optics - Why do wet objects become darker?

When something gets wet, it usually appears darker. This can be observed with soil, sand, cloth, paper, concrete, bricks...

What is the reason for this? How does water soaking into the material change its optical properties?

Answer

When you look at a surface like sand, bricks, etc, the light you are seeing is reflected by diffuse reflection.

With a flat surface like a mirror, light falling on the surface is reflected back at the same angle it hit the surface (specular reflection) and you see a mirror image of the light falling on the surface. However a material like sand is basically lots of small grains of glass, and light is reflected at all the surfaces of the grains. The result is that the light falling on the sand gets reflected back in effectively random directions and the reflected light just looks white.

The reflection comes from the refractive index mismatch at the boundary between between air $\left(n = 1.004\right)$ and sand $\left(n \approx 1.54\right)$. Light is reflected from any refractive index change. So suppose you filled the spaces between the sand grains with a liquid of refractive index $1.54$. If you did this there would no longer be a refractive index change when light crossed the boundary between the liquid and the sand, so no light would be reflected. The result would be that the sand/liquid would be transparent.

And this is the reason behind the darkening you see when you add water to sand. The refractive index of water $\left(n = 1.33\right)$ is less than sand, so you still get some reflection. However the reflection from a water/sand boundary is a lot less than from an air/sand boundary because the refractive index change is less. The reason that sand gets darker when you add water to it is simply that there is a lot less light reflected.

The same applies to brick, cloth, etc. If you look at a lot of material close up you find they're actually transparent. For example cloth is made from cotton or man made fibres, and if you look at a single fibre under a microscope you'll find you can see through it. The reason the materials are opaque is purely down to reflection at the air/material boundaries.

general relativity - Surely space-time Curvature does not explain gravity, it just describe its effects?

In special relativity co-moving objects see the other's 4-velocity as being only temporal.

When they move relative to each other they see the other's 4-velocity has rotated so that it points less in the temporal direction but now has a spatial component.

By the equivalence principle two co-moving objects falling toward a planet see each other's 4-velocity as only temporal in their own (falling) rest frame so they must think the space between them is attached to their rest frame over time. It thus seems that space has the same rotation of its own 4-vector over time (up to a constant if the objects started with a fixed velocity before falling) But surely space does not fall. Also if space-time curvature causes objects to fall, how? I'd have thought it's just a map of how objects move. not a cause of that motion, but if it does cause falling , how? Space isn't moving so as to push or rotate mater. Surely it's curvature it's just a map of the rotations in (light and) matter's 4-vector? How does something about the mass energy tensor alter geodesics or 4-velocity vectors? I see no explanation of gravity in GR merely a more detailed description of the motions it effects.

newtonian mechanics - Is the Moon in a "Freefall" Around the Earth?

The force of gravity keeps our Moon in orbit around Earth. Is it correct to say that the Moon is in “free fall” around Earth? Why or why not?

I think the answer is yes. The moon is falling towards the Earth due to gravity; but, it's also orbiting the Earth as fast as it's falling towards it. This balance between the 2 forces means the moon is essentially "freefalling" towards the Earth. Is my thinking correct? Thanks.

visible light - Why isn't it allowed to use a flash when taking pictures in a certain place?

When I go to, for example, a museum I try to take some pictures.

Sometimes the museum staffs forbid me to use a flash. Do you know the reason? I don't think it is related to photo-electric effect, right?

Answer

From Amateur Photographers in Art Galleries: Assessing the harm done by flash photography. by Martin H. Evans:

The flash built into a digital compact has, typically, a GN value of about 6 to 9 (though some manufacturers are rather coy about revealing the GN rating). If one extends the calculations used by Saunders and by Evans to these little units, then one finds that if one of these is fired at full power at about 2.5 metres (ca 8 feet), it exposes the object to about the same quantity of light as that falling on it every one-eighth of a second in a 200 lux (ca 18.6 footcandle) gallery, or every half second in a dark 50 lux (ca 4.6 footcandle) gallery. Is it worth getting steamed up about such a tiny extra quantity of light, as far as pigment fading is concerned? Several photographers have already suggested that any trifling damage done by a few hundred of these little flashes in a day could be fully offset by closing the gallery and turning off the lights a few minutes early. A ban would be justified in rare cases, where large numbers of photographers might be taking many flash photographs very close to something that could reasonably be considered photosensitive.

So it appears to me the main reason for the ban is not related to the photoelectric effect.

supersymmetry - Why should SUSY be expected naturally?

In the last 40 years (approximately) people have been "discovering", "rediscovering" and "studying" SUSY as a powerful tool and "symmetry principle".

Question:

What if SUSY is not realized in Nature at the end? Is SUSY the only path to "relate" fermions and bosons or what else? Remark: SUSY has not been discovered yet, so keep you totally conservative. What if there is no SUSY?

Bonus:

What are the merits of SUSY? What are its main issues? I do know some answers to this, but I think it could very enlightening if we "listed" pros and contras of current supersymmetric theories to see where we are NOW.

cosmology - How do scientists calculate the percentage of dark energy in the universe?

I can understand how the percentage of dark matter compared to ordinary matter is calculated, because the amount of dark matter has a clear gravitational effect on the ordinary matter in a Galaxy.

However, calculating the percentage of dark energy in the universe seems less obvious. Is it something to do with the rate of expansion of space-time?

Please explain in lay-mans terms, I've never learnt any undergraduate level cosmology.

Answer

There are (at least) four ways in which the dark energy content of the universe influences things we can observe

1. 
   

   The cosmic microwave background is formed in the early universe when atoms (of hydrogen) first formed and the universe became transparent to the radiation that was within it. There are small fluctuations in the CMB which reflect small differences in the density of regions within the universe at the time of this "decoupling" of radiation and matter. How big (in terms of an angle on the sky) these regions now look depends on the subsequent rate of expansion of space and this in turn depends on both the amount of gravitating matter (which slows the initial expansion) and the amount of dark energy (which accelerates the expansion). Hence, in broad terms, the angular size of fluctuations (about 1 degree) in the CMB allows one to infer the amount of dark energy, though it is mixed up in what the other cosmological parameters are (including the total amount of gravitating matter density).

   
   


2. 
   

   The expansion of the universe can also be tracked using standard candles. That is we can measure the brightness of something, infer how far away it must be and then measure how fast it is receding away from us. In a decelerating universe, containing only gravitating matter, then the expansion rate would have been much larger in the past and would be witnessed in the recession velocities of more distant objects that are seen as they were in the distant past. Observations of type Ia supernovae - standard candles arising from the explosions of white dwarfs at nearly a fixed mass and with an extremely consistent peak luminosity - confound this expectation. Instead it appears that the expansion of the universe is accelerating and this is attributed to dark energy. Again, this interpretation is not wholly independent of the other cosmological parameters.

   
   
   


3. 
   

   The large scale structure of the universe is dependent on both the amount of gravitating matter (especially dark matter, that was able to start clumping together even before normal matter and radiation became decoupled) and the amount of dark energy. Models of the how clusters of galaxies and filamentary structures in the universe form, suggest that the range and scale of structures we see in the universe today depends in detail on the nature of dark matter, but also on the nature of dark energy. Comparison of these models with observations leads to constraints on how much dark energy there must be.

   
   


4. 
   

   In the 80s and early 90s there was something of a crisis in cosmology. If one measure the current expansion rate of the universe and extrapolates back in time, one can calculate how long ago the big bang occurred. Performing this exercise assuming that the expansion of the universe was only decelerating with time , due to the gravitating matter within it, resulted in an age of about 10 billion years. However, the oldest stars in the universe seemed to be at least 12 billion years old! Dark energy resolves this problem by allowing the expansion to accelerate with time after an initial period gravitational deceleration. An extrapolation based on the current expansion rate thus leads to an age that is too young. Instead, a revised extrapolated age for the universe, including dark energy, is 13.7 billion years old and comfortably older than the oldest stars.

   
   

Putting all these things together leads to the so-called "concordance" or Lambda-CDM model for the universe (see image, adapted from Kowalski et al. 2008). This posits that 68% of the total energy density in the universe is in the form of "dark energy", which causes the expansion of the universe to accelerate.

water - Why does earth look blue from outer space?

I know it's more than 70% water. But what has it got to do with earth's colour ?

Answer

Quite a simple answer: Scattering of light (Rayleigh scattering would be more precise here...)

• An observer in ground sees the sky as blue due to scattering of light by air molecules present in the atmosphere. For an observer in space, The water bodies reflect the color of sky...

The water bodies (ocean, lakes, river) appear blue ('cause water is quite colorless) because of the way sunlight is selectively scattered as it goes through our atmosphere. Taking Raman effect into account, Water absorbs more of the red light in sunlight. By this way, water also enhances the scattering of blue light in the surroundings.

By Rayleigh scattering law: (It's more important here) The amount of scattering is inversely proportional to the fourth power of its wavelength. Due to the larger amount of $N_2$ and $O_2$ molecules (78% and 21%) in the atmosphere, blue light which is having shorter wavelength is scattered to a greater extent.

Thus, the earth wouldn't be blue if it doesn't have enough $O_2$ and $N_2$ molecules in its atmosphere. The scattering depends on the characteristics of gaseous molecules in atmosphere... This is applicable to other planets also. (like Mars appearing Red, Venus appears yellow, etc.)