When you are moving forward, even if the rain is coming straight down, you are riding into the droplets after they pass the brim of the hat. Large raindrops have a terminal velocity of about 20 mph. Say you are riding 20 mph and there is no other wind factor. Further assume the brim of the cap is 4 inches above the bottom of your glasses. You would need the brim to be 4 inches long for no rain to get on your glasses...roughly. The slower you ride the shorter the brim could be. The faster you ride or more wind from the front, the longer the brim would need to be. In short, you are not likely to keep your glasses perfectly dry. And slightly wet is as bad as soaked for vision.