John Kerr FRS (/kɜːr/; 17 December 1824 – 15 August 1907) was a Scottish physicist and a pioneer in the field of electro-optics.

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.

[3][4] In the Kerr effect, the difference between refractive index experienced by an ordinary and extraordinary ray is proportional to the square of the electric field.

In 1928 Karolus & Mittelstaedt used a Kerr cell to modulate a beam of light to measure its speed.