Light

Light, electromagnetic radiation that stay inside the spectrumcan area and be detected by the human eye(which is about 380 nm đến 700 nm) . Electromagnetic radiation occurs over an extremely wide range of wavelengths, from gamma rays with wavelengths less than about 1 × 10−11 metre to radio wavesmeasured in metres. Within that broad spectrum the wavelengths visible to humans occupy a very narrow band, from about 700 nanometres (nm; billionths of a metre) for red light down to about 400 nm for violet light. The spectral regions adjacent to the visible band are often referred to as light also, infrared at the one end and ultraviolet at the other. The speed of light in a vacuum is a fundamental physical constant, the currently accepted value of which is exactly 299,792,458 metres per second, or about 186,282 miles per second.

## Ligth's properties

### **Speed in the vaccum**

In vacuum, the experiment has proved in particular light, or electromagnetic radiation in general, go to the velocity does not change, often denoted as c = 299,792,458 m / s, even without additives depending on the reference system. This phenomenon has changed the views of classical mechanics of Isaac Newton and Albert Einstein found to promote the theory of relativity.

### **Energy, momentum and volume**

The energy of a photon with a wavelength λ is hc / λ, where h is Planck's constant and c is the speed of light in vacuum. The photon has no rest mass, so the momentum of the photon with its energy divided by the speed of light, h / λ. Calculated on the formula derived from the theory of relativity:

### **E2-p2c2 = m02c4** with:

**E** is the energy of the particle

**p** is the momentum of the particle

**m0** is the rest mass

## Theories

In exploring the history, there have been many theories to explain natural phenomena involving light. Here presents important theories, according to historical sequence.

### **Particle theory of light**

Particle theory of light, was made by Isaac Newton, for that is the light line movement of material particles. This theory explains the phenomenon of reflection and some other properties of light; however unexplained phenomena as interference, diffraction waves in nature.

### **Wave theory of light**

Wave theory of light, Christiaan Huygens was launched, said that the line of light is of wave propagation. This theory explains many of the phenomena of the wave nature of light as interference, diffraction; and better explain the phenomenon of refraction and reflection.

Wave theory and particle theory of light was born at the same time, the 17th century and has caused great debate between the two schools.

1817, Thomas Young proposed that light waves are transverse waves, not longitudinal waves. We oscillate perpendicular to the propagation direction, and not in the direction of transmission, as for the sound wave.

### **Electromagnetic Theory**

After the wave theory and particle theory was born, electromagnetic theory of James Clerk Maxwell in 1865, confirming again the wave nature of light. In particular, this theory connects the optical phenomenon to study electromagnetic phenomena, light shows only a particular case of electromagnetic waves.

The latter experiments on electromagnetic waves, such as Heinrich Rudolf Hertz's 1887, have confirmed the accuracy of Maxwell's theory.

### **Ether**

After the success of electromagnetic theory, the concept that light waves spread as widely accepted. The understanding of the mechanical waves, such as sound of classical mechanics, has led scientists to hypothesize that the light wave propagation as mechanical waves in the ether assumption environment, pervades the universe, but higher hardness than diamond.

Late 19th century, early 20th century, many experiments seeking the existence of the ether, as the Michelson-Morley experiment, has failed, at the same time they show that the speed of light is constant regardless of the reference system ; so the environment can not exist a fixed spread type of ether.

**Relativity**

 Albert Einstein's theory of relativity was born, in 1905, with the initial aim is to explain the phenomenon of the speed of light is not dependent reference system and the non-existence of the ether environment, by changing the mechanical constraints classic.

In relativity theory, the axioms of mechanics were changed, to ensure through transformations reference system, the speed of light is constant. This theory has unexplained movement of objects at high speed and continue to expand the theory of general relativity, which explained the motion of light in particular and in general the material in space distorted by matter.

The experiment measured the bending of the light path of the stars when passing close to the sun, the eclipse for the first time in 1919, has confirmed the accuracy of the theory of general relativity.

### **Quantum theory of light**

Quantum theory of light and matter in particular, generally when the experimental introduction of blackbody radiation is explained by Max Planck and photoelectric effect was explained by Albert Einstein are needed to hypothesize that light light is the movement of individual particles, called photons (photon).

Because of the wave nature of particles and observed at the light, and for all the material in general, quantum theory came to the conclusion about the wave-particle duality of light and matter; summarized in the formula de Broglie, 1924, contact between a particle momentum and its wavelength.