Property And Nature of Light


Refraction, is the process by which waves are bent when moving from one medium with a low index of refraction, to a medium with a higher index of refraction

Refraction of Light Diagram.  Flavio Spedalieri

Fig 1.4

Fig 1.4. Because light waves travel at differently speeds in different mediums, at the boundary of the higher index medium, the wave front must change speed, in the case above, a beam of light enters a high - index material, the wave front will slow down, and the beam will bend or what is known as refraction. Refraction only occurs at the boundary of a low - index medium to a high - index medium or vice versa.  When the beam is refracted, the wavelength changes, but is in phase and they oscillate at the same frequency, this bends the light in the direction shown. If the light beam passes back out into the same low - index medium, then the waves become longer (Speed up), and bend back to the same angle when it originally entered the medium.

If light travelling in a medium with a refractive index of n1 strikes the surface of a material with a refractive index of n2 at an angle of q 1 to the normal the light direction in the second material is given by q 2 in the equation:

n1 sin  q 1 = n2 sin q 2

The Above equation is known as Snell's law

Snell's Law of Refraction:

Where the arcsin is the angle for which the stated value is the sin, and can be re-written as:

q 2 = arcsin ( n1 sin  q 1 / n2 )  

raction of white light in a prism

Refraction of White Light Diagram.  Flavio Spedalieri

Fig 1.5

Fig 1.5. This diagram show how a glass prism separates white light into is component colours, taking the pervious details about refraction into account, we know that light waves entering a higher - index material is slowed down (refracted), also, different wavelengths travel at different speeds within a given medium. As we know white light is made up of different colours or wavelengths, with all this in mind, we can implement these theories to the above diagram, and these form the conclusion as to why a glass prism will split white light into its component colours. 

Refractive Index

Light travels more slowly in matter, air at one atmosphere will cause light to slow down. The ratio of the speed of light in a vacuum (c) to the speed of light in a material (v) is the Refractive Index:

Refractive Index = c / v

Because the speed of light is faster in a vacuum than in material, the refractive index is greater than one.

Refractive Index of common materials

Material Index
Dry Air (1 atmosphere) 1.000278
Ice (-8oC) 1.309
Water (0oC) 1.33346
Methyl Alcohol 1.3284
Ethyl Alcohol 1.3614
Ethylene Glycol 1.4318
Glass, Fused Silica 1.459
Glass, Pyrex 1.474
Pespex 1.488
Glass, Crown 1.52
Optical glass 1.51 ~ 1.81
Crystal quartz 1.544 ~ 1.553
Sapphire 1.757 ~ 1.779
Ruby 1.757 ~ 1.779
Glass, Flint 71% pb 1.805
Nd:YAG @1064nm 1.818
Cubic Zirconia 2.173 ~ 2.21
Diamond 2.419

Table 1.0