Light or visible light is an electromagnetic radiation that can be perceived by the human eye. Within the electromagnetic spectrum, it is situated after radio waves, microwaves, and infrared radiation, and before ultraviolet radiation, x-radiation or x-rays, and gamma rays. Nevertheless, it also exhibits all the fundamental properties of electromagnetic radiation, including the wave-particle duality.
The Early History of the Theories of and Research About the Properties of Light
Particle Theory vs. The Wave Theory of Light
Classical physics was divided in describing the properties of light. There were two competing schools of thought before the advent of electromagnetic radiation theory and quantum mechanics: the particle theory of light and the wave theory of light.
In n his 1675 book “Hypotheses of Light,” French philosopher, mathematician, and astronomer Pierre Gassendi stated that ir was composed of particles of matter emitted in all directions from a source. Isaac Newton also argued in his 1704 book “Optiks” that light could not be waves because it traveled only in straight lines.
Newton also used the particle theory to explain the fact that light could be polarized. In 1810, French scientist Étienne-Louis Malus developed a mathematical particle theory of polarization, and in 1812, French physicist and mathematician Jean-Baptiste Biot demonstrated how this theory explained all known phenomena of light polarization.
On the other hand, English philosopher and polymath Robert Hooke introduced the “pulse theory” in his 1665 work “Micrographia” to explain the origin of colors. He said that light could spread like waves in water.
Thomas Young, a British scientist and polymath, supported the wave theory of light beginning in the early 19th century. He ran the “interference experiment” that involved passing light through two screens placed one after the other. The first screen had a single slit while the second screen had two slits.
His experiment showed that light passed through the slits placed on the first and second screens and created a pattern of light and dark stripes on the backend. These stripes are now collectively referred to as the interference pattern.
Young concluded that light travels in a wave, similar to a ripple in a pond. Overlapping ripples, particularly when both are in step, produce a brighter light. However, when they are exactly out of step, they cancel out and leave a dark stripe.
Light as an Electromagnetic Radiation According to Classical Physics and Quantum Mechanics
General Descriptions and Definitions
Understanding what electromagnetic radiation is, as well as its specific nature, characteristics, or properties from the different standpoints of classical physics and quantum mechanics is critical to understanding the properties of light.
Because light is a form of electromagnetic radiation, classical physics describes it as energy transmitted at a speed of about 300,000 kilometers or 186,000 miles per second through oscillating electric and magnetic fields or electromagnetic waves. Note that this speed is a known constant commonly referred to as the speed of light. All other forms of electromagnetic radiation such as radio waves, microwaves, infrared radiation, x-rays, and gamma rays also travel at the speed of light.
On the other hand, based on quantum mechanics, light can also be described as a wave or stream of mass-less particles called photons of the electromagnetic field that propagate or radiate through space or a physical medium, carrying radiant energy. Photons are elementary particles. They do not have a substructure. Furthermore, while they have energy and movement, they have no mass and electric charge.
Wave-Particle Dual Properties of Light
The theory of quantum mechanics eventually describes light both as a wave and a particle. This is the widely accepted description and definition of visible light, as well as of other forms of electromagnetic radiation. Below is a rundown of the properties of light:
As an Electromagnetic Radiation
• Because it is a form of electromagnetic radiation, it is part of the electromagnetic spectrum. It has wavelengths ranging from 400 nm to 700 nm, as well as frequencies ranging roughly from 430 THz to 750 THz.
• Of course, as an electromagnetic radiation, it is essentially an oscillating electric and magnetic field. However, it does not carry a charge itself and as such, it does not attract nor repel charged particles like electrons.
• The visible light spectrum represents all electromagnetic radiation that can be seen by the human eye. Humans can qualitatively perceive and recognize its different frequencies, wavelengths, and intensities based on the following colors: red, orange, yellow, green, blue, indigo, and violet.
• Note that the color essentially varies according to wavelength and frequency. Red has the longest wavelength and lowest frequency, and violet has the shortest wavelength and highest frequency.
• Furthermore, within the electromagnetic spectrum, the longer the wavelength, the lower the frequencies. Furthermore, the amount of energy carried by a photon is directly proportional to its electromagnetic frequency and inversely proportional to its wavelength. Violet light has more energy than red light.
• A typical light would appear white. However, it separates into individual bands when it passes through a prism or similar material with prism-like properties. The best natural example is a rainbow.
As a Wave and a Particle Specifically
• Another property of light is that it transmits radiant energy at a constant speed or about 186,000 miles per second through oscillating electric and magnetic fields or electromagnetic waves. Hence, light can also be described as a transportation of energy from one part of the electromagnetic field to another.
• Because it can also be characterized as an electromagnetic wave, it shares some similarities with mechanical waves. It can be measured and quantified by wavelengths, frequencies, and amplitude. It can also be reflected, transmitted, absorbed, refracted, or diffracted when it interacts with matter.
• However, since it is an electromagnetic wave, it has differences from mechanical waves. Specifically, it can propagate through a vacuum or empty space, as well as through solid, liquid, or gas. Note that mechanical waves can only travel through a medium.
• Another property of light is that is fundamentally a wave of mass-less particles called photons carrying radian energy. It is worth mentioning that photons behave like waves and particles at the same time.
• The light the humans see is essentially a stream of photons traveling through space or a medium. The most basic description of a photon is that it is a subatomic or elementary particle made of a packet of energy that has no mass.
• It is produced when the electrons of an object heat up and entered a high-energy state. Photons are principally produced from the movement of excited electrons from their normal position to ones farther from the nucleus. Hotter objects produce more photons, as such, the brighter the light.
