# Tag Info

54

Most electromagnetic radiation is of very high frequency - the magnetic field changes many times per second. This means that the compass just doesn't have time to "follow" the magnetic field changes. The only thing that does affect a compass is a DC magnetic field - usually this is a large piece of iron etc. that gets magnetized (e.g. by the earth's ...

26

The frequency is one very good argument (and I guess the most important factor) but it might be worth also looking at the magnitudes of the fields. The earth's magnetic field has a strength of roughly $31µT$. The intensity of the sunlight hitting the earth is about $1300W/m^2$. Since the intensity is related to the electric field $E$ of an electromagnetic ...

19

Basically the same reason as what Floris said, but this also has another important aspect: Visible light has a far too small wavelength to affect a compass. Not only does the field oscillate too quickly around an average of zero – even at any single “snapshot” in time of the electromagnetic wave, there would nowhere be a large region where ...

12

Given that most green pointers are frequency doubled from a 281.8 THz infrared laser ($c$/1064 nm), it's possible that you have a two frequencies $f_1$ and $f_2$ in the original infrared laser (i.e., it is multimode). After passing through the "frequency doubling" nonlinear crystal you see three frequencies: $2 f_1$, $2f_2$, and $f_1 + f_2$. It looks like ...

8

In microwave ovens what matters is how much energy the radiation carries and how that energy is absorbed by the food. Visible light and IR are rapidly absorbed by most foods, so they would only heat the outer layer of the food. You'd get food with the outside carbonised and the inside raw. Microwaves are far less strongly absorbed by foods, so they ...

5

For light bulbs and other thermal emitters this is definitely true. Their emission follows the black body spectrum (if you neglect absorption due to the glass container). If you want to be picky: Any device, which is operated above 0 K (which applies to all devices) emit thermal radiation according to their temperature. This is not directly related with the ...

4

Under normal conditions, each photon can be thought of as a purely coherent entity with a definite polarization at all points in momentum (wavenumber) space. I discuss this notion in more depth in several other answers, notably this one here but the essential idea is this: lone photons propagate following Maxwell's equations (which are pretty much the Dirac ...

4

you see the beam of light because it lightens the molecules in the air. The photons you see have been diffused by the molecules of air. In the vacuum photons won't change of direction (they go at the speed of light) and you wouldn't see the beam passing in front of you.

4

When you first apply current to a laser diode, it does behave as an LED. Light is output across a (relatively) broad spectrum by spontaneous emission. But once the current reaches the threshold current, then positive feedback causes one (or a few) modes to oscillate. Further increases in input power will increase the ouput in those particular modes, but the ...

4

Probably green, since lemons are common objects in daily life we tend to observe them in daylight conditions often. Based on their bright yellow color (relative to daylight brightness of e.g. white paper) they probably reflect a large part of the spectrum that gives a yellow color, i.e. from red(~600nm) to green(~540nm) wavelengths. This means under most ...

4

Any energy principle is not being violated since the speed of the photon is never less than $c$ and hence the momentum is unchanging (in the classical sense). Why light travels slower than $c$ in a medium is because of the photons being absorbed and reradiated by atoms in the material. In a sense you can make the analogy of light traveling a longer path in ...

4

The colour of stars as observed by an observer on Earth varies just like the colour of our own Sun, depending on where in the sky the source is relative to the observer. However, the light of stars is generally too faint to notice this as clearly with the naked eye, because we cannot perceive colour for weak light sources.

3

No, Rayleigh scattering models the probability (and angle) of scattering as a function of wavelength and of the particle sizes. All wavelengths travel a long way but the path followed (scatter or nonscatter) varies. Since space is mostly "empty", there's little scattering. Beyond that, your understanding of stars is quite incomplete. THey do in fact have ...

3

You are correct in one thing: if an atom in an isotropic medium spontaneously emits a photon, it can do so in any direction at all, and the overall emission will be evenly spread over the unit sphere. However, lasers work using stimulated emission, which is slightly different: if an atom is excited, you can induce it to emit its energy by shining an initial ...

3

You may wish to look at the Fresnel formulas (see "Fresnel Equations" Wiki page), which are derived from the Maxwell equations.

3

Assuming I've understood your question correctly: The light from a bulb travels outwards in all directions and so hits (almost) all of the room. When it hits the walls etc, it gets reflected off of them (in most directions away from the wall), and then enters your eye. Hence your eye receives light from most of the room, so the room appears light. It's ...

3

What you are seeing is stress in the window resulting in birefringence: the speed of propagation of polarized light depends on the direction of polarization. In the setup you have, the light in the sky is partially polarized because that's how Rayleigh scattering works; this partially polarized light is transmitted through the window where it rotates ...

3

A good question, you are right the frequency remains constant (unless you have Doppler effects due to relative movement, but that's not your question). For visible light, refraction properties are quite often in question and as such it make sense to speak in terms of wavelength. As you go even higher in "frequency", physicists start talking in keV and MeV ...

3

Imagine not the direction of the column but the direction of the front row. Suppose the front row of soldiers were carrying a horizontal bar, the one on the left hitting the swamp would slow down while the one on the right was still moving quickly so the bar (=wavefront) would change direction It's a slightly bad analogy. A much better one is: Imagine you ...

2

It doesn't quite work the way you envision it (if the refraction angle is such that you can add light, it will escape the same way), but there are optical resonators that do essentially what you want: Light incident on a mirror gets added to a light field trapped between two or more mirrors. In such setups, not quite enough light usually builds up to cause ...

2

(This type of question has been asked by 4 users but in those questions they either gave an example of a wooden box or a room and they got answers that the light is absorbed by the wood or the walls of the room. But in my question its the case of mirrors.) In this case, the light would be absorbed by de "viewer". You would need some type of device ...

2

The picture of the wave you are looking at is already polarized and that is plane polarized light. But this is not the general case, waves emitted by any one molecule may be linearly polarized but an ordinary light source contains large number of molecules with random orientations,so the emitted light is random mixture of waves linearly polarized in all ...

2

Feynman: The correct picture of an atom, which is given by the theory of wave mechanics, says that,so far as problems involving light are concerned, the electrons behave as though they were held by springs. So we shall suppose that the electrons have a linear restoring force which, together with their mass $m$, makes them behave like little oscillators, ...

2

Jon Custer hinted at something, which I think is best explained via an analogy. Imagine you can walk along a pavement at 4mph. When the pavement is empty, it takes you an hour to travel four miles. But when the pavement is crowded, you're dodging around people and bumping into them. You're still walking at 4mph, but it takes you an hour and a half to travel ...

2

Due to the Structure of Glass, No Interference. To determine the thickness required to cause thin-film interference, both in light and in oil or soap bubbles, you rely on the following equations: $$2n_{film}d_{film} \cos{\theta} = m\lambda$$ $$2n_{film}d_{film} \cos{\theta} = (m-\frac{1}{2})\lambda$$ These being the equations for constructive and ...

2

No you are getting it wrong! UV filter , Will "Filter" the Ultra violet light. it means no ultra violet can get trough it. it is very cheap and you can find a UV filter every where. almost every camera has a UV filter to prevent ultra violet light to get through the lens. and also there are filters that will pass only ultra violet lights, as they absorb ...

2

When light hits a barrier, even transparent ones, some light is reflected and some is refracted. This is often described by the transmission coefficient for that material, and at that wavelength. This can happen at the macroscopic barriers and at the smaller barriers between crystals or grains within a material. It is a simple property of waves which does ...

2

Can we see a rainbow on the moon...? Usually no. A rainbow is formed by light refraction in water droplets. On earth we typically see a rainbow during rain while it's sunny. As there is no atmosphere on the moon, there will be no rain and thus no rainbows. However, if you were to spray small water droplets on the moon you might see a rainbow (but the ...

2

EV stands for exposure value. The equation to convert EV to lux is: $$L = 2.5 \times 2^{EV}$$ Since you're using a Sekonic meter note that Sekonic provide a conversion chart here.

2

Starlight, as emitted by a star, comes in a wide range of colours. For instance see the picture below. Now this is a picture, and pictures can often be tricky with their representation of colour, so you'll have to take my word for it that Betelgeuse does look significantly redder to the naked eye than say Vega until you get a chance to go look yourself on ...

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