Despite all the progress in understanding the nature of light what Samuel Johnson said about light (quoted by Boswell in 1776) still holds good :
” We all known what light is but it is not easy to tell what it is?”
It is reported that the first practical optical instrument- later called spectacles- were in use in 1270 in Florence, Italy. Progress in glass making and metallurgy was evident in the glass furnaces of Venice in the early 13 the century. A British glass-maker, George ravens croft improved the quality of glass by adding lead to the chemical composition in 1674. Four years later, Huygens suggested that light travels as a wave. No one took it seriously. In 1704 Newton famously declared that light traveled as a ray as it took a long time to counter Newton’s particle theory of light . Improved optical properties of glass triggered new ideas about the nature of light.
It was realized that unlimited magnification of images was not possible as the waves would diffract. A point source of light through a circular lens would look like a fuzzy spot with faint rings around . It is now called an Airy spot after George Airy who first described it. It was found that the spot would depend on the diameter of the optic and the wavelength of light .the airy spot would otherwise become simply bigger and not capacity of a telescope.
Young had another brilliant idea. He calculated the wavelength of colored light from the way in which light bands are spaced. Thus violet has the shortest wavelength (380-440 nano-meters), ultramarine blue, 440 -483 nm, while red has 610-705 nm. Actually these limits are given for our convenience. The colors in the visible spectrum are limitless.