# Tag Info

47

Individual photons are very small and don't have much energy. If you put a lot of them together in one place you can hurt somebody - by simply supplying enough power to melt an object (ask any spy on a table underneath a laser beam). There is another very odd feature of photons. Although lots of them can provide a lot of energy and heat an object, it takes ...

18

I have a somewhat non-physics answer for you. If you allow me to broaden your question a bit to "why doesn't light kill or otherwise make all life on Earth impossible" the answer is that the Earth is in what we call "the habitable zone". If the Sun produced so much light or light at such high energies that it would kill you, it also would heat the planet ...

10

This question is more interesting than I thought at first. I like it. There are several different parts to an answer to this question; I'll just contribute a couple that have something in common: our bodies (and everything else, it has nothing to do with bodies) also emit photons about as fast as they absorb them. On the macroscopic/thermal scale, we have ...

10

A general photon isn't too dangerous. Most photons that we encounter have the power to heat our bodies and not much else. The heat we absorb from photons daily isn't that much, so this is rarely a problem. Now, an interesting thing about photons is that two photons of a lower energy do not make a single photon of higher energy (frequency). So a million ...

7

Light rays are only a good way to describe light in the limit of very short wavelengths, as compared to all other length scales in the problem. This is called the geometric-optics limit, and there one can solve the Maxwell equations in what's called the eikonal approximation to obtain Fermat's Principle and thus a light-ray description of light. The ...

7

One way to think of a "moving shadow" is by following the last photon that was allowed through. In that case, the speed of a shadow is exactly the speed of light. On the other hand, you could also define the speed of a shadow as the speed of the boundary between dark and light. In that case there is no thing that's actually moving, so there's no bound on ...

6

Fundamentally, there are three processes that can affect a beam of light passing through a medium: absorption, emission, and stimulated emission. Emission is independent of the beam of preexisting photons, so we neglect it here. On the other hand, both absorption and stimulated emission (the latter by virtue of quantum mechanical photon statistics) are ...

4

I'm sure Emilio Pisanty's answer is fine, +1, but it goes a little over my head. It also appeals to specific properties of electromagnetic waves, whereas the ray approximation is much more general than that. Here's a simpler plausiblity argument that may be more at the level that the OP can understand. If you diffract a wave through a slit of width $w$, you ...

4

It's possible to see the wavelengths beyond the red using non-linear optics: Second harmonic generation. Basically two photons combine to form a twice more energetic photon. Green laser works this way. The opposite of this is the Spontaneous parametric down-conversion, to see beyond the blue. This involves non-linear optics again. In this process one photon ...

3

There is no magic here. To see the laser beam, the beam has to use light in the visible frequency and some of the light needs to enter your eyes. Unless the beam is pointing at your eyes you won't see it. Remember, in coherent light all the photons are traveling in essentially the same direction, with the same frequency, in the same phase. If you want to ...

3

Photons don't have mass, but they do have energy and momentum. And since they can be absorbed or reflected, they can transfer their momentum to whatever it is that reflects or absorbs. The amount of energy is proportional to the frequency $\nu$ of the light: $E = h\, \nu$, where $h$ is Planck's constant. The momentum is $p = h\, nu / c$, in whatever ...

2

When two or more sources of light combine incoherently (not in any fixed phase relation), you can only "add", and that's intensity (power) - electromagnetic field "squared" and averaged, in the appropriate sense. When you have control over phase relations between two beams, yeah, sure you "add" the two at some point. To "subtract" all you need to do is ...

2

In general light propagates in a straight line in situations where its wavelength is much smaller than the other linear dimensions of the problem. As light in the visible range has wavelengths of about a half micron, this covers most everyday circumstances, but there are exceptions. (For example, shine a laser pointer on a human hair in a dark room and ...

2

Colour theory has a lot to do with how the brain processes the signals from the retina, as well as the physics of how light is detected in the eyes. But broadly speaking, the additive and subtractive properties of colour result from the physics of light and its interaction with pigments, so if we were tetrachromatic we would experience them similarly. The ...

2

The light you see as the image of the Sun on the sky is basically undeflected. http://en.wikipedia.org/wiki/Diffuse_sky_radiation says it is 75 % when the Sun is high and the sky is clear. The frequency dependency is due to Rayleigh scattering. For the cloudy sky the fraction is much smaller, up to many orders smaller than unity (maybe 1 millionth part as a ...

1

Regarding your question about changing direction: If no light can traverse the atmosphere without interacting with air, then what fraction of it reaches the ground without significant changes to its direction Remember that air has an index of refraction different than the vacuum of space (about 1.00027 for air versus 1 for space). The index of ...

1

One visible photon has a ridiculous amount of energy to harm us: about $2\times10^{-19}$ Joules. That's about 50 000 000 000 000 000 times smaller than the energy of a raindrop falling on your head (0.01 Joules). But in one second, a sunlight beam of the size of a raindrop sends $10^{17}$ photons which makes it about as powerful as a raindrop. A sunlight ...

1

The interference patterns of light show the wave nature of light. Optical interference between two point sources for different wavelengths and source separations The photoelectric effect shows the particle nature of light, because light hits an electron and transfers its energy to the electron. This link gives an clear explanation and can be a ...

1

First, there's no perfect reflector nor absorber. In fact - even Aluminium does absorb some radiation (by which it gets heated, can be noticed at incident high frequency radiation). One more thing is that aluminium foils are designed in a way to reflect light. Here's the Wiki article quote... Aluminium foil has a shiny side and a matte side. The shiny ...

1

A lens with spherical surfaces (i.e. surfaces that are a segment of a sphere) does not focus all parallel light rays onto a single point. Instead there is a focal surface called a caustic. This causes spherical aberation. As long as the lens curvature is small, i.e. the ray is near to the optical axis (paraxial rays), the aberation is small and we ...

1

No mirror can be perfectly reflective due to quantum tunneling so that already answers your question. But even if it could be done, you would never be able to check the situation because when you look inside, the light almost instantly leaves through the peephole. This also poses a problem for your initiation method, which John M already touched on: you need ...

1

You can subtract two beams if they have a well defined relative phase to each other using an interferometer (destructive interference). Of course, energy must be conserved, so the beams will constructively interfere somewhere else (usually a second port on the interferometer). If you don't have a well defined phase relation, then no, two beams of the the ...

1

A piece of white paper is the most common device to show a laser beam. The beam is visible on both sides of the paper. Fix it on a electronically controlled motor shaft you can rotate the paper into the laser beam. If precise control is necessary: just align an iris aperture to the center of the laser beam.

1

The metal's threshold wavelength is a wavelength of light. So yes, you would use a chart converting wavelengths of light to the color to identify it. For some metals, the threshold wavelength is not visible light; it might be ultraviolet. But whatever chart you're using would identify the wavelength you have as either ultraviolet or visible, and which color ...

1

Assuming mirror to be spherical section. C is the center of sphere. See, Using trigonometry. $$x=d \times \sin(2\theta)$$ $$x=R\times\sin\theta$$ Eliminate $\theta$ and get $d$ : distance from Center of curvature as a function of $x$. Verify for small theta where $\sin\theta\approx\theta$ If you just want to see that which side ray bents then see. ...

1

A parabolic mirror is a special case of a concave mirror. The rays at the rim are refracted to the focal point. This is true in simplification of geometric optics and perfect manufactured mirror. However in modern techiques like injection molding there are more imperfections at the rims of mirrors. The question of a general concave mirror can be answered ...

1

What is the point of batteries? Obviously higher W will drain them faster. If question is wattage -> luminocity function then I think that there will be two components. One is that every watt is converted into a lumen (683 lm/W, to be exact according to Wikipedia). It is like $E=mc^2$: more mass = more energy. They are equivalent. So more power <=> more ...

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