The Particle/Wave 'Duality' of Light
The Meaning of 'Duality'
When we say that light has the 'duality' of being both a particle and a wave, what me mean is often not entirely clear. Light is a type of electromagnetic radiation. All types of electromagnetic radiation have this 'dualistic' property.
What this property means, is that sometimes light is most easily described as having the properties of a particle (we call this light particle the photon), and sometimes light is best described as a wave (like sound).
Light Best Described as a Wave
In other experiments, light clearly acts as a wave. Indeed light is treated as a wave in classical physics. Treating light as a wave more easily explains the effect lenses, reflection, refraction and the like.
The interpretation of light as a wave is the basis for optics technologies, such as glasses, contact lenses and telescopes.
While it is possible to explain the behavior of light as either a particle, or as a wave, it is not convenient to do so exclusively.
Light Best Described as a Particle
In some experiments, in is possible to measure (with very low uncertainty) the location of a 'photon'. This leads experimenters to to discuss light is such terms-- as having the properties of a particle.
Under the right circumstances, discrete 'particle-like' packets of light are observed.
The light-sensitive part of the human eye is attuned to 'feel' the effects of the packets of electromagnetic energy in the frequency range known as 'light'. The hard-to-describe nature of light is not a problem for your eyes, though, because your eyes are simply tuned to feel this particular phenomena. In fact, the genetic code for our eyes may well still encode more 'understanding' of the nature of light then science!
The human eye cannot detect a single photon, it requires a significant amount of photons for the eye to be able to 'see'. In fact it takes around 800000000000000 photons to interact with the human eye in one second to see the tiniest 'blip' of light.