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Historical theories about light, in chronological order light | Gurugrah


Light –

Light or visible light is electromagnetic radiation that can be seen by the human eye. Visible light is usually defined as having wavelengths in the range of 400–700 nanometers (nm), which lie between the infrared (with longer wavelengths) and the ultraviolet (with shorter wavelengths) at 750–420 terahertz. Corresponds to the frequencies.

In physics, the term “light” can refer more broadly to electromagnetic radiation of any wavelength, whether visible or not. In this sense, gamma rays, X-rays, microwaves and radio waves are also light. The primary properties of light are intensity, propagation direction, frequency or wavelength spectrum, and polarization. Its speed in a vacuum, 299 792 458 meters per second (m/s), is one of the fundamental constants of nature. Like all types of electromagnetic radiation, visible light is propagated largely by elementary particles called photons that represent quanta of the electromagnetic field, and can be analyzed as both waves and particles. . The study of light, known as optics, is an important research area in modern physics.

The main source of natural light on Earth is the Sun. Historically, another important source of light for humans has been fire, from ancient campfires to modern kerosene lamps. With the development of electric lights and power systems, electric lighting has effectively replaced firelight.

Electromagnetic Spectrum and visible light –

Generally, electromagnetic radiation (EMR) is classified by wavelength into radio waves, microwaves, infrared, the visible spectrum that we see as light, ultraviolet, X-rays, and gamma rays. The designation “radiation” excludes static electric, magnetic, and near fields.

The behaviour of EMR depends on its wavelength. Higher frequencies have shorter wavelengths and lower frequencies have longer wavelengths. When EMR interacts with single atoms and molecules, its behaviour depends on the amount of energy per quantum.

EMR in the visible light region consists of quanta (called photons) which are at the low end of the energy spectrum and capable of causing electronic excitation within molecules, leading to binding or a change in the chemistry of the molecule. At the low end of the visible light spectrum, EMR becomes invisible to humans (infrared) because its photons no longer have sufficient individual energy to cause a permanent molecular change (change in conformation) in the visual molecule retinal in the human retina. , which triggers the sensation of change in vision.

There exist animals that are sensitive to different types of infrared, but not through quantum absorption. Infrared sensing in snakes relies on a type of natural thermal imaging, in which tiny packets of cellular water are raised in temperature by infrared radiation. EMR in this range causes molecular vibrations and heating effects, which these animals detect.

Different sources define visible light as narrowly defined as 420–680 nm to broadly defined as 380–800 nm. Under ideal laboratory conditions, people can see infrared to at least 1,050 nm; Children and young adults can perceive ultraviolet wavelengths up to approximately 310–313 nm. Plant growth is also affected by the colour spectrum of light, a process known as photomorphogenesis.

Speed of Light –

The speed of light in a vacuum is defined to be exactly 299 792 458 m/s (about 186,282 miles per second). The fixed value of the speed of light in SI units is a result of the fact that the meter is now defined in terms of the speed of light. All forms of electromagnetic radiation travel at exactly the same speed in a vacuum.

Various 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 performed in 1676 by Ole Romer, a Danish physicist. Using a telescope, Roemer observed the motions of Jupiter and one of its moons, Io. Taking into account discrepancies in the apparent period of Io’s orbit, he calculated that it takes about 22 minutes for light to cross the diameter of Earth’s orbit. However, its size was not known at that time. If Romer had known the diameter of the Earth’s orbit, he would have calculated a speed of 227,000,000 m/s.

Two independent teams of physicists are said to have uncovered a “complete impasse” by passing it through a Bose-Einstein condensate of the element rubidium, one team at Harvard University and the Rowland Institute for Science in Cambridge, Massachusetts, and the other at Harvard-Smithsonian Center for Astrophysics, also in Cambridge. However, the popular description of light being “turned off” in these experiments simply refers to light stored in the excited states of atoms, then arbitrarily re-emitted when excited by a second laser pulse at a later time. During the time it was "turned off”, it ceased to be light.

Optics –

The study of light and the interaction between light and matter is called optics. The observation and study of optical phenomena such as rainbows and the aurora borealis provide many clues about the nature of light.

Refraction –

Refraction is the bending of light rays when passing through a surface between one transparent material and another. This is described by Snell’s law:

N1 sin O1=n2 sin O2 .

Where θ 1 is the angle between the ray and the surface in the first medium, θ 2 is the angle between the ray and the surface in the second medium and n 1 and n 2 are the indices of refraction, n = 1 in a vacuum and n > 1 in an in a transparent.

When a beam of light crosses a boundary between a vacuum and another medium, or between two different mediums, the wavelength of the light changes, but the frequency remains constant. If the beam of light is not orthogonal (or rather normal) to the boundary, then a change in wavelength causes a change in the direction of the beam. This change of direction is known as refraction.

The refractive quality of lenses is often used to manipulate light in order to change the apparent size of images. Magnifying glasses, eyeglasses, contact lenses, microscopes, and refracting binoculars are all examples of this manipulation.

Sources of Light –

There are many sources of light. A body at a given temperature emits a characteristic spectrum of black-body radiation. A simple thermal source is a sunlight, the radiation emitted by the Sun’s chromosphere at about 6,000 kelvin (5,730 °C; 10,340 °F) with a maximum in the visible region of the electromagnetic spectrum when plotted in wavelength units and roughly 44%. Is done.

The energy of sunlight that reaches the ground is visible. Another example is the incandescent light bulb, which emits only about 10% of its energy as visible light and the rest as infrared. A common thermal light source in history has been solid particles glowing in flames, but these also emit most of their radiation in the infrared and only a fraction in the visible spectrum.

Historical theories about light, in chronological order –

1. Classical Greece and Hellenism

2. Classical India

3. Descartes

4. Particle Theory

5. Wave theory

6. Electromagnetic theory

7. Quantum Theory

Use for light on earth –

Sunlight provides the energy that green plants use to make sugars, mostly in the form of starch, which releases energy into the living things that digest them. This process of photosynthesis provides almost all the energy used by living things. Some species of animals produce their own light, a process known as bioluminescence. For example, fireflies use light to locate mates and vampire squids use it to camouflage themselves from prey.


By Chanchal Sailani | November 19, 2022, | Editor at Gurugrah_Blogs.



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