why does light travel in straight lines
A straight line is 'In the eye of the beholder'. As far as light is concerned it travels in a straight line from point A to point B. However, for a distant observer the trajectory may be a bit curved. The reason is that the geometry of space is a bit warped near a massive gravitational source like a black hole or even the sun. One example of that is the verification of Einstein's theory of light from a distant star being bent by the sun's gravitational potential.
This was observed in 1919 during an eclipse of the sun. The general phenomenon is called 'Geodesic lines in curved spaces'. As a simple example in our spherical geometry of the earth, the latitudes of New York and Rome are very similar. But the shortest distance between them for an airplane is not on a direct east-west route but to travel a bit to the north-east for a while then curve back south.
If you stretch a string on a globe between these two points you find that the optimum flight trajectory takes you about 10 degrees north of the direct east-west flight path. Gravity also acts as a distortion of space, however the mathematics is a bit more complicated. (published on 11/20/2010)
When you have a light source like a candle, light travels in many straight lines in all directions.
Each light particle* travels in a straight line in a different direction. But there are so many of them that you can not perceive individual particles, so it seems like the light spreads uniformly. A light source where all light is emitted in the same direction is a. But there is one thing Isaac Newton wasn't aware of: Gravity can bend light (or rather the space through which the light travels) which causes light to travel in curves.
For more information, search for. Further, there are things like, and which cause light to change direction (but not really travel in curves. The straight line just makes a sudden change in direction) No, I am not going to go into wave-particle duality here
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