John Kerr FRS (; 17 December 1824 – 15 August 1907) was a Scottish

physicist A physicist is a scientist who specializes in the field of physics, which encompasses the interactions of matter and energy at all length and time scales in the physical universe. Physicists generally are interested in the root or ultimate cau ...

and a pioneer in the field of

electro-optics Electro–optics is a branch of electrical engineering, electronic engineering, materials science, and material physics involving components, electronic devices such as lasers, laser diodes, LEDs, waveguides, etc. which operate by the prop ...

. He is best known for the discovery of what is now called the

Kerr effect The Kerr effect, also called the quadratic electro-optic (QEO) 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 chan ...

Life and work

John Kerr was born on 17 December 1824 in

Ardrossan Ardrossan (; ) is a town on the North Ayrshire coast in southwestern Scotland. The town has a population of 10,670 and forms part of a conurbation with Saltcoats and Stevenston known as the 'Three Towns#Scotland, Three Towns'. Ardrossan is loca ...

, 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 a mathematical lecturer at the Free Church Training College in Glasgow. He died in

Glasgow Glasgow is the Cities of Scotland, most populous city in Scotland, located on the banks of the River Clyde in Strathclyde, west central Scotland. It is the List of cities in the United Kingdom, third-most-populous city in the United Kingdom ...

in 1907. Kerr's most important experimental work was the discovery of double refraction in solid and liquid dielectrics in an electrostatic field (1875) which is now known as the

. In the Kerr effect, the difference between

refractive index In optics, the refractive index (or refraction index) of an optical medium is the ratio of the apparent speed of light in the air or vacuum to the speed in the medium. The refractive index determines how much the path of light is bent, or refrac ...

experienced by an ordinary and extraordinary ray is proportional to the square of the

electric field An electric field (sometimes called E-field) is a field (physics), physical field that surrounds electrically charged particles such as electrons. In classical electromagnetism, the electric field of a single charge (or group of charges) descri ...

. Where the relationship is linear, the effect is known as the

Pockels effect In optics, the Pockels effect, or Pockels electro-optic effect, is a directionally-dependent linear variation in the refractive index of an optical medium that occurs in response to the application of an electric field. It is named after the Ge ...

. Intense light from

laser A laser is a device that emits light through a process of optical amplification based on the stimulated emission of electromagnetic radiation. The word ''laser'' originated as an acronym for light amplification by stimulated emission of radi ...

s allows the achievement of the effect using the light's own electric field, the ''AC Kerr effect''. Kerr also demonstrated a similar phenomenon for

magnetic field A magnetic field (sometimes called B-field) is a physical field that describes the magnetic influence on moving electric charges, electric currents, and magnetic materials. A moving charge in a magnetic field experiences a force perpendicular ...

s, and it is now called the

magneto-optic Kerr effect In physics the magneto-optic Kerr effect (MOKE) or the surface magneto-optic Kerr effect (SMOKE) is one of the magneto-optic effects. It describes the changes to light reflected from a magnetized surface. It is used in materials science research ...

. The Kerr effect is exploited in the ''Kerr cell'', which is used in applications such as shutters 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 In kinematics, the speed (commonly referred to as ''v'') of an object is the magnitude of the change of its position over time or the magnitude of the change of its position per unit of time; it is thus a non-negative scalar quantity. Intro ...

. Earlier measurements had used mechanical means of

modulation Signal modulation is the process of varying one or more properties of a periodic waveform in electronics and telecommunication for the purpose of transmitting information. The process encodes information in form of the modulation or message ...

achieving

frequencies Frequency is the number of occurrences of a repeating event per unit of time. Frequency is an important parameter used in science and engineering to specify the rate of oscillatory and vibratory phenomena, such as mechanical vibrations, audio ...

of around 10

kHz The hertz (symbol: Hz) is the unit of frequency in the International System of Units (SI), often described as being equivalent to one event (or cycle) per second. The hertz is an SI derived unit whose formal expression in terms of SI base uni ...

, but the Kerr cell allows frequencies of 10

MHz The hertz (symbol: Hz) is the unit of frequency in the International System of Units (SI), often described as being equivalent to one event (or cycle) per second. The hertz is an SI derived unit whose formal expression in terms of SI base u ...

and greater precision of measurement. Kerr's original cell was a glass block. Modern cells are more commonly filled with liquids such as

nitrobenzene Nitrobenzene is an aromatic nitro compound and the simplest of the nitrobenzenes, with the chemical formula C6H5 NO2. It is a water-insoluble pale yellow oil with an almond-like odor. It freezes to give greenish-yellow crystals. It is produced ...

.Steele (2004) Kerr also was an early champion of the

metric system The metric system is a system of measurement that standardization, standardizes a set of base units and a nomenclature for describing relatively large and small quantities via decimal-based multiplicative unit prefixes. Though the rules gover ...

in the UK.See Kerr, J. (1863) ''The Metric System, Its Prospects in this Country'', Effingham Wilson (publisher)

Honours

* Honorary

LL.D A Doctor of Laws (LL.D.) is a doctoral degree in legal studies. The abbreviation LL.D. stands for ''Legum Doctor'', with the double “L” in the abbreviation referring to the early practice in the University of Cambridge to teach both canon law ...

from the

University of Glasgow The University of Glasgow (abbreviated as ''Glas.'' in Post-nominal letters, post-nominals; ) is a Public university, public research university in Glasgow, Scotland. Founded by papal bull in , it is the List of oldest universities in continuous ...

(1864) * Fellow of the

Royal Society The Royal Society, formally The Royal Society of London for Improving Natural Knowledge, is a learned society and the United Kingdom's national academy of sciences. The society fulfils a number of roles: promoting science and its benefits, re ...

(1890) *

Royal Medal The Royal Medal, also known as The Queen's Medal and The King's Medal (depending on the gender of the monarch at the time of the award), is a silver-gilt medal, of which three are awarded each year by the Royal Society. Two are given for "the mo ...

of the Royal Society (1898) *

Civil list A civil list is a list of individuals to whom money is paid by the government, typically for service to the state or as honorary pensions. It is a term especially associated with the United Kingdom, and its former colonies and dominions. It was ori ...

pension (1902)

References

External links

John Kerr

Biography of John Kerr
{{DEFAULTSORT:Kerr, John 1824 births 1907 deaths Fellows of the Royal Society 19th-century ministers of the Free Church of Scotland 19th-century Scottish Presbyterian ministers Scottish physicists British optical physicists Royal Medal winners People from Ardrossan

Life and work

Honours

References

Further reading

External links