If you're starting at the lowest frequency that our eyes can see, you're at RED light.
Then, as you hike through higher and higher frequencies, you travel through orange light, yellow light, green light, blue light, violet light.
Suddenly, as you keep going to higher frequency, you can't see the light any more. You've crossed the line from violet to ULTRAviolet. Now your eyes don't know it's there. The frequency is too high. (Wavelength is too short.)
