# Tag Info

44

When objects are very small, every source of illumination will appear to be "extended" - which softens the shadows and makes it harder to see the contours of the surface. By painting highlights and shadows, you reduce the impact of the extended source. See for example Why don't fluorescent lights produce shadows?

27

In real life: A sunset is "redder" than a sunrise which makes people feel more romantic. It's mostly because the atmosphere is warmer in the evening (no pollution here, lemon, the Earth is warmer in the evening because it was naturally warmed up during the day). However, there's also a very small contribution of the Doppler shift, one that you could in ...

24

A quick look at the Del Sol How it Works page shows their explanation, HOW DOES IT WORK? The Spectrachrome® crystal reveals color upon irradiation by ultraviolet waves; i.e., sunlight. When a flower blooms, the result is the exposure of the inherent color of the flower. A Spectrachrome® crystal is similar in that an energy-shift occurs causing the ...

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The famous equation $E = mc^2$ is actually just a special case of the relativistic equation for the total energy: $$E^2 = p^2c^2 + m^2c^4 \tag{1}$$ where $p$ is the relativistic momentum and $m$ is the (constant) rest mass: $$p = \frac{mv}{\sqrt{1 - v^2/c^2}}$$ For an object that isn't moving $p=0$ and equation (1) becomes: $$E = mc^2$$ which is ...

14

As many have said, the inverse square law applies to point-sources. These are idealized light sources which are sufficiently small compared to the rest of the geometry that their size is of no importance. If a light source is larger, it is typically modeled as a collection of idealized light sources, potentially using integration. The exact definition of ...

14

There is an explanation on the Del Sol web site but it omits the technical details. This is probably because Del Sol regard it as commercially valuable confidential intellectual property, and I have to concede that they are probably correct. Anyhow, some Googling has turned up a suggestion for how it works but there is no proof for this idea so treat it as ...

13

does this equation mean masses are just condensed energy? No, it means that mass is just another form of energy, just like heat, motion, electric attraction, etc. For example, the energy of a charged sphere is $$E=\frac{3}{5}\frac{Q^2}{R}$$ This equation doesn't mean that charge is just condensed energy; it means that charged objects have energy. ...

12

The inverse square law applies to point sources. For extended sources becomes accurate at distances that are large compared to the size of the source. At large distances the source looks like a point. What "large" means depend on the application. In the case of light fixtures, the Illuminating Engineering Society and other organizations have made ...

11

The most direct answer I found is that N2 and O2 are very simple molecules. 2 atoms, tightly bound, no angles. Source From Source: Because the nitrogen gas molecule is so simple, it cannot do very much with the light energy that it absorbs. It can spin or vibrate only a little bit by stretching and pulling. Oxygen acts pretty much the same ...

6

The inverse square law applies to point sources. A real emergency light is not a point source, and therefore the law appears to not apply at close distances, because any real point is at a varying distance from different parts of the emergency light.

6

To discriminate between the other two answers. Lifetime spectroscopy would be capable of distinguishing between photochromism (in this case, reversible ultraviolet-switchable reflectance, example: hexaarylbiimidazole where the transition time is milliseconds to seconds) and ultraviolet fluorescence (where the "transition time" is zero, although the excited ...

5

You need to mix them incoherently. If the beams are from different sources, then you're pretty much there, as they won't stay in phase. You can't represent incoherent light with Jones vectors (2 components), but you can do it with density matrices (this approach has a huge importance in quantum mechanics, where incoherent mixtures are most important). So, ...

5

Quantum mechanics says that only very specific wavelengths are acceptable for exciting an atom, wavelengths that are not those (most of the spectrum) passes by the atom without interacting much (just some scattering in the case of visible light). The allowed wavelengths are the ones that have energy extremely close to the energy difference between two ...

4

Classical electromagnetism is perfectly compatible with special relativity. In classical E&M, light is an electromagnetic wave and there is generally no useful formulation in terms of particles. The most widely used technique to combine quantum mechanics with special relativity is relativistic quantum field theory. The relativistic QFT that ...

4

Short answer, the sun isn't on fire. Flames can flicker with wind or with pockets of fuel that might collect and burn in spurts or bits of water that can steam when the flame touches them, causing movement, or due to small bits of turbulence as the heat expands away from the flame. The visible surface of the sun is much more like a hot iron that has a ...

4

I believe the dyes behave similar to glow in the dark materials; They fluoresce a certain wavelength over time after being radiated. In the case of these shirts you'll have different chemicals for different colours, but which are all "activated" by UV radiation. There is a pretty nice explanation on Wikipedia, but basically this: You can excite an ...

3

Yes, it would, though not as quickly as if you were getting the full spectrum of sunlight. All frequencies of the light spectrum carry energy, so it becomes a question of how much of that energy is absorbed by the house. For example, if your house was completely black, all that visible light energy would be absorbed by the house and converted into heat. If ...

3

No, each color in the spectrum has a characteristic frequency. Every light source has a so called spectrum of frequencies. The relative intensity of these frequencies determines what color you see (or not). For example, the sun looks yellow because it's peak intensity is in the yellow wavelength. White light comes from a source consisting of a very broad ...

3

The first thing to note is that each of the slits produces a diffraction pattern the width of which is controlled by the width of the slit and the wavelength of the light. The amount of light travelling from a slit in a particular direction is controlled by the diffraction pattern due to a single slit. The light waves from each of the slits superpose ...

3

The inverse square law says that the intensity of incident light falls off in proportion to the inverse of the square of the distance from the light source. The important word here is "the distance" — the inverse square law implicitly assumes that all parts of the light source are at the same distance from the measurement point, or at least ...

2

This question raises several concerns. From a purely signal processing point-of-view, where the aforementioned pulse is not a photon, but rather a time-domain pulse of a certain duration (here less than 10^-100 seconds)--then NO: you cannot have pulses shorter than the period of wave in question--as the wave an be defined. Consider sound: when a 440Hz A note ...

2

This is caused by chromatic aberration in the eye: short waves (blue) are refracted more (by the cornea and the crystalline lens) than long waves (red light). This isn't visible in normal conditions, but can be observed using your method. A good overview of the eye can be found in this ref., see page 49 for chromatic aberration. Because of it, a ...

2

Cort and Ilmari have given good answers about the practical issue: the inverse square law is for point sources, and so a non-point source (like an emergency light) will only appear to have the same properties at some minimum distance that depends on the geometry of the real source. However, it seems nobody has mentioned a different "minimum distance" that ...

2

The quote from the reference says it all: (I added caps) "The minimum test distance IN PHOTOMETRY of these sources is called the 'minimum inverse-square distance.'" The minimum distance is therefore a photometry issue, in other words, a measurement problem. The essence of the measurement problem is how far away you have to be before you can approximate the ...

2

The microscopic mechanism of emitting photon in a solid is the transformation of kinetic energy of atoms into EM energy. If an atom is in an excited state due to collisions among other atoms, then it will emit photon when it jumps into the ground state, and the energy of the photon is  E=\varepsilon(\text{excited state})-\varepsilon(\text{ground ...

2

Given the context of the question, the fact that it seems to be about $E=mc^2$ specifically, and that the OP says he's having a hard time understanding it, i'm going to try and give a simple answer in plain english without a load more complicated formulae. I am no physicist, and although the concept may not be that easy, the formula is pretty simple, maybe ...

2

I will try to answer this question with my basic understanding of special relativity: Is matter condensed energy? It kind of is, but a better way to phrase it would be that everything that has energy, (behaves like it) has mass. Imagine you have a hollow box with the insides covered with perfect mirrors and you put it on a scale. If you shone a light ...

2

Interesting question. Taking a stab at it - not absolutely sure this is correct, but let the comments begin. In the frame of reference of Earth, the light travels straight out to the reflector, and straight back. You are asking about the case where an observer is in a reference frame that is moving with respect to Earth/moon, and the picture would have to ...

2

Agarose is a polysaccharide, and like all polysaccharides it is bristling with polar hydroxyl groups. This means the agarose chains interact very strongly with each other by hydrogen bonding. If the solution is heated to a high enough temperature to break the hydrogen bonds (around 90-100ºC) then the agarose molecules behave pretty much like any other ...

2

There is a phenomenon that generates light without generating heat as by-product*. The phenomenom is called Electroluminescence, and is used in LEDs which are used in many electric equipment (the tiny lights in your computer to let you know it's on). We can stretch it a bit more with Fluorescence and other forms of creating liminous paint Also if you will, ...

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