Pyotr Lebedev and the pressure of light

Today we celebrate the birthday of the Russian physicist Pyotr Nikolayevich Lebedev (8 Mar 1866 – 1 Apr 1912).

Working in the field of electromagnetism, Lebedev was responsible for a rather famous physics experiment in 1899 – measuring the pressure a beam of light exerts on a solid body. By doing this, he was the first to quantitatively confirm James Clark Maxwell’s theory of electromagnetism. Not only did he prove that the pressure exerted by light, although minute, is very real – he also proved that the pressure of light on a reflective surface is twice as great as on absorbent surfaces.

Thanks to Pyotr Lebedev we have proof that light exerts physical pressure on a solid body.(© All Rights Reserved)
Thanks to Pyotr Lebedev we have proof that light exerts physical pressure on a solid body.
(© All Rights Reserved)

Lebedev’s discoveries led him to postulate that it was the pressure exerted by sunlight on tiny particles of cosmic dust that made the tail of a comet point away from the Sun. However, it is now generally accepted that solar wind has more effect than light pressure in determining the direction of a comet’s tail.

Lebedev died quite young, yet his achievements was significant enough that the Lebedev Physical Institute in Moscow and the lunar crater Lebedev are named after him.

James Clerk Maxwell – the man who changed everything

Today, we commemorate the life and work of James Clerk Maxwell, the Scottish mathematical physicist who died on this day in 1879.

James Clerk Maxwell, 13 June 1831 – 5 November 1879
(wikimedia commons)

While most people will have heard of arguably the two most prominent physicists of all time – Isaac Newton and Albert Einstein – far less are likely to recognise the name of the third person on the list: James Clerk Maxwell. Maxwell, who formulated classical electromagnetic theory, has been hailed as the 19th century scientist whose work had the greatest influence on 20th century physics, and Einstein described it as the “most profound and the most fruitful that physics has experienced since the time of Newton.”

What makes Maxwell’s electromagnetic theory so important is that it is one of the great unifying theories in physics, combining the fields of electricity, magnetism and optics into a single, consistent theory. He showed that electric fields and magnetic fields both travel as waves, and they travel at the speed of light. This led him to postulate that light, electricity and magnetism behave the same, and can be described through the same equations and theories. In his own words, “We can scarcely avoid the conclusion that light consists in the transverse undulations of the same medium which is the cause of electric and magnetic phenomena,” and “The agreement of the results seems to show that light and magnetism are affections of the same substance, and that light is an electromagnetic disturbance propagated through the field according to electromagnetic laws.”

Maxwell’s electromagnetic theory has been reduced down to four fundamental differential equations, known as ‘Maxwell’s Equations’, first presented in his book “A Treatise on Electricity and Magnetism” (1873).

Another contribution by the great man, possibly less grand than his electromagnetic theory, but fundamentally important in its own way, came in the field of colour and optics. His theory of colour vision made a key contribution to colour photography.

Thanks to Maxwell we now understand that a colour image can be split into red, green and blue channels, and that the full colour image can be recreated by combining these channels.
(© All Rights Reserved)

Maxwell was the first to show that a colour image can be created by photographing the same subject through red, green and blue filters, and then projecting the three resultant images through the same colour filters onto a screen. This showed that the additive primary colours are red, green and blue and not red, yellow and blue, as was previously assumed. It introduced the principle of additive colour synthesis used to this day in colour displays.

So here’s to Scotsman extraordinaire James Clerk Maxwell, one of the greatest minds of modern times and, to paraphrase his biography, ‘the man who changed everything’.