Comment by LeoPanthera

That's true! To be more specific, the British had just invented the ground interception radar to spot incoming aircraft, and didn't want the Germans to think they had developed a technological solution.

I don't know if they ever thought the myth would catch on as well as it did, but it's still widely believed today. (Perhaps because there's no downside, eating carrots is still good for you, it just doesn't improve your eyesight.)