# Tag Info

46

Refraction of light in water droplets, leading to the formation of rainbows, is not limited to the visible range. Experimental evidence, compelling due to its simplicity, is shown in the following images taken by University of College London Earth Sciences professor Dominic Fortes. Check the alignment of the rainbow with respect to the trees in each of the ...

40

I'll do that teacher thing and turn your question around back at you. Why isn't the spectrum of the lithium atom just the spectrum of the hydrogen atom plus the spectrum of the helium atom? And, for that matter, why is the helium spectrum not simply two copies, somehow, of the hydrogen spectrum? Why do atoms have unique spectra in the first place? The ...

33

Air is normally a bad conductor of electricity, but with enough voltage it can be converted to plasma, which is a good conductor. In a plasma, the electrons constantly bind to and leave atoms. Each time an electron binds to an atom, it emits the energy in light. As a result, the plasma glows the color of a photon with that energy. There are a few different ...

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The deexcitation of nitrogen and oxygen, the primary components of air, are that of blue/purple. See http://en.wikipedia.org/wiki/Ionized-air_glow for pictures of nitrogen and oxygen in gas discharge tubes.

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The problem with the suggestion of using polarization is that you now have the reflections off the polarizers to contend with. I think the short answer is "it depends on how 'black' you want it to be". "Truly black" = reflectance of 0. I am quite sure that is impossible - there will always be some probability of light scattering off a surface. All you can ...

19

Depending on the angle of the sun, the shadow becomes elongated so that it traces out an ellipse rather than a circle. This means that the shadows of the blades must traverse a different distance across the ground but within the same time period, thus giving rise to the periodic variation in velocity. If the sun is at an angle $\alpha$ in the sky (see the ...

10

Molecules aren't just sums over their constituent atoms. There's many different kinds of bonds which involve different patterns in the overlap of electron orbitals, and which affect the energy levels those electrons can occupy - I'm assuming the QP video you watched explained how "color" relates to electron energy levels. The (hydrogen-like-)atom case is ...

9

engineer already answered it completely, I only want to add that the question is completely valid even if you already know that separation of wavelength occurs. The thing is, some materials are practically opaque or too much transparent (refractive index is equal to that of air and no separation occurs) in infrared and ultraviolet while transparent in the ...

8

The answer is that electrical excitation of air molecules is able to produce lots of excited singly ionised nitrogen ions. The electronic structure of singly ionised nitrogen has a number of allowed radiative transitions, where the outer excited valence electrons can rearrange themselves into lower energy configurations. The most prominent turn out to be ...

6

Is it possible that rainbows have ultraviolet bands and infra red bands and we are not able to see? Yes, see engineer's answer. As for whether we can see them, take a look at aphakia: "Aphakic people are reported to be able to see ultraviolet wavelengths (400–300 nm) that are normally excluded by the lens. They perceive this light as whitish blue or whitish ...

5

A force is defined as a change in momentum over time. In the Newtonian limit, this means a mass times an acceleration. But when dealing with things like photons, the formal definition is applied. Photons have no mass, only momentum. Therefore, if a force is applied to them, their momentum can be changed. This can happen in two important ways, a force can ...

5

They taught me that in high school too (i.e., that matter is "mostly empty space.") Only thing is, it's not true. Solid matter is mostly filled with electrons. Yeah, the mass is all concentrated in the relatively tiny nucleii, but the mass is not what photons interact with, and the mass is not what defines the physical and chemical properties of ordinary ...

4

The answer is that color is determined by electron transitions between different energy states. Those levels are different in molecules than they are in the component atoms where there is only a central force in atoms, whereas the multiple positive charges in molecules creates a more complex potential field for the electrons to move around within. Molecules ...

4

Newcomers to relativity tend to regard concepts like time dilation and Lorentz contraction as somehow fundamental concepts from which relativity it derived, but this is not the case. Time dilation arises because the integral of proper time along the worldline of some object will not necessarily match the coordinate time measured by a distant observer. So to ...

4

The only thing I can think of being true black would probably be a black hole. As light does not bounce off a black hole.

3

The water droplets that create a rainbow are not emitting the light that you see in a rainbow; if they were, you would see a glowing cloud of consistent color, not a rainbow. The rainbow is formed by sunlight refracting and reflecting through water droplets in the air; the water refracts through the "front" of the drop, reflects off the "back," and refracts ...

3

First of all I think a suggestion is mandatory: please don't mix units of measure. You are using lux and lumens, so it's better if you stick to SI units. So distances are measured in meters, areas in square meters and angles in radians. This is meant to help you, not to annoy. ;) Lux measure how much light (lumens) hits a square meter of surface. So if you ...

3

The spectrum of the black body radiation is given by Planck's law. The total amount of radiation is given by the Stefan-Boltzmann law. In principle there is no shortest wavelength, because the radiated intensity remains non-zero at arbitrarily small wavelengths. However, at the low wavelength end of the spectrum the radiated intensity falls exponentially ...

3

Light moves at about a foot per nanosecond, or a meter every three nanoseconds. In order to capture it propagating across a room over a few frames, you would need to gather something like a billion frames per second. No consumer camera -- indeed no camera on Earth -- is capable of this. Now there have been people playing with "fempto-photography," but they ...

3

Classically (since rob has done a thorough job on the quantum picture), the amplitude of a light wave is not related to any physical extent. It is not the size of the wave in space, it is the strength of the fields (electric and magnetic). We often draw wavy lines, but if you look closely the transverse axes will be label differently for, say, waves on a ...

2

Not quite that simple because of spectrum analysis. A light source is never "pure". So an orange (type-K) sun would make our colors here different, but bluish would still exist because of the range of wavelengths of the source. But it would be less common.

2

If you apply a constant voltage to a piece of semiconductor, you will not get light out of it. All that will happen is that the electrons that are already in the conduction band will start drifting in the applied electric field, so you will get conduction and some heating. In order to get effective recombination of electrons and holes (i.e. atoms that lack ...

2

There are three factors that need to be considered across all wavelengths: (1) the ability of the water droplet to refract and disperse the incoming light, (2) the ability of the eye to sense the wavelength, and (3) the ability of air to transmit it. The visible range we 'see' in a rainbow with our eyes satisfies all three. UV , depending on how short the ...

2

Simply put, relativistic speeds cause for events previously thought of as simultaneous to no longer be simultaneous if the velocity of the reference frame of the event changes relative to the defined observer. The best way to wrap your head around this is to pictorially trace what is happening in space time. The case you describe is v>0 Think of v in ...

2

Coherency of light in practice is not an either/or issue. Any light due to any source has some degree of coherence. Laser light has usually much higher coherence than light of a hot metal filament. Some degree of coherence means, in simple wording, that light waves at one point of space due to different parts of the source behave similarly (they have ...

2

If you twisted my arm and forced me to assign an amplitude to a single photon, I'd do it this way: The energy density of a classical electromagnetic field is \begin{align} U &= \frac12 \left( \epsilon_0 E^2 + \frac1{\mu_0} B^2 \right) \\ &= \epsilon_0 E^2 &\text{(only for light in a vacuum)} \end{align} where $E,B$ are the amplitudes of the ...

2

There are two ways to look at light, classical and quantum mechanical. Electromagnetic waves given by the classical solutions of Maxwell's equations will have interference patterns as predicted mathematically from the sinusoid form of the solutions. Are we working in the double slit argumentation with destructive interference arguments too? Young has ...

2

Light is a wave , an electromagnetic wave classically. When leaving a point source a wave expands isotropically in angle, spherically, and its intensity falls like 1/r^2 where r is the distance from the source If a point source lamp is set in a room, it will illuminate spherically all of it, as happens with the lamps hanging from the roof. A flash light ...

1

Have a look at the question How does a one-sided glass work?. The effect is due to the limited ability of the human eye to handle large contrast ranges and not to the effect of light intensity on the reflectivity of the glass. So without replacing the human eye with something else there is no way to achieve what you're asking.

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This is I guess a common problem that stems from the way we use the word "intensity". Normally, we would use intensity to mean something like the energy going through a unit area per unit time. In the case of the photoelectric effect, we instead generally mean the number of photons (per unit time), as it is this that decides how many photo-electrons are ...

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