Two centuries ago there were two conflicting theories of light. These were Corpuscles Theory suggested by Isac Newton (1642-1727) and Wavelength Theory propagated by Christian Huygens (1629-1695). The theory of Huygens was subsequently established by an English physicist Thomas Young (1773-1829). Newton suggested that a beam of light consists of tiny particles (corpuscles). And since light was corpuscles, Newton argued, it travels in straight lines and casts sharp shadows. It also explained as to why mirror-reflected light simply bounced off the glass like tennis balls off a wall. Newton thought it might be caused by the corpuscles that travel faster in glass and water than through air.
In contrast, Huygens believed that light travelled in waves like ripples on a pond. He also showed that each colour of light has a different wavelength. He proved that light travels slower through glass and water than through air. The amount of refraction, that is, the amount of light is bent depends upon the light. The shorter the wavelength the more is the bending.
In 1801, Thomas Young, based on the theory of Huygens, established the wavelength theory beyond doubt. He shone a beam of light through silts in a piece of card. The silts divided the light into two beams which when recombined, formed a pattern of alternate light and dark bands on a screen. He reasoned that the pattern was produced by interference between the waves of two beams.
Now, what is actually interference of light and how it is defined? According to Huygen’s theory, light waves spread out from their source in ever increasing circles. It tells us that where two light waves meet, they combine in some places on their way and cancel out each other. This process is called “interference of light”. When two wave-lengths of light met they reinforced each other and produced a bright band. On the other hand, where one crest (higher part of the wavelength of light) met another they cancel each other to form a dark shade.
It is interesting to note that for more than a century, Newton’s theory seemed more popular because of his name and fame. This position was changed when Thomas Young convincingly proved and established the wavelength theory of light. His experiment on “interference” was important among other experiments conducted by him in this connection.
Today, modern physicists explain the optical phenomena of light in terms of waves, e.g. interference, refraction, reflection, or diffraction.
