Understanding Light: Theories and Phenomena
Nature of Light
Newton’s Corpuscular Theory
Light has particle nature: light bulbs emit tiny particles that propagate in a straight line in all directions, colliding with our eyes, producing the sensation of light. Corpuscles, other than for color, can pass through transparent media and are reflected by opaque bodies. This hypothesis justifies phenomena such as the rectilinear propagation of light and reflection, but not refraction.
Huygens’ Wave Theory
Huygens proposed that light consists of wave propagation of a disturbance of the medium. Huygens believed that it was longitudinal waves. This hypothesis explains the phenomena of reflection, refraction of light, and double refraction. The difficulty of the wave theory was that they had not seen in the light wave phenomena such as diffraction, but we know that the wavelength is so small that these phenomena are not easily observable.
Fresnel’s Wave Theory
Fresnel proposed that light is made up of transverse waves. Foucault noted that the speed of light is greater in air than in water, which invalidates the explanation of Newton’s refraction.
Maxwell’s Electromagnetic Theory
Maxwell proposed that light is not a mechanical wave but a form of high-frequency electromagnetic wave. Light waves consist of propagation, without any support, of an electric field and a magnetic field perpendicular to each other and the direction of propagation.
Particle Nature of Light According to Einstein
Einstein proposed that light is made up of a bundle of small corpuscles or quanta of energy, also called photons. That is, the photon is concentrated wave energy rather than being distributed continuously.
Dual Nature of Light
Light has a dual nature, corpuscular and wave. It spreads through electromagnetic waves and wave phenomena are presented, but in its interaction with matter, in certain phenomena of energy exchange, it manifests a corpuscular character. Light in a specific phenomenon only behaves as a wave or a particle.
Characteristic Phenomena of Light Waves
Interference
Types:
- Constructive: If the waves are in phase, there is an intensification of the waves.
- Destructive: If the waves are out of phase, there is a weakening or even cancellation of the waves.
Young’s Double-Slit Experiment
Young’s experiment confirmed the wave model of light and allowed the measurement of its wavelength. The experiment is to provide a monochromatic light source illuminating a screen containing two slits. The slits act as emission sources, and the waves produced are consistent since they come from the same source. Interfering waves produce an interference pattern on the screen later. Here we see a central bright fringe and other parallel bright and dark fringes. The bright fringes are due to constructive interference of waves on stage, and the dark fringes are due to destructive interference of waves in phase opposition.
Diffraction
If an obstacle gets in the way of light and is discussed in detail, the shadow shows that its outline is not clear; bright and dark are seen. This is the phenomenon of diffraction.
Diffraction Pattern Produced by a Crack
A beam of monochromatic light passes through a narrow slit parallel to the wavefront of incident light to form a diffraction pattern consisting of a broad central bright fringe and sides of bright stripes alternating with narrow dark streaks. This phenomenon can be interpreted by the Huygens principle; each point of the crack becomes a tunable phase wavelet, which interferes with each other.
Polarization
Polarization shows that light waves are transverse. A light beam is linearly polarized if the electric field oscillations always occur in the same direction. The plane of polarization of an electromagnetic wave is determined by the direction of propagation and the direction of vibration. Light is not polarized, as it is formed by a large number of different wave trains from different atoms; in each of the electric fields, it oscillates in a different plane. The result is that the possible vibration directions for the field are endless and change randomly according to the instant of time considered.
Polarization by Reflection
Malus discovered that if natural light is incident on a polished glass surface, the reflected light is wholly or partly polarized, depending on the angle of incidence. Brewster discovered that polarization is total for an incidence angle such that the reflected ray and the refracted ray make an angle of 90 degrees. This angle is called the polarizing angle or Brewster angle.
Brewster’s Law
Polarization is complete when the tangent of the angle of incidence equals the refractive index of the medium in which the refraction takes place.
Polarization by Selective Absorption
Land discovered a material consisting of thin plates containing hydrocarbon molecules arranged in long chains. He called it Polaroid or polarizer. This material polarizes light through a mechanism of selective absorption made by the oriented molecules. Since electrons can only move along the molecular chains when the direction of the incident light coincides with the direction of the molecular chain, the material absorbs light energy, providing an electrical current. If the direction of the light is perpendicular to the direction of the molecular chains, it is called the axis.