Taking a selfie requires that the phone camera is pointed towards you. It’s obvious that this is necessary if you are to be in the picture. But this simple fact indicates something about the nature of light: to see an image of an object, there has to be a straight line between the object (in this case you) and the camera lens. This is usually called the ‘line of sight’. Thus light is something that propagates in a straight line from the object to the viewer.Indeed, this is what we might expect from our knowledge of certain types of light source. Eye-catching visual efects at concerts are generated using lasers, illuminating the stage and the performers with coloured light beams. Laser pointers are commonly used at talks or lectures to emphasize images or words on a screen. The beam produced by these coherent light sources is highly focused, hardly diverging at all even across a big hall. It goes in a straight line-you point the device in the direction where you want the light to go, and it does so.
Because sunlight does not obviously exhibit this characteristic, it required some thinking to determine that the propagation of light in a straight line was exactly what was needed to understand why distant objects appeared to be smaller than nearer ones, even when it was known that they were, in fact, exactly the same size physically.
5. Euclid’s construction of rays showing why objects of the same size look smaller when they are further away.
The insight that the concept of straight-line propagation could explain this efect is attributed to Euclid, working in Greece in around 300 bce. His idea, from one of the earliest books on optics, is illustrated in Figure 5. Imagine two lines-let’s call them rays-one connecting the top of the object (a pillar in this case) to the observer’s eye, and one connecting the bottom of the object to the eye. The angle between these is related to the apparent size of the image of this object that we perceive. A more distant pillar, the same physical size as the first, produces two rays whose angle of intersection at the observer is smaller-hence the pillar appears smaller. This is what we call perspective in the image.