I have been working with equations throughout the semester and using them to find cutoff frequencies and such, but when you say something like TE10, TE20, TE21, etc., I am realizing that I am having trouble getting a physical idea of what exactly these things are. Of course if you transmit at a frequency below the cutoff frequency of TE21, it will not propagate, but maybe TE10 modes can. But this is what confuses me. Let's say you are transmitting at a frequency greater than the TE21 cutoff frequency. That means you can have TE10 & TE21, among other possibilities. It seems to be in this case that both of these modes are transmitting a beam of light with the same wavelength. If that is correct, then what exactly is different about these modes? The QM I remember makes me want to say that one is in an excited state, but I wouldn't know what that really means, even if true. So what different properties would two waves (of the same wavelength, if that is true) from different modes have exactly? Finally, I am struggling to see how this applies to multimode fibers too. I heard that you can separate different modes at the end of a fiber with a grating, which makes sense if the wavelengths are indeed different. So if different modes = different wavelengths, well I am not exactly sure how these different modes run in different wavelengths anyhow... Thanks for any of these concepts you might be able to elucidate.