Propagating solutions to Maxwell’s equations in classical electromagnetism and real photons in quantum electrodynamics. A superset of thermal-radiation.

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138
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66k views

Why does NASA use gold foil on equipment and gold-coated visors?

I've read several websites about equipment covered with gold foil and astronaut helmet visors are coated with gold. However, their explanations are devoid of almost all physics content. Can someone ...
116
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6answers
53k views

If you view the Earth from far enough away can you observe its past?

From my understanding of light, you are always looking into the past based on how much time it takes the light to reach you from what you are observing. For example when you see a star burn out, if ...
91
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8answers
102k views

If photons have no mass, how can they have momentum?

As an explanation of why a large gravitational field (such as a black hole) can bend light, I have heard that light has momentum. This is given as a solution to the problem of only massive objects ...
67
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8answers
83k views

Why does a remote car key work when held to your head/body?

I was trying to unlock my car with a keyfob, but I was out of range. A friend of mine said that I have to hold the transmitter next to my head. It worked, so I tried the following later that day: ...
66
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3answers
9k views

Why doesn't the motion of a car affect the frequency of radio stations?

When we go in a car and tune to an FM radio station, why doesn't our motion disturb the frequency? Like the Doppler effect?
48
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3answers
7k views

Why doesn't light affect a compass?

In our daily life a lot of photons of visible light, infrared and radio etc move around us. We know that light is an electromagnetic radiation. So why doesn't that electromagnetic radiation affect a ...
48
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7answers
3k views

Explaining UV radiation to a 6 year old

My (just completed) PhD involved a considerable amount of research involved with the detection of solar UV radiation. This generated quite a bit of interest, especially when I was conducting my ...
48
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7answers
24k views

Why glass is transparent?

Once I asked this question from my teacher and he replied "because it passes light", "and why it passes light" I asked and he said "because it is transparent". Same question again, Why glass is ...
47
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7answers
7k views

Cyclist's electrical tingling under power lines

It's been happening to me for years. I finally decided to ask users who are better with "practical physics" when I was told that my experience – that I am going to describe momentarily – prove that I ...
41
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5answers
4k views

Scattering of light by light: experimental status

Scattering of light by light does not occur in the solutions of Maxwell's equations (since they are linear and EM waves obey superposition), but it is a prediction of QED (the most significant Feynman ...
38
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4answers
5k views

Do rainbows have ultraviolet bands and infrared bands?

We have seen that rainbows looks so colorful as we are only able to see only the visible light. But Do they also have ultraviolet bands and infra-red bands, that we are unable to see? I know someone ...
37
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5answers
6k views

Why doesn't light kill me?

Why does each individual photon have such a low amount of energy? I am hit by photons all day and I find it amazing that I am not vaporized. Am I simply too physically big for the photons to harm me ...
37
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4answers
4k views

How far out from the Sun is visible light still sufficient to read a book?

Recent pictures from the New Horizons spacecraft, shown below, seem to indicate that, at Pluto's distance, we are entering a twilight zone, with a distinct lack of colors, although that could be due ...
37
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5answers
6k views

Does a constantly accelerating charged particle emit EM radiation or not?

The Abraham-Lorentz force gives the recoil force, $\mathbf{F_{rad}}$, back on a charged particle $q$ when it emits electromagnetic radiation. It is given by: $$\mathbf{F_{rad}} = \frac{q^2}{6\pi \...
36
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10answers
77k views

Why is light called an 'electromagnetic wave' if it's neither electric nor magnetic?

How can light be called electromagnetic if it doesn't appear to be electric nor magnetic? If I go out to the sunlight, magnets aren't affected (or don't seem to be). And there is no transfer of ...
36
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4answers
5k views

How to imagine WiFi signal propagation?

When thinking about how the WiFi signal propagates through a household, can I use the following thought experiment? Assume absolute darkness. Place a strong lightbulb where the WiFi access point ...
36
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3answers
4k views

Do two beams of light attract each other in general theory of relativity?

In general relativity, light is subject to gravitational pull. Does light generate gravitational pull, and do two beams of light attract each other?
35
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10answers
22k views

Why and how is the speed of light in vacuum constant, i.e., independent of reference frame?

I was told that the Galilean relative velocity rule does not apply to the speed of light. No matter how fast two objects are moving, the speed of light will remain same for both of them. How and why ...
31
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3answers
5k views

Can I use an antenna as a light source?

Can I use a normal metal antenna to emit visible light?
31
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4answers
5k views

Is there an infinite amount of wavelengths of light? Is the EM spectrum continuous?

The electromagnetic spectrum is a continuum of wavelengths of light, and we have labels for some ranges of these and numerical measurements for many. Question: Is the EM spectrum continuous such that ...
31
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4answers
9k views

If both radio waves and gamma rays can travel through walls

and they are on opposite ends of the electromagnetic spectrum, then why can't light travel through walls which is right in the middle of the spectrum? This question has already been asked here. ...
31
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4answers
8k views

Why do prisms work (why is refraction frequency dependent)?

It is well known that a prism can "split light" by separating different frequencies of light: Many sources state that the reason this happens is that the index of refraction is different for ...
28
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6answers
3k views

Is a suit that hides a soldier's heat signature fundamentally possible?

I recently played "Crysis", a game where the protagonist wears a suit that allows the player to hide both himself and his heat signature. Then I watched Iron Man 3, where a kid suggests that Tony ...
28
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8answers
3k views

What are the various physical mechanisms for energy transfer to the photon during blackbody emission?

By conservation of energy, the solid is left in a lower energy state following emission of a photon. Clearly absorption and emission balance at thermal equilibrium, however, thermodynamic equilibrium ...
28
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2answers
1k views

Why does laser light not affect glowing materials?

I have this childrens rubber ball which glows in the dark after it's exposed to light. I "charge" it with a flash light then play with my dogs at night. I thought to try a very intense green laser, ...
28
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5answers
3k views

Does a charged particle accelerating in a gravitational field radiate?

A charged particle undergoing an acceleration radiates photons. Let's consider a charge in a freely falling frame of reference. In such a frame, the local gravitational field is necessarily zero, ...
27
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3answers
4k views

How many X-rays does a light bulb emit?

I read somewhere that most things1 emits all kinds of radiation, just very few of some kinds. So that made me wondering whether there is a formula to calculate how many X-rays an 100W incandescent ...
27
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3answers
4k views

Why do X-rays go through things?

I always heard that the smaller the wavelength, the more interactions take place. The sky is blue because the blue light scatters. So why is this not true for X-rays, which go through objects so ...
26
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5answers
7k views

Can the sun cast a shadow?

The sun's light can cast the shadow of another object, but does it ever cast its own shadow?
25
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2answers
2k views

Is it (practically) possible for a large building to be a Faraday cage?

In my sophomore year of high school, my P.E. teachers kept on complaining about how phones didn't have a network connection in our gym, regardless of model, service provider, etc. A couple of feet ...
25
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4answers
9k views

Amplitude of an electromagnetic wave containing a single photon

Given a light pulse in vacuum containing a single photon with an energy $E=h\nu$, what is the peak value of the electric / magnetic field?
25
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2answers
1k views

Why is Huygens' principle only valid in an odd number of spatial dimensions?

Apparently Huygens' principle is only valid in an odd number of spatial dimensions: http://mathoverflow.net/a/5396/21349 Huygen's principle in curved spacetimes Why is this? [EDIT] This is ...
23
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5answers
2k views

Can a photon get emitted without a receiver?

It is generally agreed upon that electromagnetic waves from an emitter does not have to connect to a receiver, but how can we be sure this is a fact? The problem is that we can never observe non ...
23
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5answers
29k views

Phase shift of 180 degrees on reflection from optically denser medium

Can anyone please provide an intuitive explanation of why phase shift of 180 degrees occurs in the Electric Field of a EM wave,when reflected from an optically denser medium? I tried searching for it ...
23
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1answer
862 views

How can my window not scramble the image of my yard?

How can an image pass through a window if the atoms in the glass randomly emit photons in any direction? I've read that glass is transparent because the atoms don't readily adsorb visible light, so it ...
22
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2answers
7k views

Is it really possible to break the speed of light by flicking your wrist with a laser pointer?

Minutephysics has a popular YouTube video called "How to break the speed of light". In the video it states that if you flick your wrist while pointing a laser that reaches the moon, that the spot of ...
22
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2answers
14k views

How do Optically Active Compounds Rotate Plane Polarized Light?

I am not sure if this is more of a Chemistry or a Physics question, but in my Organic Chem class we discussed that chiral molecules will rotate plane polarized light. However, my professor did not ...
21
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3answers
2k views

Looking at the sun through heavy fog, do invisible wavelengths get through?

This morning it was particularly foggy and my son was excitedly looking at the sun through the fog, since of course he normally can't look directly at it and see the disk. I told him to stop looking ...
21
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1answer
637 views

How do we know that these radio bursts are from billions of light-years away?

NASA just announced that they detected the first radio bursts from outside of our galaxy. Astronomers, including a team member from NASA's Jet Propulsion Laboratory in Pasadena, Calif., have ...
21
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3answers
2k views

Can light exist in $2+1$ or $1+1$ spacetime dimensions?

Spacetime of special relativity is frequently illustrated with its spatial part reduced to one or two spatial dimension (with light sector or cone, respectively). Taken literally, is it possible for $...
20
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4answers
3k views

Newton's rings: What causes the other rings?

This is from an experiment we did in physics class. We shone a sodium light at a convex lens on top of a sheet of glass - and this image was captured by a USB microscope. I know what causes the main ...
20
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4answers
947 views

Is the electromagnetic spectrum discrete?

I'm just starting to learn physics and I have a question (that is probably stupid.) I learned that energy levels that the bound electron can have are discrete. I also learned that when an electron ...
20
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2answers
997 views

Could Google scan books without opening them? [on hold]

A few years ago, I read an article about a proposal Google put forward, as part of its paper-to-digital media project, which had the advantage of greatly speeding up the process of scanning text. The ...
20
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4answers
2k views

Redshifting of Light and the expansion of the universe

So I have learned in class that light can get red-shifted as it travels through space. As I understand it, space itself expands and stretches out the wavelength of the light. This results in the light ...
20
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1answer
302 views

Should a superconductor act as a perfect mirror?

I have been told that metals are good reflectors because they are good conductors. Since Electric fields in conductors cause the electrons to move until they cancel out the field, there really can't ...
20
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2answers
23k views

How and why does accelerating charges radiate electromagnetic radiation?

Lets consider it case by case: Case 1: Charge particle is at rest. It has electric field around it. No problem. That is its property. Case 2: Charge particle started moving (its accelerating). We ...
19
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3answers
5k views

How would an X-ray scanner identify a mirror?

A mirror is under normal circumstance used to reflect Electromagnetic radiation also known as photons (light) and in airport security or medical facilities, they use X-rays to detect anomalies inside ...
19
votes
6answers
2k views

Why does matter/antimatter only produce gamma rays?

According to wikipedia, all antimatter annihilation produces gamma rays (along with potentially other elements). Why specifically Gamma rays? Why not electromagnetic waves of other wavelength?
19
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2answers
6k views

How do microwaves heat moisture-free items?

Today I learnt that microwaves heat food by blasting electromagnetic waves through the water molecules found in the food. Does that mean food with 0% moisture (if such a thing exists - dried spices?) ...
19
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2answers
1k views

Gravitational slingshot of light using a black hole/massive object

Wikipedia has this page on gravity assists using planets. In some cases this effect was used to accelerate the spacecraft to a higher velocity. This diagram shows this in a very oversimplified manner. ...