you can use a straight edge and pencil to show path of rays.
among other things
Answer:
In geomtrical optics, it's crucial to assume that line travels as a ray, because that way can form all geomtrical patterns to study it.
According to Einstein, light bends when passes near a massive object, like the Sun. Actually, light bends due to gravity. However, this phenomenon is discarded on Earth, because the bend of the light is really insignificant, as such point that it can be studied in optics as a straight line.
In geometrical optics, we study reflection and refractions, where the line is always a ray, which forms gemtrical pattern that helped to created several laws about reflection and refraction.
If we don't consider the light as a ray, all calculations would be massive, and geometrical pattern wouldn't be useful to study optics.
