This is a very fundamental question. I apologize for its simplicity. I did searches on the web but could not find a clear explanation. Whenever I read about the principles of quantum physics, I always come across a statement along the lines of: "electromagnetic energy could be emitted only in “quantized” form (i.e. restricted to discrete values rather than to a continuous set of values)" This is my confusion: If a photon could have any frequency, than the packets of energy could have any infinite number of values. Does the quantum theory imply that photons can only have certain frequency and not other? I would like to have a basic understanding of how the quantization is established. Feel free to forward me to texts online if you do not feel this question is worthy of your time. Thank you!