Light is an electromagnetic wave that propagates along a straight line in a transverse manner. The propagation of light can be understood through basic wave motion. The progression of the wave is identical to the motion of ripples in water in a pond with crests and troughs.
Figure 1: A one-dimensional representation of an electromagnetic wave
1. Wavelength: If we look into a one dimensional representation of the wave starting from the source, we can denote the maximum displacement of the wave from origin as the Amplitude (A) and the duo of one crest and one trough comprises a single wavelength (λ) as shown in Figure 1. The inverse of the wavelength is called the wave number (1/λ) in cm-1.
2. Speed: Light travels much faster than sound at a speed (c) of exactly 299 792 458 m s-1 in vacuum. The wave speed (v) gets slower when in a medium. Thus, the refractive index (n) of the medium can be identified from the ratio of the speeds by using the equation n= cv.
3. Reflection: It is quite often that we represent light in terms of its interaction with surrounding media. When a ray of light strikes a surface, it is seen to bounce off in a new direction. Its directionality is in accordance with the Law of reflection which states that the angle of incidence (θi) equals the angle of reflection (θr) i.e.θi= θr , as shown in figure 2.
4. Refraction: From a similar context, refraction is the phenomena of bending of light as it travels from one medium to another medium of a different refractive index. For positive refraction, the refracted ray is observed on the other side of the normal in contrast to negative refraction where the ray is observed on the same side as the normal. At normal incidence, the light beam passes un-deviated. Two probable scenarios in refraction (see figure 2b, 2c) are when (i) the first medium has a lower refractive index than the second. Here, light travels from a rarer to denser medium causing the refracted ray to bend toward the normal; (ii) the first medium has a higher refractive index than the second. Here, light travels from a denser to a rarer medium causing the refracted ray to bend away from the normal. Refraction and the relation between the indices is expressed in terms of Snell’s law where n1sinθ1= n2sinθ2. Here θ1 / θ2 is the angle of incidence/ refraction (Vandergriff et al., 2008).
Figure 2: (a) Reflection; (b) Refraction-(i); (c) Refraction-(ii); and (d) Total internal reflection
A special case of the second scenario leads to a condition known as Total internal reflection (TIR). When the angle of incidence exceeds a certain critical angle of the medium, the refracted light bends back into the first medium giving an appearance similar to reflection (see figure 2).
5. Dispersion: Several sources of light have emerged over the decades to meet the different applications, the most common being LEDs, fluorescent lamps and lasers. The wavelength range of light obtained from them is a mere sample of how vast the electromagnetic spectrum can be. Light can possess very short to very long wavelengths/ frequencies. The spectrum expands over a wavelength range of 108 m to 10-12 m (or frequency range of Hz to EHz). The dispersion of light is a quantity that helps distinguish wavelengths that are very close to each other by separating their spectral lines. For example, white light comprises a group of wavelengths in the visible region travelling simultaneously. They can be easily separated using common angularly dispersive elements such as prisms and gratings.
6. Polarization: The next property of light is based on the movement of its electric field vector as the wave propagates. This behavior of light, known as polarization, can be manipulated to achieve defined orientations of the electric field vector by using techniques such as reflection at Brewster’s angle, scattering and double refraction (a method that uses materials transmitting specific polarisations only). Light waves can also be modified into linear, elliptical and circularly polarizations. These are named by observing the patterns traced by the propagating electric field vector of a polarized wave along the direction of propagation (see figure 3).
Figure 3: Polarization types (a) random; (b) linear; (c) circular; (d) elliptical
As light of any aforementioned orientation passes through a linear polariser, the intensity (I) of the emergent light can be predicted by Malu’s law i.e I α cos2θ, where θ is the angle of electric field with respect to the optics’ transmission axis (Saleh et al., 1991). In addition to these properties of light, the famous wave-particle duality was concluded through a demonstration of its capability of diffraction and interference, and the photoelectric effect respectively. Huygen’s principle gave us a fair understanding of light propagation in terms of superposition and spherical wavefronts. However, the theory that surpasses all by encompassing all facets of light is Maxwell’s equations of electromagnetism.
References
Saleh, B. E., Teich, M. C., & Saleh, B. E. (1991). Fundamentals of photonics (Vol. 22). New York: Wiley
Vandergriff, L. J. (2008). Nature and properties of light. Fundamentals of Photonics, SPIE Press,Bellingham
