# Tag Info

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Why should a shadow be cast on something? Look at this picture taken at sunset: What is that dark blue band at the horizon? Answer: the shadow of the Earth itself: In a certain way, we can say that the night sky itself is the result of the shadow of the Earth being cast on the sky (think about it). Basically, when you are standing on top of the ...

33

Yes, there is a fundamental limit. It comes down to two factors: How many watts of light energy can the source produce for each watt of electrical energy? How many lumens does each watt of light energy correspond to? The first question is straightforward - by conservation of energy, 1W of electrical energy can yield at most 1W of light energy. The ...

28

Circular currents do produce EM, and indeed this is exactly how X-rays are produced by synchotrons such as the (sadly now defunct) synchotron radiation source at Daresbury. In this case the current is flowing in a vacuum not in a wire, but the principle is the same. Current flowing in loops of wire don't produce radiation in everyday life because the ...

17

Even though there is a single photon in a volume of your choice the light is still a wave. An experiment was performed which proved this. In this experiment a Michelson interferometer was set up and the incident light is so weak that only one photon was in the whole setup at a time. A photographic plate was used to detect the interference pattern. Now just ...

12

What we perceive as colour is basically the frequency, or combination of frequencies, of the light shone on our retinas. The light that comes out of a light bulb (but, note, not of an LED) is a mixture of lights of many different frequencies, actually, of all frequencies in the visible range. This is what we call "white". Filters have the properties of ...

12

If you consider that electric current is actually the flow of individual charged electrons, then as John Rennie pointed out, the radiation exists but is negligibly small. But if you were to imagine breaking the current into more and more point particles with less and less charge while holding the linear charge density $\lambda$ fixed, then the radiation ...

10

What is this shadow being cast upon? The sun was directly behind me when I took the picture and it appears to be casting a shadow on.... the sky itself? It's the volumetric shadow of the mountain itself being cast on the sky itself. A mountain is not necessarily needed. Sometimes even a bit of smoke can create a nice volumetric shadow: Source: https://en....

9

Light does not slow down during a reflection. Light is a signal disturbance in electric and magnetic fields. These disturbances propagate through space at a fixed speed $c$ in vacuum. The situation is completely analogous, in a mathematical sense, to a wave pulse that is sent along a string. When the pulse encounters a boundary, it flips direction, and may ...

8

"Color" is how we sense visible light with different mix of wavelengths. Each wavelength of light in the visible range is a particular color. White light is all the wavelength mixed together at the right balance. What filters do is attenuate selected wavelengths of light. A red filter blocks most blue light, for example. If you start with white light ...

7

Plain and simple answer: yes, the sky itself. Unlit sky is black, the color of the sky comes from scattering of sunlight. So the dark parts are the ones where it's night already (they don't see the sun because the mountain is in the way), the rest still has evening.

7

The daylight sky has a brightness of about magnitude 3 per square arcsecond. The brightest stars have an integrated intensity of about zeroth magnitude. If your eyes had an angular resolution approaching 1 arcsecond then you would easily be able to see bright stars in the daylight sky - they would be about 10 times as bright as the sky. Unfortunately, the ...

6

Update 1: 1) Note added in proof: The photon stress-energy densities obtained below more or less heuristically are identical to those obtained in more rigorous approaches from the electromagnetic stress-energy density tensor. 2) The physical reason why the stress-energy argument retrieves the detailed balance result in the OP, but is inequivalent to simply ...

6

Yes there is the thermodynamic law of Conservation of Energy. Light is a form of energy, and you can't get more energy out than is put into the system. This paper http://physics.ucsd.edu/~tmurphy/papers/lumens-per-watt.pdf puts the number at about 250 lm/W for "white" light

6

The reddening of the sun has to do with Rayleigh scattering as the sun passes through more atmosphere. (see picture). This is in a sense, related to less energy but not the primary cause. The reason we get less solar energy per square meter is that the angle of the sun in the sky affects how spread out the light is. (see updated picture). Ignoring ...

6

In this case, it would be useful to not consider light in its particle form as photons, but instead to consider it as a wave - see this wikipedia page. Then, the wave is simply reflected from the surface, without us having to consider the kinetics of any particle. The wave, in a vacuum, would continue to propagate at the speed of light, regardless of the ...

5

For a definition of the UV index and a rough discussion of influencing factors see e.g. https://en.wikipedia.org/wiki/Ultraviolet_index. You can compare the model shown there against actually measured data for the US: http://www.cpc.ncep.noaa.gov/products/stratosphere/uv_index/uv_annual.shtml. Both sources will support your statement that UV exposure in ...

5

This may be a phenomenon due to beer's law. The amount of light a material absorbs is related to the path length of the light. Since we're looking at the liquid from above the one on the left is a length of, say, 2cm ....whereas the one on the right is a path length of, let's say, 15cm. Since the light is traveling through a much shorter length when it's ...

5

The light spectrum is continuous, RGB is about how you perceive light: You have 3 detectors in your eyes, each one is sensitive to a different range: one in the short wave lengths (Blue), one in the long (Red) and one in the middle (Green). When yellow or orange or any other light gets to your retina, each one of the detectors sends a signal and by the ...

5

Correction, a single photon does not have a circular polarization. It has spin +1 or -1 to the direction of its motion. Qualitatively Left and right handed circular polarization, and their associate angular momenta. The way the classical wave emerges from the quantum mechanical level of photons is given in this blog entry, and it needs quantum ...

5

Let me explain in purely classical terms (not the description of reality, but easy to imagine). You realized that when a ball bounces off the wall, at a certain point, it has no momentum. However, it must still have all the energy of the movement (neglecting losses to environment) - where did that energy go? The ball is formed of many discrete atoms, bound ...

4

Assuming that each sensor sees only the light from the other night light, and assuming that each night lights is bright enough to reliably trigger the other's sensor, then you have discovered a configuration that computer engineers call a "flip-flop". https://en.wikipedia.org/wiki/Flip-flop_%28electronics%29 It has another name, "bistable multivibrator." "...

4

I believe the question has been answered (earlier), but not explicitly. The question seems to be if an incandescent bulb requires 100 watts to produce 1690 lumens and an LED 23 watts to produce 1600 lumens, what is the theoretical minimum number of watts required to produce 1600 lumens. Using Jason and Sergei's answers (and Tom Murphy's paper already linked ...

4

To back up John Rennie's answer, consider the Bremsstrahlung formula for velocity perpendicular to acceleration: $P= {{q^2a^2\gamma^4}\over{6\pi\epsilon_0c^3}}$. For all practical purposes $\gamma=1$, so we can simplify this to $P\approx ({q \over \mathrm{C}})^2 ({a\over \mathrm{m/s^2}})^2 {1\over{18.85\times 8.85\times 10^{-12}\times 2.7\times 10^{25}}}\... 3 The blue end of the spectrum is higher frequency, meaning shorter wavelength. A cage that can block blue is going to block (technically attenuate) the given wavelength and any that are longer (lower frequency). If you start blocking at blue, you'll be blocking the entire visible spectrum and on down into the IR, microwave, and radio. To filter out UV, the ... 3 This is essentially the same as the narrowing of the light cones that happens as you approach the event horizon of a Schwarzschild black hole, and it occurs for the same reason i.e. the coordinate velocity of light tends to zero as you approach the horizon. There is nothing physically interesting in this. It is a result of the coordinates we are using. If ... 3 "Burning" effect happens when you get a sufficiently high power per unit area. So what you need is a source of power, and a way to get all that power on a small area. In good sunlight, you can use a magnifying glass to get this kind of burning effect quite readily (wear sunglasses - you will be looking at a very tiny hot image of the sun if you do this ... 3 If I understand your question right you want to know if there is a way to illuminate a room without being able to locate the source of light. This rules out usual sources like light bulbs, fluorescent lamps, candles, etc... I imagine that you have in mind a situation like in a computer graphics scene where you can see a room, even if no obvious light source ... 3 Let us look at coherent states $$|\alpha\rangle~=~e^{-|\alpha|^2/2}e^{\alpha a^\dagger}|0\rangle$$ $$e^{-|\alpha|^2/2}\sum_{n=0}^\infty \frac{(\alpha)^n (a^\dagger)^n}{n!}|0\rangle$$ If you have a classical system it means overlap between states is small. We then look at over lap$\langle\alpha'|\alpha\rangle\$ \langle\alpha'|\alpha\rangle~=~e^{-(|\...

3

You are correct that LEDs just produce a mixture of 3 wavelengths that our eyes and brains combine into one "color." However, that doesn't make it less real in all cases. There are some "colors" like pink which don't exist in the rainbow because they are actually caused by a mix of two different frequencies of light being seen at the same time (a bluish ...

2

Can Light Really be Matter? I suppose it depends on what you mean by "be". Matter will be created from light within a year, claim scientists It just is counter-intuitive to me. This is the fallacy of reasoning from personal incredulity. It isn't a good guide. I find most of QM and relativity counter-intuitive - it doesn't seem to be a simple ...

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