Wave Properties and the Dual Nature of Light
Wave Properties
A maximum value is reached, varying in magnitude. For example, in simple harmonic motion, amplitude is the maximum displacement of a particle. In an electromagnetic wave, the amplitude is the maximum value of the electrical or magnetic field.
- Wavelength (L): The distance between two successive points having the same state of disturbance. It gives us the spatial periodicity of the wave. It is measured in meters (m).
- Period (T): The time taken to complete one cycle of the disturbance. Put another way, it’s the time it takes for the disturbance to advance one wavelength. It gives us the temporal intervals, measured in seconds (s). We can relate the period and the wavelength through the speed of wave propagation.
- Frequency (F): The inverse of the period. It gives the number of oscillations in a second, measured in Hertz (Hz).
- Angular Frequency (ω): The number of radians swept in a second.
Huygens’ Principle (HP)
HP is a simple mechanism for the construction of wave fronts from previous fronts. A wavefront is one of the surfaces that pass through the points where a wave oscillates with the same phase. The principle states that:
The points in a wavefront become sources of secondary waves, whose envelope is a new primary wavefront.
How to apply it: Small circles of radius are plotted with centers at different points of a wave front, and then the envelope of the circles is plotted, which is the new wavefront.
One consequence of Huygens’ principle is that all rays take the same time between two consecutive wave fronts. Rays are lines perpendicular to the wave fronts and correspond to the line of wave propagation. Although Huygens’ principle was formulated for matter waves, which were the only ones known at the time, it is valid for all types of waves. Kirchhoff extended the method to electromagnetic waves once they were discovered.
The Nature of Light (NL)
The question of what is the nature of light has been a problem from antiquity to the twentieth century. Throughout history, people have developed two main competing theories:
- The Particle Theory: States that light is composed of particles or corpuscles. Newton was its main representative.
- The Wave Theory: Argues that light behaves as a wave.
Both theories explain the phenomena of reflection and refraction. However, only the wave theory could explain satisfactorily the phenomena of interference and diffraction and the fact that the speed of light is greater in less dense media. This, together with the development of electromagnetism by Maxwell, reinforced the wave theory as valid.
In the nineteenth century, the issue was settled, and it was acknowledged that light was an electromagnetic wave. But in the early twentieth century, Einstein had to resort back to the particle nature of light to explain certain phenomena of emission and absorption of light by matter, such as the photoelectric effect.
After that, the wave-particle duality of light was introduced in physics, which means that light has two natures: in some phenomena, it behaves like an electromagnetic wave of some frequency; in others, it behaves like a stream of particles called photons with a given energy.