ABSTRACT
The properties of light and radiation are considered in detail in standard works on optics. Vision is a result of interactions between photoreceptors in the retina of the eye and light, which is a form of radiant energy. Detailed theoretical work on the wave nature of light replaces the simple oscillating particle model by one considering propagated radiant energy as two orthogonal, transverse harmonic waves representing electric and magnetic fields, whose behaviour is generalized by J. Maxwell’s equations. The small spectral band corresponding to integrated wavelengths known as ‘white light’ is particularly narrow. Light and radiation sources of thermal types emit energy over extended or discrete spectral regions to give continuous or line spectra characterizing the stimulated movements of electrons in the outer molecular shells of the materials. Photometry is the measurement of light energy, and is a division of the science of radiometry, which is the measurement of electromagnetic radiation in general, and whose basic unit is the watt.
