Question: What is an achievement of James Clerk Maxwell, a Scottish scientist who helped to develop the concept of coherence?
Context: An Elementary Treatise on Electricity is a book by James Clerk Maxwell.  The origin of the book are lecture notes Clerk Maxwell gave to members of the Cavendish Laboratory, which he founded. James Clerk Maxwell {'1': ", '2': ", '3': ", '4': "} (13 June 1831 – 5 November 1879) was a Scottish scientist in the field of mathematical physics.  His most notable achievement was to formulate the classical theory of electromagnetic radiation, bringing together for the first time electricity, magnetism, and light as manifestations of the same phenomenon.  Maxwell's equations for electromagnetism have been called the "second great unification in physics" after the first one realised by Isaac Newton. Glenlair House, near the village of Corsock in the Scottish Council area of Dumfries and Galloway, was the home of the physicist James Clerk Maxwell (1831–1879).  The original structure was designed for Maxwell's father by Walter Newall; Maxwell himself oversaw the construction of an extension in the late 1860s, and further improvements were made by his heir, Andrew Wedderburn-Maxwell.  Fires in 1899 and 1929 left the house gutted, but a project to preserve and stabilize the remains has been undertaken by the Maxwell at Glenlair Trust. The upper-convected Maxwell (UCM) model is a generalisation of the Maxwell material for the case of large deformations using the upper-convected time derivative.  The model was proposed by James G. Oldroyd.  The concept is named after James Clerk Maxwell. Katherine Clerk Maxwell (née Dewar) 1824 – 12 December 1886, was a Scottish physical scientist best known for her observations which supported and contributed to the discoveries of her husband, James Clerk Maxwell.  Most notable of these are her involvement with his colour vision and viscosity of gases experiments.  She was born Katherine Dewar in 1824 In Glasgow and married Clerk Maxwell in 1859.  Her contributions are largely recorded in writings on her husband, partly due to a fire at the Maxwell family estate which destroyed many of the family papers.  Katherine had no children and died in 1886. The James Clerk Maxwell Prize in Plasma Physics is an annual American Physical Society (APS) award that is given in recognition of outstanding contributions to the field of the Plasma Physics.  It was established in 1975 by Maxwell Technologies, Inc, in honor of the Scottish physicist James Clerk Maxwell.  It is currently sponsored by General Atomics.  The prize includes a $10,000 USD monetary award and recognition at the annual American Physical Society Division of Plasma Physics conference. The James Clerk Maxwell Foundation is a registered Scottish charity set up in 1977.  By supporting physics and mathematics, it honours one of the greatest of physicists, James Clerk Maxwell (1831–1879), and works to increase the public awareness of science.  It maintains a small museum in Maxwell's birthplace which is in the ownership of the Foundation. A coherent derived unit is defined as a derived unit that, for a given system of quantities and for a chosen set of base units, is a product of powers of base units with no other proportionality factor than one.  The concept of coherence was developed in the mid-nineteenth century by, amongst others, Kelvin and James Clerk Maxwell and promoted by the British Association for the Advancement of Science.  The concept was initially applied to the centimetre–gram–second (CGS) and the foot–pound–second systems (FPS) of units in 1873 and 1875 respectively.  The International System of Units (1960) was designed around the system of coherence. Coherence is defined as the ability of waves to interfere.  Intuitively, coherent waves have a well-defined constant phase relationship.  However, an exclusive and extensive physical definition of coherence is more nuanced.  Coherence functions, as introduced by Glauber and others in 1960s, capture the mathematics behind the intuition by defining correlation between the electric field components as coherence.  These correlations between electric field components can be measured to arbitrary orders, hence leading to the concept of different orders of coherence.  The coherence encountered in most optical experiments, including the classic Young's double slit experiment and Mach-Zender interferometer, is first order coherence.  Robert Hanbury Brown and Richard Q. Twiss performed a correlation experiment in 1956, and brought to light a different kind of correlation between fields, namely the correlation of intensities, which correspond to second order coherence.  Higher order coherences become relevant in photon-coincidence counting experiments.  Orders of coherence can be measured using classical correlation functions or by using the quantum analogue of those functions, which take quantum mechanical description of electric field (operators) as input.  While the quantum coherence functions might yield the same results as the classical functions, the underlying mechanism and description of the physical processes are fundamentally different because quantum interference deals with interference of possible histories while classical interference deals with interference of physical waves. Maxwell’s thermodynamic surface is an 1874 sculpture made by Scottish physicist James Clerk Maxwell (1831–1879).  This model provides a three-dimensional plot of the various states of a fictitious substance with water-like properties.  This plot has coordinates volume (x), entropy (y), and energy (z).  It was based on the American scientist Josiah Willard Gibbs’ graphical thermodynamics papers of 1873.  The model, in Maxwell's words, allowed "the principal features of known substances [to] be represented on a convenient scale."
Answer: classical theory of electromagnetic radiation