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Why are some things darker when wet?

aryankashyap.com

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Re: Why are some things darker when wet?

#11

Post by Khoth »

k__ wrote: post #6
Zenst wrote: post #5

Short version: It changes the refraction index and less photons bounce of in the direction of the observer, so it looks darker.

How does it manage to direct the photons away from the observer? I mean, it obviously doesn't know where they stand.

The actual short version is: diffuse reflection from the surface means that some of the light leaving the surface is at angles which makes it get totally internally reflected at the boundary between the water film and air, giving it another chance to be absorbed by the surface

Re: Why are some things darker when wet?

#12

Post by iforgotpassword »

SamBam wrote: post #9

> Once you spill water on the shirt, that part of the shirt is now covered with a thin film of water. So, any light which has to reflect off that part of the shirt has to go through water. > Before water is spilt, 100% of the light travelling towards that part of the shirt will hit the surface. But now only a fraction of the light moving towards it will hit its surface. This is because the light now has a layer of wa…

Yes, wouldn't that only apply to soaking wet cloth? If it's just a little wet it still looks darker but not shiny, and there is no specific angle you can look at it and see the light source reflected. So it appears to still be diffuse in that state. Where does the light go?

Re: Why are some things darker when wet?

#13

Post by Asterate »

SamBam wrote: post #9

> Once you spill water on the shirt, that part of the shirt is now covered with a thin film of water. So, any light which has to reflect off that part of the shirt has to go through water. > Before water is spilt, 100% of the light travelling towards that part of the shirt will hit the surface. But now only a fraction of the light moving towards it will hit its surface. This is because the light now has a layer of wa…

Refractive index of a material is the ratio between speed of light in vacuum and speed of light in that material. Light tends to bounce back when encountered with a sharp change in refractive index. Being wet means that there's a water film covering the material, mediating the change in refractive index, resulting in reduced reflection.

Apart from index mediation, the water film does something else. For rough/fibrous surfaces, the reflection will be diffuse, i.e. visible from all directions. When a water film is present, the surface becomes smooth, and the reflection will be specular, and only visible in one direction. So in most directions, the material will appear darker.

Conductors are a completely different beast. The reflection off of metals are not solely dictated by the refractive index.

Re: Why are some things darker when wet?

#14

Post by [deleted] »

tartoran wrote: post #10
fbelzile wrote: post #8

Wow, is this the same reason why you're more prone to sunburn if you have water on you or are in a swimming pool? Does the light refracted under the surface of the water bounce around and hit your skin multiple times?

Yes, the water drops on the surface of the skin act as mini lens.

[deleted]

Re: Why are some things darker when wet?

#15

Post by RealityVoid »

tartoran wrote: post #10
fbelzile wrote: post #8

Wow, is this the same reason why you're more prone to sunburn if you have water on you or are in a swimming pool? Does the light refracted under the surface of the water bounce around and hit your skin multiple times?

Yes, the water drops on the surface of the skin act as mini lens.

I am unconvinced by this argument. After all, the same light flux is hitting your body. Only thing a lens would do is make it focus in different spots than it otherwise would, but then you'd get spots with burns and spots without burns. This is not what I see happening at all.

Re: Why are some things darker when wet?

#16

Post by mannykannot »

Start with a dry surface and consider the cone of light, scattered from a point on it, that will pass through your iris. If, as shown in the final diagram, the water, when added, forms a flat surface, and you are looking at it fairly perpendicularly, then none of the rays that are totally internally reflected would have been in that cone and ended up entering your eye anyway - they are too oblique. They do not contribute to the brightness either before or after the water is added.

On the other hand, when water is added, the above cone of light is broadened, on account of refraction, so it is no longer the case that all the light within it will enter your eye.

These considerations must be modified (I am not sure how) if the water forms a thin, capilliary-adhering film that conforms to the roughness of the substrate.

Finally, if the substrate is transparent or translucent (e.g. sand (quartz crystals)), the change of refractive index at its surface is lessened in the presence of water, causing more transmission / absorbtion and less reflection / scattering there.

Re: Why are some things darker when wet?

#17

Post by aryankashyap »

SamBam wrote: post #9

> Once you spill water on the shirt, that part of the shirt is now covered with a thin film of water. So, any light which has to reflect off that part of the shirt has to go through water. > Before water is spilt, 100% of the light travelling towards that part of the shirt will hit the surface. But now only a fraction of the light moving towards it will hit its surface. This is because the light now has a layer of wa…

Thanks for your feedback. Will make the changes shortly!

Re: Why are some things darker when wet?

#18

Post by aphextron »

This is a bit hand-wavey in its' description of light reflecting. One of the most mind blowing bits of physics I've ever learned is that photons do not actually "bounce" off of a surface like little balls. They are always absorbed and re-emitted. The actual photons that hit an object are not the same ones that eventually enter your eyes. The atoms in an object are stimulated by photons hitting them to then emit a new photon via the photo-electric effect, which we perceive as light "bouncing".

Re: Why are some things darker when wet?

#20

Post by aryankashyap »

aphextron wrote: post #18

This is a bit hand-wavey in its' description of light reflecting. One of the most mind blowing bits of physics I've ever learned is that photons do not actually "bounce" off of a surface like little balls. They are always absorbed and re-emitted. The actual photons that hit an object are not the same ones that eventually enter your eyes. The atoms in an object are stimulated by photons hitting them to then emit a new…

Could you please elaborate on how it's hand-wavey. Happy to take constructive criticism and improve :)
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