LIGHT TRAVELS IN STRAIGHT LINES
We see things if either they GIVE OUT light (like a light bulb) or they reflect light.
The light travels from these things to our eyes in waves (electromagnetic waves). If we think in 2 dimensions
the waves travel out from a point like ripples in a pond when a stone is dropped into it...they are circles. In 3 dimensions, of course,
these waves are really spherical. The line which we draw to represent the wave is called the "wavefront"
BUT.......a POINT on a wavefront travels in a straight line....it travels directly away from the source of light. We sometimes draw lines
to represent the direction in which these points travel and call them RAYS (you can think of rays as being very thin beams of light). So we can consider that LIGHT RAYS TRAVEL IN STRAIGHT LINES.
If we think again in 2 dimensions, the wavefronts are circular and the rays go away from the centre..ie they are radii of the circle, so we see that light rays are always at right angles to the wavefront.
If we look at a small part of a wavefront a LONG distance from the source, we are looking at a tiny section of a circle and it is effectively a straight line: we call this wavefront a "plane wave" and again
the rays are at right angles to this wavefront