Dr. Redish has observed people with cameras undergoing some strange behavior. (a

Question

Dr. Redish has observed people with cameras undergoing some strange behavior.

(a)When I was at a conference in North Carolina, one of the physics graduate students (who should have known better!) was trying to take a picture of the overheads projected on a white screen. Since the room was darkened, he used a flash. Explain why this is a bad idea and what his pictures are likely to show.

(b)I mentioned to him that he probably should not be using his flash so he turned it off. He then proceeded to try and take pictures of the participants in the darkened room! Explain why this is a bad idea and what his pictures are likely to show.

(c)I once observed a woman on an airplane at night with a camera. As we flew low over Washington DC she was impressed with the view of the city lights in the dark. She stood back in the aisle and tried to take a picture through the window using her flash. Explain why this is a bad idea and what her pictures are likely to show.

Explanation / Answer

Part A) The overhead projector uses light to project an image on a white screen. You see the image as the background light is dimmed. The darker the background, the easier it is for you to see the image. When the camera flash goes off, it will flood the area of the image with light at the instant the shutter exposes the film. The film will therefore see only a flood of the light overwhelmed the light from the projected image. The image, then, on the film will just be a bright flash of light. Part B) The opposite effect is true for part B. In the darkened room, there is no ambient light to expose the film. The flash would be needed to illuminate the subjects at the instant the shutter exposes the film. If there is no light, nothing can expose the film, and the pictures will all be dark blobs. Part C) For this part, when the woman takes a picture with the flash at the window, we know that the flash will be reflected from the window. The reflected flash will be much brighter than what she is trying to take outside of the window, so the flash will flood the film with light at the instant it is exposed. The image on the film will be the reflected flash of light off of the airplane window.

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