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 ca ...

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 p ...

. 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 cha ...

Life and work

John Kerr was born on 17 December 1824 at

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'. Ardrossan is located on the east shor ...

, 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 Glasgow ( ; sco, Glesca or ; gd, Glaschu ) is the most populous city in Scotland and the fourth-most populous city in the United Kingdom, as well as being the 27th largest city by population in Europe. In 2020, it had an estimated pop ...

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

. In the Kerr effect, difference between

refractive index In optics, the refractive index (or refraction index) of an optical medium is a dimensionless number that gives the indication of the light bending ability of that medium. The refractive index determines how much the path of light is bent, o ...

experienced by ordinary and extraordinary ray is proportional to the square of the electric field. Where the relationship is linear, the effect is known as the

Pockels effect The Pockels effect or Pockels electro-optic effect, named after Friedrich Carl Alwin Pockels (who studied the effect in 1893), changes or produces birefringence in an optical medium induced by an electric field. In the Pockels effect, also known ...

. 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" is an acronym for "light amplification by stimulated emission of radiation". The firs ...

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 is a vector 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 to its own velocity and t ...

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 everyday use and 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 scalar quantity ...

. Earlier measurements had used mechanical means of

modulation In electronics and telecommunications, modulation is the process of varying one or more properties of a periodic waveform, called the '' carrier signal'', with a separate signal called the ''modulation signal'' that typically contains informat ...

achieving

frequencies Frequency is the number of occurrences of a repeating event per unit of time. It is also occasionally referred to as ''temporal frequency'' for clarity, and is distinct from '' angular frequency''. Frequency is measured in hertz (Hz) which is e ...

of around 10

kHz The hertz (symbol: Hz) is the unit of frequency in the International System of Units (SI), equivalent to one event (or cycle) per second. The hertz is an SI derived unit whose expression in terms of SI base units is s−1, meaning that one h ...

, but the Kerr cell allow frequencies of 10

MHz The hertz (symbol: Hz) is the unit of frequency in the International System of Units (SI), equivalent to one event (or cycle) per second. The hertz is an SI derived unit whose expression in terms of SI base units is s−1, meaning that one h ...

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 organic compound 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 on a large scale from benzene as a precursor ...

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

metric system The metric system is a system of measurement that succeeded the decimalised system based on the metre that had been introduced in France in the 1790s. The historical development of these systems culminated in the definition of the Intern ...

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

Honours

* Honorary LL.D from the

University of Glasgow , image = UofG Coat of Arms.png , image_size = 150px , caption = Coat of arms Flag , latin_name = Universitas Glasguensis , motto = la, Via, Veritas, Vita , ...

(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, r ...

(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 for "the most important ...

of the Royal Society (1898) * Civil list 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 Scottish physicists Optical physicists Royal Medal winners People from Ardrossan

Life and work

Honours

References

Further reading

External links