Light Phenomena and Theories
Dispersion of Light
The index of refraction of a substance is a function of the wavelength of the incident light. It decreases with increasing wavelength. As a result, if a beam of light containing different wavelengths is incident on a refracting material, each wavelength is deviated at a different angle. This effect is called dispersion.
Interference
When two waves reach a point simultaneously, the resulting vibration at that point is the sum of the vibrations that each wave would have produced individually. Constructive Interference: If the waves are in phase, the resultant amplitude is the sum of the individual amplitudes, and the intensity, proportional to the square of the amplitude, is maximized. Destructive Interference: If the waves are out of phase, the resultant amplitude is the difference of the individual amplitudes, and the intensity is minimized.
Diffraction
Light waves bend around obstacles and reach points that would otherwise be in shadow.
Polarization
A light beam is linearly polarized if the electric field oscillations always occur in the same direction. A wave is unpolarized if all oscillation directions are equally likely or if the wave is a superposition of waves with different vibration directions. Polarized waves can be rectilinear, circular, or elliptical.
Reflection
When a wave reaches the interface between two media, a portion of its energy is returned to the first medium. The incident ray, the normal to the surface at the point of incidence, and the reflected ray lie in the same plane. The angle of incidence equals the angle of reflection.
Refraction
When a wave reaches the interface between two media, a portion of its energy enters the second medium. The refracted ray, the normal, and the incident ray lie in the same plane. The ratio of the speeds of light in the two media is called the refractive index.
Theories of Light
Newton’s Corpuscular Theory
Newton proposed that light consists of tiny particles emitted by sources and traveling in straight lines. This theory struggled to explain why light travels faster in some media than in others.
Huygens’ Wave Theory
Huygens proposed that light is a wave propagating through a medium, similar to sound waves. This theory explained reflection, refraction, and double refraction but faced challenges explaining diffraction.
Fresnel’s Wave Theory
Fresnel’s work on interference, diffraction, and polarization supported the wave theory. He proposed that light waves are transverse.
Maxwell’s Electromagnetic Theory
Maxwell’s theory established that light is a high-frequency electromagnetic wave, not a mechanical one. This theory gained wide acceptance.
Einstein’s Photon Theory
Einstein explained the photoelectric effect by proposing that light consists of quanta of energy called photons. This theory revived the particle aspect of light.
Dual Nature of Light
Light exhibits both wave-like and particle-like properties.
Doppler Effect
The Doppler effect is the change in frequency of a wave due to the relative motion between the source and the observer.