Consider a shiny block of metal, and a dull block of concrete, both in direct sunlight. The metal will reflect more of the sunlight than the concrete, due to its reflective surface. However, when touching the two blocks, the metal will feel hotter. How is it that the metal is absorbing more energy from the sun, even though it is reflecting more back?
[Physics] Why does metal simultaneously reflect light and heat up
energymaterial-sciencethermodynamicsvisible-light
Related Solutions
First, there's no perfect reflector nor absorber. In fact - even Aluminium does absorb some radiation (by which it gets heated, can be noticed at incident high frequency radiation). One more thing is that aluminium foils are designed in a way to reflect light.
Here's the Wiki article quote...
Aluminium foil has a shiny side and a matte side. The shiny side is produced when the aluminium is rolled during the final pass. It is difficult to produce rollers with a gap fine enough to cope with the foil gauge, therefore, for the final pass, two sheets are rolled at the same time, doubling the thickness of the gauge at entry to the rollers. When the sheets are later separated, the inside surface is dull, and the outside surface is shiny. This difference in the finish has led to the perception that favouring a side has an effect when cooking. While many believe that the different properties keep heat out when wrapped with the shiny finish facing out, and keep heat in with the shiny finish facing inwards, the actual difference is imperceptible without instrumentation. The reflectivity of bright aluminium foil is 88% while dull embossed foil is about 80%.
The shiny and not-shiny surfaces are totally a favor of production technology (credit goes to the rollers). Now, to the "why" question.
As a physics parameter, we use reflectivity to address the shininess. As we can see, the reflectivity is quite high for the bright surface, compared to dull one. The unreflected light (as you say) can go anywhere. It can go inside the aluminium foil (i.e) it's absorbed and hence the 12% & 20% loss...
Response to comment (based on edit): That's a nice strange idea. With some perfection (I mean, there should be very less allowance of any sorta radiation inside), the room will be relatively cooler. But, in reality (where we can't expect idealistic things), there will always be some radiation inside. But, it keeps the room warmer compared to the outside. But, always be careful when playing with such things because, any sort of harmful radiation (if any- what about a heater or even an electric iron?) inside the room will be reflected back to you by the matte side - which can be very harmful...
Because radiated heat is heat transferred by light (that is, radiation). The paint is in physical contact with the radiator, so the paint gets heating by conduction, and the paint then cools by radiating heat (as well as a little conduction with the air, and a lot of convection).
Unless the radiator is getting hot enough to glow in optical wavelengths the optical color doesn't matter. If it is hot enough to glow, then black will perform better than white because the ability to emit radiation is the same, from a physics perspective, as the ability to absorb it. If the radiator isn't that hot, then what matters is the "color" in wavelengths you can't see, and shiny metal is good at reflecting light in many wavelengths, so is bad at absorbing/emitting light.
Best Answer
The feeling depends on three factors:
What happens is that once your finger is on the material, heat is exchanged. The molecules at the surface of your finger are warming up, the molecules at the surface of the material are cooling down until both materials have the same temperature.
Now stone has a smaller specific heat, in fact six times less than water (the main material your finger consists of). The contact area of your finger and the material are obviously the same. But that means for every degree your finger molecules are warmed the stone must lose six degrees, so even if we start with the same temperature, water will always feel hotter. It also explains why water burns are so dangerous: Water can transmit much more heat to your body until the temperature comes down to safe levels.
The second thing is thermal conductivity. I can in fact touch the hot surface of a tile stove without burning myself. Why ? The thing is that heat is continously replenished or dissipated from the surrounding molecules of the contact surface, but the speed depends on conductivity. Touching a 200°C tile will cool it down from my 37°C finger to somewhat 70°C (remember the 1:6 factor), but stone has abysmal conductivity, so the stone molecules heat is not replenished, but my finger's are dissipated. Not so fast, but still more than the stone, so the resulting temperature will be something like 50-55 °C. (In fact, those purists which use cooled "whiskey stones" because they do not want water dilution by ice are deluding themselves; the cubes are not working). Metals on the other hand have a very good conductivity, so energy is easily replenished. I can therefore touch a very, very thin metal plate, but touching a 200°C thick plate with the same temperature as the stone tile will result in severe burns. So metal feels hotter (and colder) than other materials.
The last important thing is temperature. The more temperature, the hotter, but the equilibrium temperature of materials in sun under the same conditions depends on their color: Black materials have higher surface temperatures than bright materials. Unpolished metal should have higher reflectance than concrete and should be cooler, but the two other factors are dominant.