logearav said:
Sir, i don't understand clearly. Can u give some other example, please?
I have been thinking more about this. Maybe I am reading too much into your question, but I am not crazy about my answer either. First let me try to be clearer about what is meant by saying light traveling in a straight line creates a sharp shadow.
I am assuming you get that "sharp" shadow has very clear edge: you can say clearly "Here is the line where it stops being dark and starts being light". There is no gray between lit and unlit.
How would straight beams of light cause that? I think http://en.wikisource.org/wiki/File:PSM_V04_D690_Formation_of_a_shadow.jpg" shows that well: those are straight lines of light that graze the edge of the sphere. The shadow is created where the light rays are prevented from falling. If light moved in a path that
curved around the edges of the sphere, some light would fall in the area shown shadowed in that image.
Here is the problem with the answer: suppose the light
did curve around the sphere. If all the light bent, it looks like there could still be a sharp (albeit smaller) shadow. So is this really evidence that light moves in a straight line?
Having thought through that by writing it, something else occurs to me. If the beams tangent to the sphere bent one degree and the "adjacent" beams went straight, there would be a gray ring around the shadow and the shadow would n longer be sharp. And if those beams bent, you could make the same argument about the next ones over...