My apologies if I am posting this in the wrong forum, and if this question has already been asked. This is my first post. First off: I'm a first-year undergraduate student. My program is Philosophy of Physics. I am trying to understand (more fully) black body radiation. I am having trouble understanding the experiment involving a cavity with a small hole in it. From what I understand, a cavity with a small hole in it can be approximated as a black body. Photons enter the the hole and become trapped in the cavity, and all their energy is absorbed. The cavity will eventually reach "thermal equilibrium" with the radiation, which means that the amount of radiation absorbed is equal to the amount of radiation emitted. (Please, correct me if I'm wrong.) There are several things about this experiment that I don't understand: 1) The Ultraviolet Catastrophe: I don't understand this at all, to be honest. Here is what I think: EM radiation with a very high frequency will have a very high energy. When this kind of radiation enters the cavity, such as ultraviolet light, the amount of energy absorbed will be very high and consequently the amount of energy emitted will be infinite. Like I said, I'm not sure if what I'm saying makes sense. Also, what does this have to do with "the number of modes that can fit inside the cavity"? What does that even mean? 2) Thermal Radiation: The energy that is emitted is in the form of EM radiation. But what does this resulting energy have to do with the original energy absorbed in the form of photons? If, for example, a "red" photon entered the cavity, what would happen to the energy it was carrying? Does it necessarily have to be re-emitted as a red photon? 3) What is the relationship between energy and radiation? For example, let's take a microwave. Why makes the microwaves "intense" enough to heat the food? Is it because there are MANY photons? Because we are exposed to micro waves all the time, but we aren't cooked. What makes a microwave oven different? 4) What does it mean for EM radiation to have a "temperature"? I know that heat can be transferred through a vacuum as radiation. But the actual radiation does not really have a temperature, does it? It is only when it hits something that is able to detect it that we can measure its temperature. Is that correct? Thanks in advance for your help. I'm trying my best to understand this stuff, but it's very confusing to me.