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Q & A: When Light Curves . . .

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Most recent answer: 10/22/2007
I was always told that light travels in straight lines - it cannot curve around corners etc which is why there are shadows. Why then is it possible for the light of a distant star to be ’bent’ around a body of high gravity (such as the Sun) slightly, and appear to be in a different position?
- Chris (age 18)
St Clement Danes School, Chorleywood, Bucks, England
It's always great to get a question like this, because not everybody remembers the different things they've heard and understands them well enough to know when they're contradictory. Of course, it's also a little scary to give an answer to someone who will actually remember what you say.

Anyway, the main answer here is really simple- don't always believe what you're always told! When people say that light doesn't bend, they mean that those gravitational effects aren't noticeable under most conditions. When you do take them into account, as Einstein did
in General Relativity, the situation gets even worse than you think. There just aren't any old-fashioned straight lines in our universe.

For simple Earthly experiments, it STILL isn't quite true to say that light travels in straight lines. Everybody is familiar with how light bends when it goes from air to water or glass, etc. Of course, the people who told you that light travels in a straight line
probably assumed you wouldn't apply that to such situations. Even leaving those situations out, and ignoring gravity, the statement that light travels in straight lines still isn't quite true.

Waves have a tendency to spread out. Even the straightest light beam you can get, from a good laser, will gradually spread out. If you look carefully at the shadows of light from objects with sharp edges, you'll find that the shadow won't have a sharp edge, even if
the light all came from a tiny region. Sometimes this effect- called diffraction- produces amazing results, such as making a bright spot in the middle of the shadow of a round object.

Of course if I'm saying not to believe what people tell you, you might wonder why you should believe me. You could do some of these experiments yourself. Some of the easiest involve shining laser beams through little slits, and looking at the interesting patterns produced.


(published on 10/22/2007)

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