II.

Nature of Light

Light is emitted from a source in straight lines and spreads out over a larger and larger area as it travels; the light per unit area diminishes as the square of the distance. When light strikes an object, it is either absorbed or reflected; light reflected from a rough surface is scattered in all directions. Some frequencies are reflected more strongly than others, and this gives objects their characteristic colour. White surfaces scatter light of all wavelengths equally, and black surfaces absorb nearly all light. Image-forming reflection, on the other hand, requires a highly polished surface such as that of a mirror.

Defining the nature of light has always been a fundamental problem in physics. The English mathematician and physicist Sir Isaac Newton described light as an emission of particles, and the Dutch astronomer, mathematician, and physicist Christiaan Huygens developed the theory that light travels by a wave motion.

It is now believed that these two theories are essentially complementary, and the development of quantum theory has led to the recognition that in some experiments light acts like a series of particles and in other experiments it acts like a wave. In those situations in which it travels in wave motion, the wave vibrates at right angles to the direction of travel; therefore light can be polarized in two mutually perpendicular planes (see Optics).