What is Light?
Light is a kind of wave, somewhat like ocean waves or sound waves. Waves carry energy from one place to another. But light waves don’t need water or air or anything to travel. They can move even in empty space (unlike sound waves).
Topics:
- is Light a Matter or Energy:
— — |Light is a Particle
— — |Light is an Electromagnetic Wave
- Light as Wave
- Light as Electromagnetic Radiation
- The Speed of Light
- Quantum theory of Light
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- Light is a form of electromagnetic radiation that shows properties of both waves and particles. It is a form of energy. Light also keeps the Earth warm. Light exists in tiny energy packets called photons.
- Light waves are made of a mixture of electricity and magnetism so they are called electromagnetic waves. These waves travel very quickly, about 186,000 miles (300,000kilometers) per second. This means a beam of light could go 7 ½ times around the world in one second.
Light is electromagnetic radiation within the portion of the electromagnetic spectrum that is perceived by the human eye.
Also, when we say light, we actually mean visible light which is a tiny part of the electromagnetic spectrum:
- Energy in form of electromagnetic radiation. Electromagnetic radiation consist of an enormous range of wavelengths and frequencies. Gamma rays have the smallest wave lengths because they are the highest energy photons. But most gamma rays are just under ten picometers, which is still way smaller than a hydrogen atom. For reference, a hydrogen atom compared to a cent is about as big as a cent compared to the Moon.
is Light a Matter Or Energy?
Light is both a particle and a wave. Light has properties of both a particle and an electromagnetic wave but not all the properties of either. It consists of photons that travel in a wave like pattern.
The debate has raged for generations amongst the giants of the physics community regarding the nature of light, namely whether it is a particle or an electromagnetic wave. For centuries, this mysterious and elusive phenomenon left scientists baffled because with each experiment conducted to define its nature, it seemed to change the way it behaved.
In simple terms, light is one of nature’s freaky exceptions, and is considered to be both a wave and a particle. This variability is also one of the fundamental tenets of the theory of Quantum Mechanics. Let’s look at what happened over the years as people came to this important conclusion.
Light is a Particle:
The idea that light may be a particle was first advocated by Sir Issac Newton, but the idea didn’t catch on particularly well until the 19th century, when Albert Einstein revived the view. He argued that properties such as the reflection and refraction of light could only be explained if light was made up of particles.
Waves do not travel in straight lines and cannot exhibit those properties outlined by Newton and Einstein. However, if that’s true, then why was light rejected as a particle? The partial answer is that it did not fulfill or have all the properties that define a particle. A particle is a minute fragment or a quantity of matter with certain properties, such as mass and volume. The smallest unit of light is considered to be a photon, which does not have mass. Also, results of experiments by other researchers during the period between Newton and Einstein showed light having wave-like properties, which made them conclude that light was energy, instead of matter.
No!, Light is an Electromagnetic Wave:
A number of scientists, including Fresnel, Young and Maxwell, are credited with investigating the wave-like properties of light. A wave is a transfer of energy from one point to another without the transfer of material between the two points. Young performed the single-slit experiment, which was instrumental in establishing the wave-like properties of light, such as interference and diffraction. He passed a beam of light through a slit and observed the image it formed on the screen placed behind the slit screen.
If the corpuscular theory of light (light is a particle) proposed by Newton was true, then the pattern on the screen should have been light in the shape and size of the slit. However, the light pattern on the screen was more diffused/ diffracted, which indicated that light has an interference property, just like those exhibited by energy waves. Interference is a phenomenon in which two waves (considered to be linear systems) either have an additive or subtractive effect on each other’s intensity, which make the resultant wave either greater or lower in amplitude.
Light as a wave
Isaac Newton’s corpuscular model of light (see Early particle and wave theories) was championed by most of the European scientific community throughout the 1700s, but by the start of the 19th century it was facing challenges. About 1802 Thomas Young, an English physician and physicist, showed that an interference pattern is produced when light from two sources overlaps. Though it took some time for Young’s contemporaries fully to accept the implications of his landmark discovery, it conclusively demonstrated that light has wavelike characteristics. Young’s work ushered in a period of intense experimental and theoretical activity that culminated 60 years later in a fully developed wave theory of light. By the latter years of the 19th century, corpuscular theories were abandoned. Before describing Young’s work, an introduction to the relevant features of waves is in order.
Light as electromagnetic radiation
In spite of theoretical and experimental advances in the first half of the 19th century that established the wave properties of light, the nature of light was not yet revealed — the identity of the wave oscillations remained a mystery. This situation dramatically changed in the 1860s when the Scottish physicist James Clerk Maxwell, in a watershed theoretical treatment, unified the fields of electricity, magnetism, and optics. In his formulation of electromagnetism, Maxwell described light as a propagating wave of electric and magnetic fields. More generally, he predicted the existence of electromagnetic radiation: coupled electric and magnetic fields traveling as waves at a speed equal to the known speed of light. In 1888 German physicist Heinrich Hertz succeeded in demonstrating the existence of long-wavelength electromagnetic waves and showed that their properties are consistent with those of the shorter-wavelength visible light
The Speed of light
The speed of light in a vacuum is defined to be exactly 299 792 458 m/s (approx. 186,282 miles per second). The fixed value of the speed of light in SI units results from the fact that the metre is now defined in terms of the speed of light. All forms of electromagnetic radiation move at exactly this same speed in vacuum.
Different physicists have attempted to measure the speed of light throughout history. Galileo attempted to measure the speed of light in the seventeenth century. An early experiment to measure the speed of light was conducted by Ole Rømer, a Danish physicist, in 1676. Using a telescope, Rømer observed the motions of Jupiter and one of its moons, Io. Noting discrepancies in the apparent period of Io’s orbit, he calculated that light takes about 22 minutes to traverse the diameter of Earth’s orbit.[15] However, its size was not known at that time. If Rømer had known the diameter of the Earth’s orbit, he would have calculated a speed of 227 000 000 m/s
According to physicist Albert Einstein’s theory of special relativity, on which much of modern physics is based, nothing in the universe can travel faster than light. The theory states that as matter approaches the speed of light, the matter’s mass becomes infinite. That means the speed of light functions as a speed limit on the whole universe. The speed of light is so immutable that, according to the U.S. National Institute of Standards and Technology, it is used to define international standard measurements like the meter (and by extension, the mile, the foot and the inch). Through some crafty equations, it also helps define the kilogram and the temperature unit Kelvin.
Quantum theory of light
By the end of the 19th century, the battle over the nature of light as a wave or a collection of particles seemed over. James Clerk Maxwell’s synthesis of electric, magnetic, and optical phenomena and the discovery by Heinrich Hertz of electromagnetic waves were theoretical and experimental triumphs of the first order. Along with Newtonian mechanics and thermodynamics, Maxwell’s electromagnetism took its place as a foundational element of physics.
However, just when everything seemed to be settled, a period of revolutionary change was ushered in at the beginning of the 20th century. A new interpretation of the emission of light by heated objects and new experimental methods that opened the atomic world for study led to a radical departure from the classical theories of Newton and Maxwell — quantum mechanics was born. Once again the question of the nature of light was reopened.
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