Light does appear to travel in straight lines
In a dusty atmosphere it is sometimes possible to see light travelling and it does appear to be moving in a straight line. The fact that light travels can be demonstrated by putting an object in its path. If the object is opaque the result is a degree of blackness on the other side of it which is due to the absence of the light. The dark area is a shadow.
As light does appear to travel in straight lines then light is usually modelled as straight lines in drawings. This can sometimes be very confusing because to make the explanatory diagrams easier to understand the light from source is usually depicted as just coming from the top and bottom of the source when it is in fact coming from the whole source. For example in the diagram below the photograph in the pinhole camera is shown by just two lines when of course it is generated by billions of lines of light coming from the object.
Shadows can provide evidence that light travels in straight line. Think of the shadows that are formed on a cloudless sunny day, they are very well defined shadows. Supposing at this point that light did not travel in straight lines what would the shadows look like? They would probably be very fuzzy just like you would get on a sunny day with lots of clouds in the sky. When the sky is cloudy the sunlight is reflected of the clouds and they become secondary sources of light so on such a day the shadow is formed from lots of different light sources coming from a variety of directions.
When light come into contact with the surface of an object it can do one of four things.
· It could be absorbed by the surface (more about this later but suffice it to say at this time that this is a characteristic of black objects).
· It could be scattered by the object.
· It could be partly scattered and partly absorbed.
· It could be reflected.
The difference between reflection and scattering is dependant upon the nature of the surface of the object. A ‘rough’ surface causes the light to bounce off in all directions (scattering) while a smooth surface means the light will all bounce off travelling in the same direction.
Reflection of light from a smooth surface can be compared to a billiard ball being bounced off the side of the billiard table. If you hit the ball so that it hits the wall at 900 to the table edge then the ball should travel backwards in exactly the same direction. If the billiard ball hits the edge at an angle of 300 from the vertical then the ball will bounce back at 300 from the other side of the vertical (the angle of incidence equals the angle of reflection). Light will behave in exactly the same way when it bounces off a flat surface like that of a mirror.