How can we realize that light travel in straight line ?​

Light travels in a straight line can be observed by keeping an object in the path of light. In an atmosphere which is bit dusty, we can see light traveling in a straight line. Light emerging from the torch, train and lamps always travel in a straight line.

waves moving from a fast medium to a slow medium will experience a reflection as light travels more slowly than red light. total internal reflection occurs when the critical angle is.