Why does an object immersed in water look less shiny than in air? I was surprised to learn today that objects look less shiny when immersed in water. Presumably it's because of the different index of refraction of the transparent medium, but why?

Source: https://twitter.com/3Dmattias/status/1134454234466922496
 A: This problem is needs a little bit more explanation. I will give you a:
I am mostly going to talk about visible wavelength photons, because that is what you are asking about.
1.
Classical answer, where the IOR for air is 1.0, and for water is 1.33, and the apple's is somewhere around 1.5. Now classically, what happens is that the light from the lamp above your head on the ceiling (I am assuming that is the sharp reflection on the apple), gets easily reflected by the apple's tip that is closest to your eyes, and the intensity of photons is so high, that it creates a almost mirror image of the lightsource, a yellowish reflection on the center of the apple. The light from the lamp hits directly the apple, and gets reflected directly into your eyes when in air. Air molecules will scatter off and deter very little ratio of the original photons from the lamp. That is why you get a high intensity reflection directly from the apple in air. Now in water, the water molecules will scatter off way more photons in different directions, and the intensity decreases and you will not get a sharp high intensity reflection of the original lightsource. Please see here: https://www.mdpi.com/2304-6732/3/3/50/pdf
If you submerge a mirror into water, you will still see a mirror image of yourself. The mirror's lattice structure does not change, and it will still work as a mirror even under water. This effect of submerging does not change the reflection structure of the material at all. What does change, is the intensity of the direct light into your eyes. Because water scatters off way more photons on the way through the water and backwards too. These scattered off photons will have an altered trajectory and will not add to the high intensity direct reflection of the lightsource.
What is very important to understand is that the shiny center area on the apple in air, is not a mirror like specular reflection. It is a high intensity, diffuse reflection of a 
lightsource directly into your eyes. So the apple is not so much shiny as a mirror, it does not create a mirror image like a metal, but it will only create a diffuse high intensity reflection.
2.
QM explanation, needs more explanation. When photons hit the surface atoms, they do:


*

*elastic scattering, the photon keeps its energy and phase

*inelastic scattering, the photon keeps part of its energy and phase

*absorption, the photon gives all its energy to the atom
As per QM, all three happen always, but the ratio is different.
Now with a mirror, it is mostly 1., elastic scattering. The ratio of inelastic scattering, and absorption is very little.
But in your case, with the apple, it is mostly 3., absorption, and some more 2. inelastic scattering, and very little 1., elastic scattering.
Now it is because the apple has a different lattice structure from a really shiny metal, and the apple's surface's lattice atoms will mostly absorb the photons, and re-emit them at certain wavelengths (this by the way creates the color of the apple).
Now when you have a direct high intensity lightsource directed at the apple, those photons mostly will be absorbed by the atoms in the surface of the apple, and get re-emitted. But as per QM, some of them will be reflected (elastic scattering), creating an ever increasing shiny area (that grows with the intensity of the lightsource), and some will be inelastically scattered (transforming into the vibrational energies of the inside of the apple, heating it up).
As you increase the intensity:


*

*the number of photons will increase that will be absorbed by the surface atoms, and re-emitted directly into your eyes

*the number of photons elastically scattered (reflection) will grow too, creating an ever growing intensity shiny area in the center of the apple's surface, where you will see this diffuse reflection

*the number of photons inelastically scattered will grow too, heating up the inside of the apple
Now it is very important to understand that what you see as a shiny reflection is not a mirror like specular reflection. A specular reflection is where the energy and phase and relative angle of the photons is kept, you will get a mirror image. Diffuse reflection is where the photons will scatter off in random angles.
With this diffuse reflection, you will see a high intensity, shiny area, but the original lightsource is not recognizable. It is not a mirror image. 
So in your case, when in air, the apple will have this diffuse reflection of shiny area at the center of the surface.
It is very important to understand that this shiny reflection grows with intensity. If you could increase the intensity of the light, without burning the apple, you would get an ever increasing intensity shiny reflection.
So in your case, it is very important to understand that this apple will never be able to create a specular, mirror like reflection. What you see is just a diffuse, high intensity reflection directly from the lightsource. Some of these reflected photon in random angles will get into your eyes. As you increase the intensity of the light, as per QM, more will get randomly into your eyes, you will see an ever more shiny area.
Why it is important? Because that is the answer to your question why it does not happen in water.
As soon as you submerge the apple into water, the diffuse reflection becomes dimmed. Water molecules' atoms will scatter off the direct light from the source, and very little direct light will reach the surface center area of the apple. Most of the light that reaches the apple will zig zag through the water. 
Now out of the photons that do reach the apple directly from the source, some will be (very little) elastically scattered, creating a diffuse shiny reflection, and most of these will be scattered off on the way back in water, so that they will have an altered trajectory (zig zag back) and will never reach your eyes.
If you use a very shiny, high intensity spotlight, and move it close to the apple, the apple might have some little shiny area even under water.
A: The matter that reflects the light has an index of refraction similar to that of water. Light reflects much less at the interface water to apple.than at air to apple. This type of phenomena is well described by Fresnel's equations. 
A: If the reflection is definitely getting dimmer it might be because the refractive index of water is greater than that of air.
https://en.wikipedia.org/wiki/Fresnel_equations
Read about the Fresnel Equations and how the intensity of the reflected wave changes with the ratio of refractive index of media. (I think the refractive index of apple is a bit more than water but still quite close? It scatters a lot so it's hard to intuit the exact number). It'd make sense that an apple in water would have a dimmer reflection. And an apple in vacuum should be even shinier.
A: There are multiple effects. Water will absorb light on the way to the apple and light reflected from the apple, making the reflection dimmer. Another effect is what you mention, scattering of light because of the different trajectories as light leaves the apple. 
Water might also effect the surface of immersed objects distorting the surface's reflective properties so more light is absorbed and reflections are more diffuse. 
