Why diamond glitters and gives white as well as coloured light?

Why diamond glitters and gives white as well as coloured light?

PROF WAQAR HUSSAIN

           

Diamond literal meaning unbreakable is an allotropic form of carbon with face-centered cubic structure called diamond lattice. It is the hardest known natural material with high optical dispersion (ability to disperse light of different colour), high refractive index (2.417), etc.

       

          Diamonds glitter due to total internal refraction (explanation is given below as 1) It is cut in a way to get number of faces so that if a ray of light enters in it, it must strike the face at an angle greater than its critical angle (for diamond it is approximately 24.4^o degree).When light strike a surface at angle greater then critical angle, it is reflected back instead of going out as refraction. Greater are the reflections, greater is the glitter. The brilliance also depends on the cutting pattern of a diamond .Greater are reflective surfaces, greater are reflections and in turn greater is glitter. So, its brilliance also depends on the diamond cutter's skill. It is the art of the lapidary, cutting numerous facets which enable the material to make the most of its reflective powers.

                  

         Or have another dimension of the above discussion. Diamonds have a high refractive index (explanation is given below as 2) which means that light travelling through them will be bent more than in any other substances. Bending of light is due to fact that the speed of light is slowed down in a diamond. When it hits the face, when the speed is drastically reduced, it tends to bend significantly. So, light instead of going out, it is reflected back and again it strike other face and is reflected and it strikes many times and meet the same fate. Finally it finds the way out either as white light or as band of seven colour. It is analogous to carrom board striker which when struck with full force; keep on striking different sides of carrom board. This entire phenomenon appears to observer as glitter. It is called an adamantine luster.

1) Total internal reflection. When, a beam of light passes from a denser to a rare medium, refracted ray bends away from the normal and the angle of refraction is greater than the corresponding angle of incidence. As the angle of incidence increases, the angle of refraction also increases till for a certain value of angle of incidence, the corresponding angle of refraction equal to 90⁰ .The angle of incidence for which the angle of refraction is 90⁰ is called the critical angle.

           When the value of angle of incidence becomes greater than the critical angle, the ray of light no longer suffers refraction but the whole of it, is reflected back in the denser medium. Such a reflection of light is called total internal reflection.

           Let’s have an analogous example, striker in carrom board shows total internal reflection.

2) Refractive index, also called index of refraction, measure of the bending of a ray of light when passing from one medium into another. If i is the angle of incidence of a ray in vacuum (angle between the incoming ray and the perpendicular to the surface of a medium, called the normal;  and r is the angle of refraction (angle between the ray in the medium and the normal), the refractive index n is defined as the ratio of the sine of the angle of incidence to the sine of the angle of refraction; i.e., n = sin i / sin r. Refractive index is also equal to the velocity c of light of a given  wavelength in empty space divided by its velocity v in a substance, or n = c/v.( Encyclopedia Britannica)

          It is the measure of degree of speed of lessened in a material relative to speed of light in vacuum. Its high value indicates that the said material has reduced the speed of light to a greater extent.