It is often stated that because the energy in an em wave is $E=hf$, the energy comes only as multiples of $h$, ie quantized. But we know that $f$ is a real number, and you could have fractions of one Hertz. The difference in energy of two such waves can be a fraction of $h$ in this case. Is this a correct thinking. I can accept that absorption and emission of energy is quantized, but this is not the same thing- as in this case the quantization is a property of the material not the wave.