Light

Principle

Light is the part to the electromagnetic spectrum that is visible to the animal eye. The study of light and the interaction of light and matter is termed optics.

The elementary particle that defines light is the photon. The three basic dimensions of light (or better all electromagnetic radiation) are:

· intensity (or amplitude), which is related to the human perception of brightness of the light;

· frequency (or wavelength), perceived by humans as the color of the light, and

· polarization (or angle of vibration), which is only weakly perceptible by humans under ordinary circumstances.

Due to the wave-particle duality of matter light simultaneously exhibits properties of both waves and particles.

Here follows a description of the most important features of light.

Speed of light The speed of light in a vacuum is exactly 299,792,458 m/s (fixed by definition).

Refraction When light goes from the one to another medium, it is refracted (see Light: refraction) and reflected (see Fresnel equations).

Dispersion Since refraction is frequency dependent, the refracted beam is decomposed in its various frequencies (or wavelengths) which all have their own angle of refraction. The classical way to achieve this is with a prism, see Fig. 1.

Fig. 1 Dispersion of a light beam in a prism.

The visible spectrum (see Fig. 1).

Electromagnetic radiation from 400 to 700 nm is called visible light or simply light. However, some people may be able to perceive wavelengths from 380 to 780 nm. A light-adapted eye typically has its maximum sensitivity at around 555 nm, which is in the green region (see: Luminosity function). In day light, so with photopic vision, the different wavelengths are detected by the human eye and then interpreted by the brain as colors. The spectrum does not, however, contain all the colors that the human eyes and brain can distinguish. For instance, brown and pink are absent. See Color vision to understand why.

The optical spectrum includes not only visible light, but also ultraviolet (UV) at the short wavelength (high frequency) end and infrared (IR) at the long wavelength end. Some animals, such as bees, can see UV radiation while others, such as pit viper snakes, can see IR light.

Fig. 1 The part of the optical spectrum visible to the human eye.

Polarization With reflection and refraction light is also polarized to some extent (see Light: polarization). Polarization describes the direction of the electric oscillation in the plane perpendicular to the direction of propagation.

Diffraction This refers to phenomena associated with wave propagation, such as the bending, spreading and interference of waves emerging from an aperture on the order of the wavelength (pinhole and narrow-split experiments). For more explanation see Light: diffraction.

Absorption When light propagates trough a medium some of its energy is absorbed by the medium (see Lambert-Beer law). In general, all or most of the absorbed energy is transformed to heat. The part that is not transformed to heat can be emitted as radiation (see Chemoluminescence and Bioluminescence, Fluorescence, Phosphorescence) or transformed to electric current (the photoelectric effect, see More Info).

Scattering Scattering of is a process whereby light (and sound or moving particles), are forced to deviate from a straight trajectory by one or more localized non-uniformities in the medium through which it passes. This also includes deviation of reflected radiation from the angle predicted by the law of reflection (called diffuse reflections). An example is scattering of light in the eye lens and intraretinal scatter. See further Light: Scattering.

Theories about light

Classical particle theory (Newton)

Light was assumed to be composed of corpuscles (particles of matter) which were emitted in all directions from a source. This theory cannot explain many of the properties of light. It wrongly assumed a higher speed in a denser medium. The classical particle theory was finally abandoned around 1850.

Classical wave theory (Huygens)

Light was (and is) assumed to be emitted in all directions as a series of waves in a medium (Fig. 3). As waves are not affected by gravity, it was assumed that they slowed down upon entering a denser medium. It can explain phenomena such as refraction, polarization, dispersion and diffraction. It was wrongly assumed that light waves would need a medium for transmission (like sound waves indeed need).

Fig. 3 Interference of the waves emitted by two sources.

Fig. 4 A linearly-polarized light wave frozen in time and showing the two oscillating components of light; an electric field and a magnetic field perpendicular to each other and to the direction of motion.

Electromagnetic theory

The angle of polarization of a beam of light as it passed through a polarizing material could be altered by a magnetic field, an effect now known as Faraday rotation. It is one of the arguments that light is a high-frequency electromagnetic vibration, which could propagate even in the absence of a medium such as the “ether”. The frequency range of light is only a very small part of the whole electromagnetic range. Other parts of the electromagnetic spectrum are applied in e.g. radio, radar, television, electromagnetic imaging (see Spectroscopy).

Application

Medical Many medical instruments and apparatus are based on light for imaging, as do also prostheses like spectacles etc. All this things will not be discussed. Here some applications based on UV and IR light are mentioned. .

UV radiation is not normally directly perceived by humans except in a much delayed fashion, as overexposure of the skin. UV light can cause sunburn, or skin cancer. Underexposure can cause vitamin D deficiency. However, because UV is a higher frequency radiation than visible light, it very easily can cause materials to fluorescence visible light.

Thermography is performed with a camera using IR light. In general heating of the skin or the whole body by radiation is caused by IR light. However, any intense radiation can have the same effect. Other examples are UV and IR spectroscopy (see Spectroscopy).

Technical IR cameras convert IR light to visible light. Depending on their application we distinguish night-vision binoculars, cameras. These are different from image intensifier cameras, which only amplify available visible light.

More info

The Special Theory of Relativity

The wave theory explains nearly all optical and electromagnetic phenomena, but some anomalic phenomena remained that could not be explained:

- the constant speed of light,

- the photoelectric effect,

- black body radiation.

The constant speed of light contradicted the mechanical laws of motion, which stated that all speeds were relative to the speed of the observer. This paradox was resolved by revising Newton's laws of motion into Einstein’s special theory of relativity.

The photoelectric effect, being the ejection of electrons when light strikes a metal surface, causing an electric current to flow out. The explanation is given by the wave-particle duality and quantum mechanics.

A third anomaly involved measurements of the electromagnetic spectrum emitted by thermal radiators, or so-called black bodies (see Wien's displacement law and Body heat dissipation and related water loss). The explanation is given by the Quantum theory. The theory of black body radiation says that the emitted light (and other electromagnetic radiation) is in the form of discrete bundles or packets of energy. These packets were called quanta, and the particle of light was given the name photon, just as other particles, such as an electron and proton. A photon has an energy, E, proportional to its frequency, f:

E = hf = hc/λ,

where h is Planck's constant ( = 6,623∙10^-34 Js), λ is the wavelength and c is the speed of light. Likewise, the momentum (mass times speed) p of a photon is also proportional to its frequency and inversely proportional to its wavelength:

p = E/c = hf/c = h/λ.

Wave-particle duality and of quantum electrodynamics

The modern theory that explains the nature of light is the wave-particle duality, founded by quantum theory. More generally, the theory states that everything has both a particle nature and a wave nature, and various experiments can be done to bring out one or the other. The particle nature is more easily discerned if an object has a large mass, but also particles, such as electrons and protons exhibited wave-particle duality. The quantum mechanical theory of light and electromagnetic radiation culminated with the theory of quantum electrodynamics, or QED.