# Tag Info

178

Your brain adjusts your perception of color to compensate for lighting that is strongly tinted. This was the reason for the violent conflict some time back about a certain dress. Depending on whether people perceived the dress was being lit by yellow-tinted or blue-tinted light, they saw either a black and blue dress or a white and gold dress. Here's an ...

78

Your misunderstanding is very common and quite easy to make. Basically, what students are usually introduced to first is the thermodynamics of ideal monoatomic gasses. This is good because it is simple and easy to understand, but can be problematic because features specific to the simple substance can be misunderstood as general features of all substances. ...

59

Well, the answer you usually get is half right. They do disappear (more on this in a second). I'd hesitate to say they turn into "heat energy," both because we don't use the term "heat" that way in a technical sense and because most of the time we like to talk about atoms absorbing photons. In this case the energy of the photon becomes potential energy of ...

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(photograph credit: Efram Goldberg) [Note: left-most ampule is cooled to -196°C and covered by a white layer of frost.] $NO_2$ is a good example of a colorful gas. $N_2O_4$ (colorless) exists in equillibrium with $NO_2$. At lower temperature (left in Wikipedia photo), $N_2O_4$ is favored, while at higher temperature $NO_2$ is favored. For a gas to have ...

49

[Already said] A blackboard is not porous, i.e. it actually never takes up much water from the sponge in the first place (and if you were to squeeze out more than a little, it would just run down to the bottom). [Already said] Yet the surface is hydrophilic, i.e. the water that does stay on the board forms a very thin film instead of droplets (as you'd get ...

41

First of all, gas molecules are not invisible. There are plenty of elements whose gaseous state is quite colored, but these (iodine, e.g.) are in such rare amounts in the atmosphere that the net effect is not discernable to the eye. Next, if you Google for "atmospheric transmission curves," you'll see all sorts of spectral absorption going on, again at ...

25

Introduction The transfer of momentum gets included properly when one incorporates the motion centre-of-mass $\mathbf R$ of the atom as a dynamical variable. Performing the dipole approximation allows one to treat all the electrons as interacting with some field at the centre of the atom, $\mathbf F(\mathbf R,t)$, but now $\mathbf R$ is an operator on the ...

24

When you turn on a lightbulb, you easily create many photons. They can go away just as easily. That's because they are bosons and they have no charge. Think of waves on a pond. Where do they "come from" when you throw a stone in? Where do they go when they dissipate? That's actually a very good analogy in some ways because the math that describes ...

24

You don't. You actually hear the high frequency notes from headphones. The bass really doesn't travel at all well, but the attack noise from the drum or bass guitar is what leaks from headphones. This is why on the tube you hear "tsss tsss tsss tsss" and very little else. From @leftaroundabout's answer on the post that valerio92 linked: Normal ...

21

One way is through the Zeeman effect. The presence of a magnetic field in the region where the absorption line originates can split the energy levels of an atom into multiple components. Transitions between these split energy states then lead to absorption lines with multiple components at separate wavelengths. The number of components and the size of the ...

19

As has been said by many answers; all gases aren't colourless, for example chlorine gas is a pale yellow; which is a good thing as it's very dangerous. So the gases in our atmosphere are colourless. But this is completely the wrong way round to look at it. If our eyes operated at frequencies that were blocked by gases in the atmosphere they wouldn't work ...

19

You have to distinguish, which interactions take place, when electromagnetic radiation passes through a solid and interacts with it. There is a nice plot on Wikipedia, showing the dielectric response of solids for different wavelengths/frequencies. Basically, as the frequency gets higher, the wavelength becomes shorter, and the molecules or atoms are no ...

19

Although flat the blackboard has a texture so that when a damp cloth is rubbed across it the water adheres to the board and isolated droplets are not formed. So you have a fairly uniform thin film of water across the board which is ideal for evaporation.

18

The other answers cover almost everything, but I would like to add that at any temperature above absolute zero, there is a degree of line broadening caused by Doppler shift: some of the atoms are moving towards you, and others away, and that will mean that in your reference frame they can absorb many different frequencies of light. This is important in ...

15

Some gases actually are visible (nitrogen dioxide for instance). The air is invisible, because its molecules don't absorb the visible light. These molecules simply don't have useful vibration modes available to absorb these wavelengths, or the electrons in their orbitals can't utilize the frequencies of visible light to move to higher orbital (the energy ...

15

The wavelengths that stimulate vitamin D production are between 280nm and 320nm, which is called UVB. You would need to use a detector capable of measuring light in this wavelength. However there is no need, because normal windows are made from soda-lime glass and this transmits no wavelengths shorter than about 350nm. Some Googling will find you the ...

13

Light is composed out of a large ensemble of photons, and photons are quantum mechanical elementary particles. Matter is composed out of atoms and molecules , which have small dimensions and are in the quantum mechanical range. The quantum mechanical "size of interaction region" is given by the Heisenberg uncertainty relation. Even though a photon is an ...

12

You must distinguish between reflection versus absorbtion-and-emission. When incident light $I$ hits a surface, some portion is reflected $R$ and some is absorbed $A$: $$I=R+A$$ The reflected portion $R$ of the incoming light is spreading out depending on the surface roughness and texture. A rough surface reflects scattered light while a very plane and ...

12

Rob Jeffries already has an excellent answer. I shall just add an image and some numbers to that answer. The spectrum in the presence of magnetic field will split certain absorption lines. The splitting $\Delta$ is proportional to the magnetic field strength. This splitting depends on the magnetic moment of the parent atom which is generally of the order of \$...

11

Dale and Arpad already gave great answers, but I want to correct something that you said that also contributes to your confustion: transmission occurs when the energy of an incident photon does not correspond to any electron's energy transition within the material. Therefore, the photon does not interact with the atoms / electrons and is transmitted ...

11

Seeing thin absorption lines is difficult. You need pretty good equipment to see them over an extended body. If you're just looking at it with a prism, it will overlap enough that such lines are obscured and the spectrum appears continuous. We describe sunlight as a continuous spectrum even though there are absorption lines from elements in places above ...

10

According to Bohr model, the absorption and emission lines should be infinitely narrow, because there is only one discrete value for the energy. There are few mechanism on broadening the line width - natural line width, Lorentz pressure broadening, Doppler broadening, Stark and Zeeman broadening etc. Only the first one isn't described in Bohr theory - it's ...

10

This is in the x-ray region and beyond. The wavelength of the light is smaller than the size of the electron orbitals, and decreasing when the photon energy goes up. When the electric field oscillates a lot on the length scale of the wave function, positive and negative contributions to the integrals in the transition to the excited state (the ...

10

I think the blackboards absorb very little if any water and, being smooth and vertical, there is just not that much water there after they've been wiped. Also, the blackboards I'm most familiar with are made of painted steel. Probably their good thermal conductivity helps keep them at room temperature and not cool much as the water evaporates.

10

As already explained in other answers, it is about perception, not physics. If you take a piece of white paper outdoors on a clear sunny day and look at it, it looks white. If you photograph it with an old-fashioned film camera, the paper will appear white in the finished print. If you repeat the same exercise indoors let's say at night with only ...

9

Radiation can be several things, but since you specifically mentioned lead shielding, let's look at X-rays - a lot of what you learn applies to other radiation as well. To stop radiation it needs to interact with "something" that makes it give up its energy and momentum. This is how you get the radiation to stop going in the direction it was going. Now X-...

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How are photons created? An accelerating charged particle generates photons tangentially as well as a decelarating one. Where do these photons come from? From the energy carried by the electron. In this sense photons are just a packet of energy which is associated with the electromagnetic field. This type of interactions of electrons and ions with fields ...

9

The answer is yes, the atom does absorb radiation that does not exactly match the transistion frequency. This is due to the Doppler effect that everyone knows from an ambulance with siren driving by. The frequency you hear is higher if the ambulance moves towards you and lower if it drives away from you. It's the same with the atom. If the atom moves (and ...

9

It is called dark matter because the equations of motion of galaxies as seen with our instruments cannot be explained unless there is extra mass than derived from the luminous mass seen. There are also other observations that require this extra dark mass. There are two main proposals for what this mass is composed of, MACHOS and WIMPS. MACHOS : A ...

8

Basically, absorption lines exist because absorbed photon are not re-emitted in the same direction, so dark lines can be observed. There are various reason causing this. For example, the extra energy can be dissipated as phonon in solid or strongly interacting system. Excited states can also emit multiple low frequency photon if there are meta-stable states....

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