What makes the light a wave?

Light has a frequency and can be made to interfere with itself, causing interference patterns. This means it must have properties of a wave.

Wavelength is simply velocity divided by frequency to get a distance. Typical visible light wavelengths are 390-750 nanometres.


Our eyes react chemically to different frequencies of light. We have three types of receptors. Each one reacts to a different range of frequencies: red green and blue.

 

One important reason which leads us to believe that light is a wave is that when you write down the differential equations that describe the interactions of electric and magnetic fields (Maxwells Equations), we see that a couple of the equations are the same as wave equations, in other words, the equations which describe time-oscillating electric and magnetic fields are mathematically identical to wave phenomena. The speed of the wave is derived from it's mathematical description. It turns out that the speed of the waves that we calculate from Maxwells equations is identical to the speed of light that is experimentally observable. Thus it leads one to conclude that light is the same an electromagnetic wave. Obviously there is a lot more to nail down than just drawing a connection between the speeds of these things, but this was one of the first discoveries which led to the postulation that light is a wave. Thus to understand its wave nature you must understand electromagnetism.

 

Also, this may not be totally correct, since I have not verified this with a professional, but I have always assumed that the eye can observe light due to the same reason that personal radios can observe music emitted from a radio station. When an electromagnetic wave passes over a conductor, such as a wire or antenna, it generates a current in the wire and this current is then generated into music from the circuitry and mechanics of the radio. I draw an analogy (not exceptionally rigorous) between the radio and the eye. When an electromagnetic wave (light) passes over or through our bodies, some of it passes into our eyes and current is generated along nerve cells (conductors) which is then physiologically transformed into visual-perception.

Like I said, not 100% sure this is how it all works.

 

Wiki has a good basic description about how light is converted into a nerve signal:
http://en.wikipedia.org/wiki/Photoreceptor_cell#Phototransduction

 

I read the question as WHY is light a wave, not WHY do we know it is a wave. Maybe that would be fun to discuss.

 

Perpendicular electric and magnetic fields oscillating in phase. If this electromagnetic wave oscillates in a particular frequency range, we see it as visible light.

 

Actually light is NOT a wave! Light is known to consist of discrete particles of energy known as "photons". But this is a forum in classical physics so here we discuss the older wave theory. Just because the wave theory isn't quite correct doesn't mean it isn't useful! That is because when you have a large number of photons in a beam, somehow they act as if they were an electromagnetic wave traveling along. Somehow all the same wave characteristics are exhibited such as frequency, wavelength, diffraction, refraction etc. So thus, if one has a reasonably bright light, the classical theory will give you answers that describe what you observe quite accurately.

And in any event science doesn't ask "why" things are the way they are. Science simply does experiments to find out how things work and that is the final authority. Probing the mind of God or nature for motivations is not what science does.