Ultraviolet Rays
noun
The purple outside line is the color spectrum that's outside purple. Those are ultraviolet rays (the lines are the rays).
This is a jukugo word that uses the on'yomi readings of the kanji. You didn't learn the on’yomi reading of the 紫 kanji, so here's a mnemonic to help you remember:
Do you know what animal has the best protection from ultraviolet rays? Sheep (し), obviously. The thick fur of wool is the best UV protection. I mean, you've never seen sheep tanned and sunburned, right?
あんたはまずその紫外線恐怖症を克服しなきゃいけないね。
You must first conquer your fear of the ultraviolet rays.