### My Yahoo! Answers Science Post

I am my dad's personal Yahoo! answers person when it comes to things science and physics. So when I received a question on the dual nature of light and matter, I wrote him back a nice long (LFP!) email in answer. I thought I'd share.

Question:

Sound travels in waves and needs an "ether" for transmission.
Light is also a wave but doesn't need an ether. Is light in space "wave" or "particle"? because when light bends do particles bend or this this one of the examples of light as wave?

Sound is a pressure wave. It is the compressing of molecules. It is the compression that is the sound.

Light is a wave of electromagnetic fields, fields in this case being force representations. The light wave is a constant of nature in which a electric force changing will produce a magnetic force which will cause an electric force in a stable, repeating way traveling forward. There are other places where electricity creates magnetic forces and vice versa, but they are not stable and not self-contained.

When material is present, those electromagnetic forces exert themselves into a single point, which is what is classically defined as a particle. Light in space is then always a "wave", since that is the way it propagates itself and travels.

When light is in a perfect vacuum, its speed is constant at c. When light is traveling through a material, the speed is slower than c, based on a calculation related to its "index of refraction". This quantity is the change at which an electromagnetic wave will move forward while remaining self-contained and stable.

The 'index of refraction' is the change in velocities between material boundaries causing an apparent difference in the way that light is 'seen' (this is velocity that have direction now, not speed which is a directionless quantity). This is the reason you put a pencil in a glass of water, it seems to fracture or "refract" at the air-water boundary since the speed of light in each material is different.

So light never "bends". What light bending is usually described that way because it is passing through things in which the velocity of the electromagnetic wave is different. This is the case of seeing mirages of water when the light from the sky bends up; the index of refraction of the air is temperature dependent. 