Does light heat transparent mediums? I was thinking that if a photon is travelling in the vacuum at $c$, then enters a transparent medium so it's speed becomes $0.8c$, the photon has lost energy. What is that energy transformed into? Is the surface being heated?
 A: The energy carried by light is entirely independent of the speed it travels at - it is simply given by the light's intensity, which is equal to the photon flux (number of photons carried per second) times the photon energy (equal to Planck's constant times the frequency). If the light is slowed down in a medium, then at constant energy flux the local energy density will increase (the energy is 'squeezed together', if you will), but that is all that happens. 
In a transparent medium, no light is absorbed and the intensity does not change: the photon flux remains constant, and the frequency is not altered. Thus, there is no energy loss, either by the light beam as a whole nor from the individual photons that comprise it.
A: No, Light doesn't imparts heat to the transparent medium. As far as the decrease in speed of light is concerned, energy of the 'visible light waves' depends upon the frequency of light not on  speed. As the reduction in speed is nullified by the reduction in wavelength. (ref. Einstein's Photoelectric Equation)
Note: Light here means Visible Light, i.e around 400-800 nm wavelength.
A: Let’s have a closer view about the medium.


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*Semi-transparent medium


If one say that glass for example is a transparent medium, this is a simplification. Take a lot of glass panes together and the loss of light is obvious. Some photons of the ingoing light interact with the atoms of the medium and get converted at the end into photons of lower frequency. The momentum transfer to the atoms increases their kinetic energy. They vibrate stronger, which is nothing else as the increase of the temperature of the medium.
An astronaut could see the earths surface from 100 km away. The atmosphere has to be transparent. This is a simplification again. From your point of view the sky is blue and in the afterglow it is red. So some part of the light from the sun gets blocked in the atmosphere and some part gets converted into longer wavelengths.
There is a technology of industrial imaging, where behind an object a flashlight pulses light and a highspeed infrared camera makes a lot of images. In dependence from the material the IR light pulse gos through the material more or less fast and from this can be concluded for example about blowholes. For our question it is interesting that the photons that are ingoing into the object are not the outgoing photons. The atoms of the object absorb the photons and the re-re-....-re-emission is coming out on the camera side. To talk in this case about c is a simplification again. The measured speed of the light through the object is a statistical value from all the delays during absorptions and emissions and the flight of the photons between the atoms.
Conclusion 1: In a semi-transparent medium light heats the medium.
To draw a fuller picture one remember the laser cooling. So a medium can be cooled by light!
Conclusion 2: Sometimes in a semi-transparent medium light cool down this medium. 


*Transparent medium


Transparent media do not exist. But let us - only theoretical - suppose that the photons get not absorbed inside the medium. For example let us suppose that the electric and magnetic field components of the photon interact with this fields of the electrons of the medium. Like a swing-by (Gravity assist). The photon does not transfer energy at the end to the electrons but is wiggling or zigzagging through the medium. The path is not a straight anymore and the speed of light inside the medium becomes lower c (compared to the straight path).

if a photon ... enters a transparent medium so it's speed becomes 0.8c,  the photon has lost energy

As you could see for the example above this is not the case.
