An Attempt to Explain Cyclops' Optic Blasts' Properties
Cyclops' optic blasts for me are a mystery that has been explained in part by the Marvel Universe. So what kind of beams are they? Provided that Marvel Comics is fiction, sometimes you have purposely written bad science, there was a description handed over by the 1983 Marvel Handbook that Cyclops' eyes are gateways to some kind of dimension, and that his optic blasts are not of this dimension.
So Cyclops' beams are said to be concussive force with no recoil and almost no heat. Though they emit light, they apparently possess no heat (or not) but has been seen to be able to weld objects, suggesting that it does contain some heat if narrowed down. In spite of its power, wearing the ruby-quartz glasses blocks the light from Cyclops' eyes, which causes him not to release the optic blasts. His combat visor however allows him to control the degree of the optic blasts with varying height and intensity. He can use it to merely strike a melee blast or it can go as far as to reach 2 gigawatts of energy. In some cases, his optic beam can be reflected on shiny surfaces though at times, the blasts won't bounce off them but rather puncture them like it would do on other objects. The intensity is variable because at one time, it was able to hurt the Blob and Mr. Sinister.
What can be seen is also that Cyclops' optic blasts are a result of his body involuntary absorbing the ambient energy of his surroundings which are stored into his cells. Using the optic blasts can drain him of his body's energies, which in turn he needs to be recharged. So it would be a disadvantage if he stayed too long in dark areas. Also he is immune to his brother Havok's powers but he was at one point, recharged by the latter.
Is Mr. Sinister truly weak against it? Now just thinking about it, Mr. Sinister is not totally vulnerable to it. Even after Mr. Sinister was destroyed by Cyclops' super charged optic blast, he was more or less still able to reassemble himself. In a sense, Cyclops was only able to hurt Mr. Sinister but never fully destroy him. Remember Cyclops was able to hurt Blob's skin, so it also hurts Mr. Sinister. Yet Cyclops' rivalry with Mr. Sinister is far unevenly matched. Mr. Sinister still has a huge advantage over Cyclops in terms of mutant powers though as he later gained immunity from it. =P