Consider photons which are reflected from some point on a tree — say, from a leaf at the very top. Focusing by the lens at the front of the eye means that every photon which reaches the lens from that particular point on the tree is directed towards the same spot on the retina. Similarly, photons from every other leaf on the tree are brought together at certain corresponding points on the retina. An 'image' of the tree is formed at the back of the eye, which is recognised by the brain as 'a tree'. Recognition of any object is a job for the brain rather than the retina. As babies grow, their brains develop the ability to reconstruct objects such as 'my fingers', 'mother' and 'teddy'. They also learn to allow for the fact that the image in the retina is upside down with respect to the object!
So far, we have represented light by 'rays', and have not been concerned with either particle or wave properties of light. In fact, we have freely interchanged concepts of 'photon', and 'wave' in the preceding paragraphs, depending on context. Later we will formalize the various aspects from which the subject may be approached.
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