What is light? – Physical models of lightI. Moreno, M. M. Sánchez-López, September 1st, 2017http://tecnopto.edu.umh.es

What is light?Light is part of the electromagnetic radiation spectrum and is a form of energy.

Light bending at SunsetImage Credit: Thomas DeHoff - http://lightexhibit.org/bio_image14.html

Light is usually considered to be the visible part of thespectrum. However, in physics, light is defined as allportions of the electromagnetic scale, includinginvisible forms such as infrared, ultraviolet, x rays, radio waves, and more.

Light energy can be described as a wave, a particle (orphoton), or a combination of both (called the wave-particle duality).

The observations of how light behaves with matterdemonstrate its various properties as a wave, particle, or ray.

Physical models of light

“The study of light has resulted in achievements of intuition and imagination that are unmatched in any field of human mental activity. It also illustrates better than any other branch of Physics vicissitudes of theories.” 1925, Sir Joseph J. Thomson (1856-1940), Physics Nobel Prize (1906) for the discovery of the electron.

Optics is the branch of science thatstudies light phenomena

Ray Optics Wave (EM) Optics Quantum Optics

Alhacen965-1940

Snelius1580-1626

Newton1643-1727

Young1773-1829

Fresnel1778-1827

Maxwell1831-1879

Planck1858-1947

Einstein1879-1965

Townes1915-2015

What is light? – Physical models of lightI. Moreno, M. M. Sánchez-López, September 1st, 2017http://tecnopto.edu.umh.es

Ray Optics – Geometrical Optics

Pierre de Fermat1601-1665

Willebrord Snell1580-1626

Ray Optics, or Geometrical Optics, describes light propagation in terms of rays. The ray is an abstraction useful for approximating the paths along which light propagates.

It is valid as long as light does not encounters objects with sizes close to the wavelength of light (micron scale).

It describes optical phenomena on the basis of Fermat principle, including reflection and refraction (Snell law), and instruments such as mirrors, lenses, even some aspects of optical fibers.

What is light? – Physical models of lightI. Moreno, M. M. Sánchez-López, September 1st, 2017http://tecnopto.edu.umh.es

Ray Optics

Despite being an old theory, ray optics continue to be the basis of many modern instruments, like microscopes, telescopes, solar concentrators or lithographic systems for microelectronics.

http://www.agefotostock.com/en/Stock-Images/Rights-Managed/B20-1081688

http://www.mitechworld.com/Pro_Images/20110215103620M10XB-PC.jpg

http://www.raycotechnologies.org/tracking_solar_concentrator.htm http://nanotechnologytoday.blogspot.com.es/2012/03

/nanoscale-metal-lithography-into-three.html

What is light? – Physical models of lightI. Moreno, M. M. Sánchez-López, September 1st, 2017http://tecnopto.edu.umh.es

Ray Optics

Isaac Newton 1643-1727

http://upload.wikimedia.org/wikipedia/commons/thumb/c/cc/NewtonsTelescopeReplica.jp

g/250px-NewtonsTelescopeReplica.jpg http://colt-rane.com/wp-content/uploads/461px-newton-opticks.jpg

One of the major contributors to the development of the ray optics theory was Isaac Newton, through his treatise “Optiks”.

He developed a number of instruments, including the Newtonian telescope configuration, still in use, and the discovery of the color decomposition of white light.

However, his adoption of a particle based view of light delayed the development of the wave theory of light for around a century.

What is light? – Physical models of lightI. Moreno, M. M. Sánchez-López, September 1st, 2017http://tecnopto.edu.umh.es

Wave Optics

Wave nature of optics was proposed by Huygens, but remained unconsidered until the milestone experiment by Young: the double slit interference experiment.

Thomas Young 1773-1829

Christiaan Huygens1629-1695

http://www.kirksville.k12.mo.us/khs/teacher_web/alternative/wave_interference.jpg

http://homepages.physik.uni-muenchen.de/~Otto.Schaile/macros/waves.gif

http://www.itp.uni-hannover.de/~zawischa/ITP/bildchen/spalt02.png

Interference of waves in waterWhat is light? – Physical models of lightI. Moreno, M. M. Sánchez-López, September 1st, 2017http://tecnopto.edu.umh.es

Wave Optics

Wave optics is the basis of many modern technological techniques including metrological systems to obtain extremely precise measurements, optical filters, optical discs, photonic crystal fibers, spectrometers….

What is light? – Physical models of lightI. Moreno, M. M. Sánchez-López, September 1st, 2017http://tecnopto.edu.umh.es

Electromagnetic Optics

The electromagnetic nature of light waves was discovered by Maxwell, derived from his famous four equations.

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xy

James Clerk Maxwell 1831 -1879

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Wave equation:

Maxwell equations in vacuum

Propagation speed:

What is light? – Physical models of lightI. Moreno, M. M. Sánchez-López, September 1st, 2017http://tecnopto.edu.umh.es

Electromagnetic Optics

Visible light is only a small portion of the electromagnetic spectrum. Nowadays light is also considered in other forms of invisible light such as infrared, ultraviolet, x rays, or microwaves.

The vectorial and transversal nature of the EM wave is used in many technological modern systems such as displays or 3D glasses.

http://www.digital-digest.com/images/newsimages/RealD-polarized-3d-glasses.jpg

http://image.made-in-china.com/2f0j00wKptsBLGAQkJ/65-Inch-LCD-CCTV-Monitor-Liquid-Crystal-Display-TV.jpg

http://infothread.org/Science/Physics/

What is light? – Physical models of lightI. Moreno, M. M. Sánchez-López, September 1st, 2017http://tecnopto.edu.umh.es

Quantum Optics

At the beginning of XX century the Quantum theory is developed, starting with the explanation of the blackbody radiation by Max Plank.

Light radiation is discovered to be quantized in energy packets of energy hf.

Max Planck 1858-1947

Physics Nobel Prize -1918

What is light? – Physical models of lightI. Moreno, M. M. Sánchez-López, September 1st, 2017http://tecnopto.edu.umh.es

Solar spectrum: light emitted by theSun follows a blackbody radiatiorat the temperature of 5800K.

Photon energy: the quentization of light energy led to the concept of photons, radiation packets of energy hf. Its interaction withelectrons explain for instance how cameras work, including visible and infra-red (thermal) cameras.

Quantum Optics

Quantum optics provides the description of interaction of light and matter.

Bohr introduced the atomic era by explaining the light emission form the hydrogen lamp, and introduced the concept of spontaneous emission.

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n=1

n=2

n=3

e

r1

r2

r3

Niels Bohr1885-1962Physics Nobel Prize - 1922

Bohr’s model for the hydrogen atomand its light emission spectrum

What is light? – Physical models of lightI. Moreno, M. M. Sánchez-López, September 1st, 2017http://tecnopto.edu.umh.es

Spontaneous emission: emission of a photon when anexcited electron decays to a lower energy level.

Quantum Optics

Einstein explained the photoelectric effect based on the light quanta concept.

He also introduced the concept of stimulated emission, that later led to the development of masers and lasers.

What is light? – Physical models of lightI. Moreno, M. M. Sánchez-López, September 1st, 2017http://tecnopto.edu.umh.es

Albert Einstein 1879-1955

Physics Nobel Prize - 1921

Photoelectric effect: electrical current generated byillumination of a cathode

Stimulated emission: emission of a photon when an excited electron decaysto a lower energy level in the pressence of another photon; the twophotons are identical.