I was starting to type up the next Laser Smackdown entry, when it occurred to me that this was a good point to talk about a neat little thing from optics. It further occurred to me that this would be a good poll/quiz topic, to see what people think before I give you the real answer.
So, here’s the scenario: You have a brightly illuminated object, the canonical example being a vertical arrow (it could also be a shadow cast in a beam of light). You take a lens, and put it in front of the arrow, projecting an image of the arrow on a screen, like so:
As you can see from the picture, the image that is formed on the screen is inverted– the arrow points down, rather than up.
Now, imagine taking a card (the black rectangle below the lens) and positioning it right in front of the lens, so it blocks the lower half of the lens. What happens to the image?
This is a classical problem, so you have to pick one and only one answer. You can easily check your answer, if you have access to a flashlight and a magnifying glass, but please don’t do that until after you enter your answer, just so we can see what people arrive at from pure reason.