Visible light is an EM wave just like radio waves are

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Im not sure of this, but visible light is an EM wave just like radio waves are, right?

If I am right, does that mean mirrors reflect all ranges of EM waves, radio included? Also what is it about a mirror that reflects photons?
 
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  • #2
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The material from which the mirror is made happens to reflect a certain range of EM wavelengths.
You are right to assume that the mirror will not only reflect visible light, but it will also reflect other EM waves, but that does not seems that it will reflect ALL kinds of EM waves (some EM waves like Radio waves can easily get through it).
 
  • #3
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Some radio telescope antennae are just grids of metal bars. These behave as a 'mirror' for radio waves (because these have a large wavelength).
There are also X-ray telescopes in which X-rays are deflected at very small angles in order to intersect at a focus. I think it's called Bragg-reflection.
What a 'mirror' is, depends on the kind of radiation you want to reflect.

As for your 2nd question: There is no such thing as a 'reflection of a photon'. A photon can't be reflected since it's not a corpuscle but a quantum of e.m. energy.
 
  • #4
chroot
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Oh? Photons can't be reflected because they're waves? You need to learn a bit more physics before you make statements like this.

- Warren
 
  • #5
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Originally posted by chroot
Oh? Photons can't be reflected because they're waves?
Hi chroot, I didn't say photons are waves. Do you think a photon is a wave? If so, what makes you think so?
 
  • #6
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I think, IIRC in my first physics class, my teacher said that for an electromagnetic wave to be reflected, the distance between the 'bumps' or more correctly, non-smooth parts of the mirror has to be equal to or less than 1/8th the wavelength of the wave. I'm not sure if that's the case or not.
 
  • #7
chroot
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Originally posted by arcnets
Hi chroot, I didn't say photons are waves. Do you think a photon is a wave? If so, what makes you think so?
I'd like for you to explain for me what a 'quantum of e.m. energy' is.

- Warren
 
  • #8
FZ+
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Ok... maybe time to cut through the semantical discussion here...

arcnets is correct. Photons are not "reflected" in the conventional sense. They don't just "bounce" off the mirror, photons cannot be accelerated - their motion is ALWAYS at c. Instead, the case in light reflection is that the energy of the photon is absorbed and re-emmitted in reflected direction, in a case of specular reflection - all the photons are deflected in the same angle, maintaining the image.

Photons are both waves and particles (or in Newtonian lingo, corpuscles) They exhibit the properties of either depending on how you test them.
 
  • #9
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I haven't heard photons called corpuscles in a long time. They are merely wave-trains if you will. They carry information about the group of photons they travel with, and such. Due to quantum fuzziness, they are both wave and particle, but as pointed out, it depends on how you test them as to how they manifest themselves.
 
  • #10
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Originally posted by chroot
I'd like for you to explain for me what a 'quantum of e.m. energy' is.
I think electromagnetic energy comes in discrete portions called quanta or photons. Example, you can't see infrared light no matter how intense, because the photons it contains can not be absorbed by the receptors in your eye.
 
  • #11
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Originally posted by arcnets
Example, you can't see infrared light no matter how intense, because the photons it contains can not be absorbed by the receptors in your eye.
Hmmm...this seems strange. If you shine a IR laser into your eye, you will quite literally, cook it. But if it is not absorbed by your receptors, how can it cook it? Note the reason is not that the receptors do not absorb IR wavelengths while the rest of the eye does (thus allowing cooking to still take place), it is still possible to get blinded by an IR laser just by staring into it for a moment, way before the eyeball actually starts steaming
 
  • #12
chroot
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Originally posted by arcnets
I think electromagnetic energy comes in discrete portions called quanta or photons.
Ah, so photons are 'quanta of e.m. energy,' and quanta of e.m. energy are photons! Oh so helpful! [zz)]

- Warren
 
  • #13
Integral
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There was a therad concerning the nature of Radio Frequency Radation on PF2 over a year ago. Tom and other very knowledgeable posters argued the existance of the RF photon. I remained unconvinced (Sorry Tom :wink: ).

Considering that the source of the radiation from an antenna is due to macroscopic currents which exist well above the quantum limit, I am uncertian as to how they can be considered quantum in nature. Just my lack of understanding speaking so perhaps I can learn something.

A RF dish reflector is ideally parabolic in shape, so the weak signal being recieved can be reflected to the focus where the reciever horn is located. The source of the reflected wave is again macroscopic currents generated by the incident wave in the conducting medium of the reflector. This of course is analogous to a visible light mirror, which is a conducting surface (the "silver") which has many weak bonded electrons free to move under the influence of the incident light. The "reflected" wave is a good reproduction of the incident due the low loss in the conductive surface.


Why does the eye not see the infrared light even though it is burning the retnia? Simple, because the retina is not sensitive to that wavelengt. By sensitive I mean the ability to transform the incident energy into a signal the brain can interpret as light.
 
  • #14
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I just want to be clear on one part that Stai mentioned regarding the reflection of other EM waves. I was under the impression that if something is reflective for visible light that it would certainly be reflective for any frequency of the spectrum below it i.e. light waves reflect off a mirror thus a radio wave 'would' to.

I made this grand assumption based on how light reflects off of different surfaces. Whatever type of EM rays reflects off a surface depending on how the ray 'percieves' the surface it falls incident upon, polished or rough. If the EM rays falls incident upon a polished surface it will reflect 'mirror-like' and the most of the rays will have the same angle of reflection as angle of incidence thus yielding a perfect 'V' shape of EM rays (no cris-crossing of waves on the reflective side but all rays are parallel). If the EM ray falls incident upon a rough surface the rays will reflect in many directions (diffuse reflection).

Now whether a particular EM ray will 'percieve' a surface rough or smooth depends on the differences in elevation across the surface it falls incedent upon. Roughly speaking for a ray to 'percieve' a surface polished the difference in elevation across the surface should be no more than 1/8 it's wavelength. If the EM rays fall incident upon a surface w/ a difference in elevation greater than 1/8 it's wavelength it will percieve the surface as rough and it will diffusely reflect.

Now if visible-light deems a reflective surface as polished (mirror) b/c the elevation difference is less than 1/8 it's wavelength then any (reflective) surface elevation less then about .00 000 000 000 5 meters is polished for visible waves. Anything less then .375 meters is polished for a radio wave, at above 100 Mhz. So wouldn't a mirror also reflect a radio wave as well as visible light since 5 * 10 ^-12m < .375 m?

(wavelength of visible light is approx 400 to 700 billionths of a meter: 1/8 of 4-7*10 ^ -11 = 5-8.75 * 10^-12)

(wavelength of a 100 Mhz radio wave is about 3 meters: 1/8 (3m)= 37.5 cm or .375 m)
 
  • #15
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I was under the impression that if something is reflective for visible light that it would certainly be reflective for any frequency of the spectrum below it i.e. light waves reflect off a mirror thus a radio wave 'would' to.
No, this is definitely wrong, as shown by the fact than we can see Blue objects (which reflect blue light but not read). You must remember than surfaces also Absorb radiation.
 
  • #16
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Thanks, I just made the connection.:wink:
 
  • #17
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why does a blue object reflect blue light, or should I say why does a particular wavelength makes us visualise as blue?
 
  • #18
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Originally posted by FZ+
No, this is definitely wrong, as shown by the fact than we can see Blue objects (which reflect blue light but not read). You must remember than surfaces also Absorb radiation.
So, eventually (taking all factors), does the normal mirror reflect Radio Waves or not ?
or should I say why does a particular wavelength makes us visualise as blue?
Because the receptors on your retina will send a signal that says this point is blue to the brain when a blue wavelength comes to it.
What actually happens in the brain to intercept this signal as blue is not really clear (so far).
 
  • #19
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I thought a mirror can reflect any wavelength of electromagnetic wave?
 
  • #20
neutroncount
A mirror is just polished metal, or plastic with a thin layer of metal coating. If you want to reflect at a certain frequency range wiht a high certainty, you have to make sure the surface can handle the wavelength you throw at it. For instance, radio waves can be reflected with a metal grid that doesn't necessarily need to be fine. Most low frequency radar dishes are mostly open space. Microwaves can be reflected by a simple metal sureface. Metal waveguides can direct microwaves to a target. A simple bathroom mirror reflects the visible spectrum. But when you start getting into the higher frequency range, the penetrating power of the photons really start to show. X-rays can be reflected by mirrors but the grazing incidence must be very steep. Gamma rays are far too energetic to really reflect as far as I know.
 
  • #21
Integral
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You "mirror" must approiatly sized. How much of a ocean wave does a teacup reflect? Likewise if the mirror is much smaller then the wavelength of the RF energy it will simply be passed over with no reflection. For effecient reception or transmission there must exist a integral relationship between the size of the antenna and the wavelength of the RF wave.
 

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