185
votes
Accepted
Why does a yellow object turn white under a yellow light? Shouldn't it turn yellow instead?
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 ...
- 24.7k
86
votes
Accepted
How can an object absorb so many wavelengths, if their energies must match an energy level transition of an electron?
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 ...
- 109k
63
votes
Why is water a good neutron absorber?
Water is useful for neutron shielding, even though water is not an especially good neutron absorber. Oxygen nuclei are basically invisible to neutrons, since oxygen-16 is a spinless doubly-magic ...
rob♦
- 94.2k
55
votes
Accepted
Why can't sunlight reach the very deep parts of an ocean?
Light is an electromagnetic wave and when it passes through (cold) matter it can be absorbed via reaction with the electrons in the atoms that make up the matter. Typically, the absorption is an all-...
- 3,321
49
votes
Accepted
Why does a blackboard dry very quickly?
[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 ...
- 15k
23
votes
Why is water a good neutron absorber?
Water is not a good neutron absorber compared to other materials. Water is a good neutron moderator; water slows neutrons down due to scattering collisions. Once slowed, the cross section for ...
- 9,441
21
votes
Accepted
How does a spectral line tell us about the magnetic field of a star?
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 ...
- 136k
19
votes
Why does a blackboard dry very quickly?
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 ...
- 99.9k
19
votes
Why can't sunlight reach the very deep parts of an ocean?
Consider that a cloud is made up of transparent water droplets. When light interacts with a water droplet, it changes direction without being absorbed. However, the interaction with very many such ...
rob♦
- 94.2k
18
votes
How can an object absorb so many wavelengths, if their energies must match an energy level transition of an electron?
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 ...
- 539
17
votes
Aren't places where geodesics end singularities?
Do not confuse geodesics (the abstract geometrical things) with the paths of particles. Particles follow geodesics, but the fact that a particle is created at some point and destroyed at another doesn'...
- 10.7k
15
votes
Why can Einstein coefficients be derived based on thermodynamically equilibrium relations when they are basically intrinsic?
How can it be that intrinsic microscopic properties
(which might be calculated by quantum mechanics on n-level systems
including radiation fields) can be derived so "easily" based
just on ...
- 41k
15
votes
Aren't places where geodesics end singularities?
This doesn't seem to be so because geodesics are an intrinsic property of space time and not simply the path of a light ray or quantum. Indeed we might say that the light is no longer traveling along ...
- 4,609
13
votes
Why does a yellow object turn white under a yellow light? Shouldn't it turn yellow instead?
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 ...
- 4,154
12
votes
Accepted
Why do colours of object change due to incident light?
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 ...
- 52.3k
12
votes
How does a spectral line tell us about the magnetic field of a star?
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 ...
- 9,463
12
votes
Why are spectrums of incandescent light bulbs continuous despite the presence of Argon around them?
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 ...
- 42.3k
12
votes
Accepted
What is cross section?
"Who cares"? You care. You are shooting at a swarm of bees of known number density n and depth d (the foil thickness) trying to infer the size (surface area) of each combined with your ...
- 66.3k
12
votes
Accepted
Could the solar shield on the James Webb telescope have been pitch black or does it need to be highly reflective?
Emissivity is equal to absorptivity at a given wavelength. But the emissivity/absorptivity of any surface does vary with the wavelength of the light in question. In this case, the Webb telescope is ...
- 51.6k
11
votes
Why don't absorption and emission lines cancel out in our Sun?
The key point missing from most efforts to answer this question are that the Sun has a temperature gradient with depth. If it were (somehow) isothermal, then indeed the absorption and emission ...
- 136k
11
votes
How can an object absorb so many wavelengths, if their energies must match an energy level transition of an electron?
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 ...
- 4,200
11
votes
Why is water a good neutron absorber?
Water is a good moderator (good at slowing neutron) because of the two hydrogen atoms. Recall that, in the collision of two billiard balls where the target is initially at rest, the incident ...
- 47.8k
10
votes
Why does a blackboard dry very quickly?
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 ...
- 8,920
10
votes
Accepted
Sunscreen protection, SPF, and amount applied: relationship?
A sunscreen's sun protection factor (SPF) is defined as $$\frac{\text{minimal erythemal dose in sunscreen-protected skin}}{\text{minimal erythemal dose in non-sunscreen-protected skin}},$$ so its ...
- 907
9
votes
Absorption of dark matter by black holes
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 ...
- 235k
9
votes
Accepted
Is a detailed absorption spectrum available for carbon dioxide from 300-1100nm?
Here are the absorption spectra of some atmospheric gases
for wavelengths from $0.1\ \mu$m to $100.0\ \mu$m.
(image from "Cold Facts on Global Warming" by T.J.Nelson)
Especially you can see,...
- 41k
9
votes
Why can't sunlight reach the very deep parts of an ocean?
Photons get absorbed on their way down, due to the finite interaction cross section $\sigma\;[cm^2/g]$ in a material of density $n\; [g/cm^3]$. The photon mean-free path is then $l=1/(n\sigma)$.
This ...
- 1,701
8
votes
Accepted
Why don't absorption and emission lines cancel out in our Sun?
I think that this is a very good question.
In my answer I will only mention the formation of one of the absorption lines, the 589 nm of sodium, and I will call the photon associated with that ...
- 99.9k
8
votes
Why do colours of object change due to incident light?
In an ideal situation, i.e. if
really all colors except one are absorbed
your red light is perfect (one wavelength)
the objects should actually appear black, i.e. no light is reflected.
In the ...
- 4,322
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