How is energy quantized? I'm at the moment studying quantum physics and I came across $$E=hf.$$ My question is: how come there are only certain energies allowed if in this formula $f$ can be any number, and therefore, energy could be any number? Are there only certain values of $f$ allowed?  What am I not understanding correctly?
Is the electromagnetic spectrum discrete? I'm just starting to learn physics and I have a question (that is probably stupid.)  I learned that energy levels that the bound electron can have are discrete. I also learned that when an electron transitions from one level to another, a photon with a specific wavelength (energy) is created and released into the wild.  My question is this: are there holes in the electromagnetic spectrum, values of frequency that the created photon can't possibly have?
Why do universal constants have the values they do? This is meant to be a generic question of the type that we get repeatedly on this site, in different versions:        Why do universal constants have the values they do? Can we predict their values theoretically? Do they change over time? How would the world be different if a particular constant had a different value?