wavength of light is a distance between which two points in physical reality? i know it is the distance b/w 2 consecutive crests/troughs on a graph paper, but what is it actually and how does it propagate?

Wavelength is distance divided by cycles. Frequency is cycles divided by time. Multiply the two, the cycles cancel out, and you get distance divided by time, or velocity.For instance, if you look at a 90MHz FM radio wave (that`s 9 x 10^7 cycles per second), the wavelength is about 3 1/3 meters (that`s 3.333 meters per cycle). Multiply them together, and you get 3 x 10^8 meters per second. Bingo: speed of light.