# Tag Info

24

The Apollo 11 flag was included almost as an afterthought. It was just a month or so before liftoff, and someone at NASA slapped themselves on the head and said, "we need an American flag to plant at the landing site!" Someone rushed out to a local store (Sears?) and bought a standard nylon flag, which went to the Moon. Besides being bleached out by solar ...

15

White light does not have a single frequency, it is a mixture of all of the colors. Similarly pink does not have a single frequency it is a mixture of red with white i.e. white with extra red. If the color does not appear in the color spectrum (rainbow) then it is not a single frequency but a mixture of different frequencies. In an LED light is produced by ...

15

Yes, there are. Such materials are called "saturable absorbers," and are (or at least, have been) used as switches in some laser designs. The one I recall is a nickel acetate dye, although there are others. Basically, the molecules absorb single photons at the laser wavelength, but when the intensity is great enough that two photons are absorbed ...

5

$400$ - $700\text{ nm}$ corresponds to about $430$ - $750\text{ THz}$ ($10^{12}\text{ Hz}$), not $\text{MHz}$ ($10^6\text{ Hz}$). To convert from wavelength to frequency, use $$f = \dfrac{c}{\lambda},$$ where $\lambda$ is the wavelength, $f$ is the frequency and $c$ is the speed of light. So, for $400\text{ nm}$, this is:  f = ...

4

At its heart, an LED is a diode fabricated out of semiconducting materials. There are fixed bandgaps for a given material (or mixture of materials at a given ratio), so only certain energy states are available. This means only certain wavelengths of light can be generated, since wavelength is a function of photon energy. In theory, if you could design some ...

4

You are confusing additive and subtractive colour mixing. If you mix paints together you should get black, not white. In additive mixing (as used in TVs and monitors), you create light, which is then mixed. When you mix the three primary colours (red, green and blue), you produce white. Other mixes produce other colours, for example red and green combine ...

4

Sure, that is what the wavy motion over a hot car is, and also the cause of mirror-like mirages on deserts and hot roads. Stars also twinkle due to the same effect. Not sure about street lights per se, but I'm sure there are combinations where air refraction of light would show up there also.

4

I would make a flag from iron oxide (red), platinum (white), and lazurite (blue). It won't wave in the wind, but it will retain the color. The base would be a platinum plate, of course. I would made a really large one, so that people wouldn't complain that it was too cheap.

3

I'm not sure I've understood your question but I think you're asking if a big wave can have wave-features on its large features. If so, sure, why not? You can add waves of different frequencies to achieve results like:

3

The "color" of a body is not a property of the body nor of the light it reflects or emits, but rather of the human eye and brain that receive and process the light. Of course, this is based on a property of the light, for this is how the eye receives the information that there is some object in there in the first place. This property is the spectrum of the ...

3

The other answers to the effect that one needs big optics to see fine detail are indeed true for are true for conventional imaging optics that sense the electromagnetic farfield or radiative field i.e. that whose Fourier component at frequency $\omega$ can be represented as a linear superposition of plane waves with real-valued wave-vectors ...

3

Yes. Fluorescence absorbs a color (which we don't see) and emits it as another color (which we do see), so we see what wasn't absorbed plus what was emitted. Non-fluorescent absorbance just absorbs a color (which we don't see), so we just see what wasn't absorbed. Most physical objects exhibit non-fluorescent absorbance.

3

The answer is that we can't see a black hole, which is I would guess what you were leading up to. There are only two ways we can detect a black hole using light (of various wavelengths): if it occludes something behind it if it's surrounded by an accretion disk Point (1) is actually quite complicated because a black hole doesn't simply mask whatever is ...

3

This is simply the law of reflexion: if you trace a ray diagram for objects reflecting in your iPad screen, you are looking at light from a virtual image that is as far beyond your screen as the real source of light is from the iPad screen. So if the lamp is, say, 3 metres behind your shoulder, when you look at the iPad screen the reflected light is exactly ...

2

According to most sources such as this university page on albedo and this modeling paper on albedo versus wavelength, the typical albedo of snow is virtually 1 throughout the visible region, so it seems unlikely that it is the snow itself which is causing this effect. Also, I've never personally observed this as John Rennie stated, so perhaps it's caused ...

2

Actually, there is a type of LED that does directly produce white light. It was developed in the early days of blue LED development (mid 1980s) and consisted of a diode in zener breakdown and a blue lens. A diode in breakdown produces, for lack of a better word, light noise (white noise = white light). I doubt that these type of white LEDs are available ...

2

This article has some relevant results based on a study of bird plumage (it also happens to be cited in the abstract of the Nature paper mentioned in one of the other answers), and is summarized in simpler terms here. I'll attempt to summarize the summary. Black and fluffy/loose fitting clothing is best if it is hot out and there is any (\$>3 ...

2

This is effectively just a diffraction problem. I.E. you get out a diffraction pattern on the left similar to the one you expect to get on the right in the first case, and one of the peaks will be at the original light source. Why will we get multiple peaks instead of just one corresponding to our original light source? In the case of the first ...

2

This sound is most likely caused by the choke coil which is inside the lamps housing. It is needed for lamp starting and operation. Starting works like this: After initially the starter circuit allows for current flow through the heaters in the tube, it interrupts the the current after an initial period. This causes a high voltage impulse to be created by ...

2

This is due to Rayleigh scattering, i.e. the shorter wavelengths (those near the violet end of the visible spectrum) are more deflected by dust particles than the longer wavelengths (those near the red end of the visible spectrum). When the sun is near the horizon, it the path of the light through the atmosphere (and in particular through layers which have ...

2

The white colour of clouds is due to Mie scattering. This arises because the refractive index of water is different from air. If the particles are large enough all wavelengths are scattered equally so there is no change in the colour of the scattered light. This is the case in clouds where droplet sizes are typically 10 to 20 microns. The light scattering ...

2

Imagine each torch gives off marbles instead of light. And instead of a light detector we use a cup. In this model, the number of marbles entering the cup per second is like brightness. If you have only one torch on, only marbles coming from that torch enter the cup. Now turn on the second torch. There will be more marbles hitting the cup every second. The ...

2

The wavelength of light, and for any wave in general, is measured along the direction of propagation. It has every bit of the intuitive meaning that the wavelength of a water surface wave does. One of the most meaningful ways to visualize light is as an oscillation of the electric and magnetic fields over space and time: (Image source) The electric ...

2

All materials emit thermal radiation (such as light). The hotter the material, the more the radiation is shifted to high frequencies (shorter wavelengths). The radiation comes from oscillating electrons (regardless of whether there is an electric current). Welding reaches temperatures high enough to cause significant emission of UV light. Oxyacetylene and ...

2

You are right that the sun appears to emit a continuous spectrum, and that's due to the fact that it's a black body to reasonable approximation. We don't even have to consider the spectral response of our eyes, or the color mixing in our brains. Measurements with 'cameras' of one type or another are routinely made to produce plots like this. Discrete ...

1

Optical systems not involving magnetic fields are symmetric. So, if the display passes light in one direction, it will pass light in the other. Putting a mirror at the back of the TFT and lighting it from the front is therefore equivalent, expect that some light will be attenuated on the way in as pointed out by @CarlWitthoft in the comments. As a ...

1

A couple things: this site is not specifically for engineering questions, so there's a small chance this may be closed by mods. Second, many commercial high-power LED's are listed not by luminous flux, but by nominal diode power consumption; a 1 watt LED actually emits far less than 1 watt of radiant power, so you'll need to think about what radiant flux ...

1

If we define "pure white light" as the combination of all the frequencies if the visible spectrum, then the possibility of one diode generating it, is practically nil. If we define "white light" as the combination of 3 frequencies (red, green, & blue), then we can generate it with 3 small and "closely spaced" LEDs with the appropriate color and amount. ...

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