Well, corruption is better than crimson

It works because of triangles. Just start drawing some diagrams and you’ll quickly figure out how on your own. Which is more fun than having someone give you a geometric proof.

Try it. Look at a mirror and put a ruler on the mirror, the size of your face on the mirror will always be the same, no matter how far you are. It will be half the size of your actual face.

When you see your own face in the mirror, no matter how far you are, the portion of the mirror showing your face is always half the size of your actual face.