PHOTON

The photon is an elementary particle.
The quantum of the electromagnetic interaction and the basic unit of light and all other forms of electromagnetic radiation. It is also the force carrier for the electromagnetic force. Like all elementary particles, photons are best explained by quantum mechanics and exhibit wave–particle duality.

The human eyesight depends on photons reflecting and refracting from surfcaes before entering the visual cortex. Every photon that hits the cortex is unique, a piece of energy leaving its mark as an image. The image of stars in the sky is the work of photons, sometimes travelling for millions of years before entering the eye.