## Saturday, January 22, 2022

### Basics of Light: UNIZOR.COM - Physics4Teens - Waves - Properties of Light

Notes to a video lecture on http://www.unizor.com

Basic Characteristics of Light

What is Light?

When we talk about light, we mean electromagnetic waves (oscillations of the electromagnetic field) that our eyes can detect.
Not all the oscillations of electromagnetic field are sensed by our eyes, but only within a visible spectrum of frequencies. This spectrum of frequencies of visible light varies for different people, but, in general, it's usually defined as from flow = 4·1014Hz to fhigh = 8·1014Hz.

With the speed of light in vacuum approximately c = 300,000,000 m/sec, using the formula for the wavelength λ=c/f, we can approximate the low and high wavelengths for visible light:
λlow = c/fhigh = 750·10−9(m) =
= 750(nm)

λhigh = c/flow = 375·10−9(m) =
= 375(nm)

Colors

Traditionally, we divide the visible spectrum of light based on the difference in how we sense it in terms of different colors.
Though different people see colors slightly differently, here is the division by colors, as is traditionally defined, as a function of the wavelength in nanometers:

As you see from the picture above, the color becomes almost black, when we approach high and low boundaries of visible spectrum, that is the light becomes almost invisible for the eyes, though younger people usually have more sensitive eyes and see a slightly broader spectrum of light.
Invisible for an eye light with wavelength of less than 375 nm is called ultraviolet.
Invisible for an eye light with wavelength of greater than 750 nm is called infrared.

Speed

The speed of light mentioned above as 300,000,000 m/sec is an approximation. The exact speed depends on the substance where the light propagates.
In vacuum it's the fastest.

In vacuum it's exactly 299,792,458 m/sec. We emphasize the exactness of this speed because in SI system of units meter is defined through a speed of light, as the length traveled by light in vacuum during the time
T = 1 /299,792,458 sec.
Speed of light in water is slower than in vacuum by, approximately, 1.33 times and equals to 2.25·108m/sec. Obviously, it depends on the chemical composition of water.
Analogously, speed of light is different in all translucent substances, but always slower than in vacuum.
According to the Theory of Relativity by Albert Einstein, the speed of light in vacuum is the fastest speed possible to achieve.

Source

There are many different sources of light.

Chemical reaction can produce a visible light. For example, coal or wood burning is a chemical reaction between carbon in the coal or wood and oxygen in the air, producing carbon dioxide and energy in a form of heat and visible light.

Electric current can be a source of light, when a sufficiently strong flow of electrons passes through a conductor, producing heat and light.
This can be observed in the incandescent lamps.

Nuclear reactions of fission and fusion, occurring within stars, including our Sun, produces visible light.

Luminescence is a general term that encompasses close in their nature but slightly different sources of light:
fluorescence,
phosphorescence,
chemiluminescence.
They all involve absorption of light energy in some form and its emission as a visible light of different wavelengths immediately after absorption or at a later time.

Recently new way of producing light is light emitting diodes (LED).

Theories

There have been many theories of light, each one explaining this or that property of light. Discovery of each new property of light was the cause to re-evaluate the concept of light and, in most cases, developing a new theory.

Particle or corpuscular theory of light was developed, primarily, by Pierre Gassendi, Isaac Newton and other scientists. According to this theory, light consists of particles (corpuscles) emitted by the source and flying in all directions.
The corpuscular theory explained many properties of light, but had problems explaining certain observable phenomenons, like interference. Eventually, this theory was rejected by scientists.

Wave theory explained quite well such property as interference, but required a medium for wave propagation - aether. Many scientists contributed to this theory, including Hooke, Huygens and others. Numerous experiments, however, contradicted the concept of aether and, eventually, this theory was rejected as well.

Electromagnetic theory of light became the dominant because of work by Faraday, Maxwell and Hertz. According to this theory, light is the oscillations of electromagnetic field with variable electric and magnetic components causing each other. This theory is the foundation of contemporary usage of radio waves, including TV, cell phones, remote controls etc.

Quantum theory complemented the electromagnetic theory and provided a better explanation of certain corpuscular properties of light, like photoelectric effect. Works of Planck, Einstein and other physicists were essential to developing the quantum theory of light, which is now considered as the current model.