bit to make reception possible.

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Old 16th Feb 2019, 2:50 pm   #20
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Default Re: The electromagnetic spectrum

As David implied back in #5, this wave/particle duality stuff is just the result of physicists not fully understanding the universe at the subatomic level. Waves and photons are alternative ways of describing phenomena metaphorically (ripples in ponds, billiard balls) and neither is a very satisfactory description of what's actually going on 'underneath'. Particle physics has become increasingly mathematical during the last century, and it may be impossible to explain the universe in a way that can be understood from everyday human experience.
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