Hello Larry, good question.
Both lightning and thunder are caused by an electrical spark between a cloud and the earth or, sometimes, between two clouds. The spark arises due to the accumulation of electrical charge caused by the rubbing friction between falling raindrops and the air. It's a phenomenon similar to running a comb through your hair or scuffling your feet on a rug on a dry day. When the charge gets big enough it causes a spark to occur.
The sequence of events goes something like this. First, the spark creates local heating of the surrounding air causing it to glow. The resulting light flash then travels outward at the 'speed of light'. This is what you see. Meanwhile the intense temperature generated by the electrical discharge heats up the air and causes it to expand very rapidly. It's similar to an explosion. The associated sound waves are what you hear as thunder. They travel at the speed of sound 0.343 km/sec (about 768 mph). The speed of light is vastly higher, 300,000 km/sec (about 186,000 miles per second!). The wavelengths are different too. The sounds of thunder are in the audio region with corresponding wavelengths of a few tenths to a few meters whereas the wavelengths of visible light are in the range of 400 to 750 billionths of a meter.
In sum, the light and the sound of a lightning strike have really different properties. By the way, Wikepedia has nice articles on both lightning and thunder.
(published on 12/14/2011)