ABSTRACT
Light and dark Light is a form of energy propagated by electromagnetic waves travelling at an immense velocity – some 300 km/ms – and carried in discrete packets called quanta or photons. As you can see above, only a very small range of all the wavelengths of electromagnetic radiation is visible. The longest waves that we can just see, forming the red end of the spectrum, are some 0.7 mm in length, slightly less than twice as long as the shortest waves at the blue end. In nature, most electromagnetic radiation is generated by hot objects: the hotter they are, the more of this energy is radiated at shorter wavelengths. The peak of the spectrum of light from the sun – an exceedingly hot object – corresponds quite closely with the range of wavelengths seen by the cone receptors in the eye (the expanded graph above). Of man-made sources of light, many, like the
ordinary incandescent electric lamp, radiate as hot bodies and have a smooth and broad emission spectrum; others are quite different, and emit light only at a few discrete wavelengths. The older sodium lights used for street lighting, for example, are effectively monochromatic, their energy being concentrated in a very narrow band in the yellow region. Domestic fluorescent lamps have a spectrum consisting of a number of emission lines superimposed on a continuous background.
