John Kerr (physicist)
Encyclopedia
John Kerr FRS was a Scottish
physicist
and a pioneer in the field of electro-optics
. He is best known for the discovery of what is now called the Kerr effect
.
, Scotland. He was a student in Glasgow from 1841 to 1846, and at the Theological College of the Free Church of Scotland, in Edinburgh, in 1849. Starting in 1857 he was mathematical lecturer at the Free Church Training College
in Glasgow. He died in Glasgow
in 1907.
Kerr's most important experimental work was the discovery of double refraction in solid and liquid dielectrics in an electrostatic field (1875) and of the so-called Kerr effect
. In the Kerr effect, a change in refractive index
is proportional to the square of the electric field
. Where the relationship is linear, the effect is known as the Pockels effect
. Kerr also demonstrated a similar phenomenon for magnetic field
s, and it is now called the magneto-optic Kerr effect
. Light from laser
s allows the achievement of the effect using the light's own electric field, the AC Kerr effect.
The Kerr effect is exploited in the Kerr cell, which is used in applications such as shutter
s in high-speed photography
, with shutter-speeds as fast as 100 ns. In 1928 Karolus & Mittelstaedt used a Kerr cell to modulate
a beam of light to measure its speed
. Earlier measurements had used mechanical means of modulation
achieving frequencies
of around 10 kHz, but the Kerr cell allow frequencies of 10 MHz and greater precision of measurement. Kerr's original cell was a glass
block. Modern cells are more commonly filled with liquid
s such as nitrobenzene.
Kerr also was an early champion of the metric system
in the UK
.
Scotland
Scotland is a country that is part of the United Kingdom. Occupying the northern third of the island of Great Britain, it shares a border with England to the south and is bounded by the North Sea to the east, the Atlantic Ocean to the north and west, and the North Channel and Irish Sea to the...
physicist
Physicist
A physicist is a scientist who studies or practices physics. Physicists study a wide range of physical phenomena in many branches of physics spanning all length scales: from sub-atomic particles of which all ordinary matter is made to the behavior of the material Universe as a whole...
and a pioneer in the field of electro-optics
Electro-optics
Electro-optics is a branch of technology involving components, devices and systems which operate by modification of the optical properties of a material by an electric field...
. He is best known for the discovery of what is now called the Kerr effect
Kerr effect
The Kerr effect, also called the quadratic electro-optic effect , is a change in the refractive index of a material in response to an applied electric field. The Kerr effect is distinct from the Pockels effect in that the induced index change is directly proportional to the square of the electric...
.
Life and work
John Kerr was born on 17 December 1824 at ArdrossanArdrossan
Ardrossan is a town on the North Ayrshire coast in south-western Scotland. The name "Ardrossan" describes its physical position — 'ard' from the Gaelic àird meaning headland, 'ros' a promontory and the diminutive suffix '-an' - headland of the little promontory...
, Scotland. He was a student in Glasgow from 1841 to 1846, and at the Theological College of the Free Church of Scotland, in Edinburgh, in 1849. Starting in 1857 he was mathematical lecturer at the Free Church Training College
Free Church Training College
The Free Church Training College was established by the Free Church of Scotland in 1845 as a college for teacher training.In 1836, David Stow had established a Normal School in Glasgow but, following the Disruption of 1843, a legal ruling of 1845 compelled adherents of the Free Church to resign...
in Glasgow. He died in Glasgow
Glasgow
Glasgow is the largest city in Scotland and third most populous in the United Kingdom. The city is situated on the River Clyde in the country's west central lowlands...
in 1907.
Kerr's most important experimental work was the discovery of double refraction in solid and liquid dielectrics in an electrostatic field (1875) and of the so-called Kerr effect
Kerr effect
The Kerr effect, also called the quadratic electro-optic effect , is a change in the refractive index of a material in response to an applied electric field. The Kerr effect is distinct from the Pockels effect in that the induced index change is directly proportional to the square of the electric...
. In the Kerr effect, a change in refractive index
Refractive index
In optics the refractive index or index of refraction of a substance or medium is a measure of the speed of light in that medium. It is expressed as a ratio of the speed of light in vacuum relative to that in the considered medium....
is proportional to the square of the electric field
Electric field
In physics, an electric field surrounds electrically charged particles and time-varying magnetic fields. The electric field depicts the force exerted on other electrically charged objects by the electrically charged particle the field is surrounding...
. Where the relationship is linear, the effect is known as the Pockels effect
Pockels effect
The Pockels effect , or Pockels electro-optic effect, produces birefringence in an optical medium induced by a constant or varying electric field. It is distinguished from the Kerr effect by the fact that the birefringence is proportional to the electric field, whereas in the Kerr effect it is...
. Kerr also demonstrated a similar phenomenon for magnetic field
Magnetic field
A magnetic field is a mathematical description of the magnetic influence of electric currents and magnetic materials. The magnetic field at any given point is specified by both a direction and a magnitude ; as such it is a vector field.Technically, a magnetic field is a pseudo vector;...
s, and it is now called the magneto-optic Kerr effect
Magneto-optic Kerr effect
Magneto-optic Kerr effect is one of the magneto-optic effects. It describes the changes of light reflected from magnetized media.-Definition:The light that is reflected from a magnetized surface can change in both polarization and reflected intensity...
. Light from laser
Laser
A laser is a device that emits light through a process of optical amplification based on the stimulated emission of photons. The term "laser" originated as an acronym for Light Amplification by Stimulated Emission of Radiation...
s allows the achievement of the effect using the light's own electric field, the AC Kerr effect.
The Kerr effect is exploited in the Kerr cell, which is used in applications such as shutter
Shutter (photography)
In photography, a shutter is a device that allows light to pass for a determined period of time, for the purpose of exposing photographic film or a light-sensitive electronic sensor to light to capture a permanent image of a scene...
s in high-speed photography
Photography
Photography is the art, science and practice of creating durable images by recording light or other electromagnetic radiation, either electronically by means of an image sensor or chemically by means of a light-sensitive material such as photographic film...
, with shutter-speeds as fast as 100 ns. In 1928 Karolus & Mittelstaedt used a Kerr cell to modulate
Modulation
In electronics and telecommunications, modulation is the process of varying one or more properties of a high-frequency periodic waveform, called the carrier signal, with a modulating signal which typically contains information to be transmitted...
a beam of light to measure its speed
Speed of light
The speed of light in vacuum, usually denoted by c, is a physical constant important in many areas of physics. Its value is 299,792,458 metres per second, a figure that is exact since the length of the metre is defined from this constant and the international standard for time...
. Earlier measurements had used mechanical means of modulation
Modulation
In electronics and telecommunications, modulation is the process of varying one or more properties of a high-frequency periodic waveform, called the carrier signal, with a modulating signal which typically contains information to be transmitted...
achieving frequencies
Frequency
Frequency is the number of occurrences of a repeating event per unit time. It is also referred to as temporal frequency.The period is the duration of one cycle in a repeating event, so the period is the reciprocal of the frequency...
of around 10 kHz, but the Kerr cell allow frequencies of 10 MHz and greater precision of measurement. Kerr's original cell was a glass
Glass
Glass is an amorphous solid material. Glasses are typically brittle and optically transparent.The most familiar type of glass, used for centuries in windows and drinking vessels, is soda-lime glass, composed of about 75% silica plus Na2O, CaO, and several minor additives...
block. Modern cells are more commonly filled with liquid
Liquid
Liquid is one of the three classical states of matter . Like a gas, a liquid is able to flow and take the shape of a container. Some liquids resist compression, while others can be compressed. Unlike a gas, a liquid does not disperse to fill every space of a container, and maintains a fairly...
s such as nitrobenzene.
Kerr also was an early champion of the metric system
Metric system
The metric system is an international decimalised system of measurement. France was first to adopt a metric system, in 1799, and a metric system is now the official system of measurement, used in almost every country in the world...
in the UK
United Kingdom
The United Kingdom of Great Britain and Northern IrelandIn the United Kingdom and Dependencies, other languages have been officially recognised as legitimate autochthonous languages under the European Charter for Regional or Minority Languages...
.
Honours
- Honorary LL.D from the University of GlasgowUniversity of GlasgowThe University of Glasgow is the fourth-oldest university in the English-speaking world and one of Scotland's four ancient universities. Located in Glasgow, the university was founded in 1451 and is presently one of seventeen British higher education institutions ranked amongst the top 100 of the...
(1864) - Fellow of the Royal SocietyRoyal SocietyThe Royal Society of London for Improving Natural Knowledge, known simply as the Royal Society, is a learned society for science, and is possibly the oldest such society in existence. Founded in November 1660, it was granted a Royal Charter by King Charles II as the "Royal Society of London"...
(1890) - Royal MedalRoyal MedalThe Royal Medal, also known as The Queen's Medal, is a silver-gilt medal awarded each year by the Royal Society, two for "the most important contributions to the advancement of natural knowledge" and one for "distinguished contributions in the applied sciences" made within the Commonwealth of...
of the Royal Society (1898) - Civil listCivil list-United Kingdom:In the United Kingdom, the Civil List is the name given to the annual grant that covers some expenses associated with the Sovereign performing their official duties, including those for staff salaries, State Visits, public engagements, ceremonial functions and the upkeep of the...
pension (1902)
Further reading
- Obituaries:
- The TimesThe TimesThe Times is a British daily national newspaper, first published in London in 1785 under the title The Daily Universal Register . The Times and its sister paper The Sunday Times are published by Times Newspapers Limited, a subsidiary since 1981 of News International...
, August 19, 1907, 12c
- The Times
- See also
- Kerr, John (1867) An Elementary Treatise on Rational Mechanics, William Hamilton (publisher)
- Steele, R. rev. Anita McConnell (2004) "Kerr, John (1824-1907)", Oxford Dictionary of National Biography, Oxford University Press, accessed 19 June 2005