Turning Points in Physics

Main

Cathode Rays

Plucker

1/1/1859 - 2/1/1859

Observed that the gas glows and that the area of fluorescence can be moved by a magnet. Also, that the tube gets hot

Varley

1/1/1871 - 2/1/1871

Cathode rays must be negatively charged due to the way that they are deflected by a magnetic field

Goldstein

1/1/1876 - 2/1/1876

Shows that rays are emitted perpendicular to the cathode. If the cathode was concave, the rays could be directed at a point

Crookes

1/1/1879 - 2/1/1879

Showed that the gas inside the tube changed the colour of the glow. Eg. Neon was red and oxygen was a faint purple

Hertz & Lenard

1/1/1891 - 2/1/1891

Showed that cathode rays can pass through thin metal foil without puncturing it, so they must be much smaller than atoms.

Jean Baptiste Perrin

1/1/1895 - 2/1/1895

Put a metal can in the tube, connected to a negatively charged electroscope. The leaf rises further - providing conclusive proof of negatively charged particles.

Electrons

J.J. Thomson

1/1/1897 - 2/1/1897

Measured the specific charge of an electron, and is usually credited with their discovery

Robert Milikan

1/1/1901 - 2/1/1901

Measured the charge on an oil drop and observed that their charges were always multiples of 1.6x10-19, hence discovering the charge on an electron.
This meant that mass could also be calculated using Thomson's specific charge

Light

Christiaan Huygens

1/1/1629 - 1/1/1695

Huygens believed that light was made of waves.
His evidence is that when two rays of light cross, they do not scatter, instead they emerge unaffected (like waves)
He proposed that space is filled with an invisible medium called the Ether, through which light can travel.
He predicted that light would move slowed in water than in air

Isaac Newton

1/1/1642 - 1/1/1727

Newton believed that light consists of streams of corpuscles travelling in straight lines
His evidence is that light casts sharp shadows, doesn't diffract (later disproven) and that no wave can travel through a vacuum, but light can.
He predicted that light would move faster in water than in air

Thomas Young

1/1/1801 - 1/1/1807

Young observed that light did in fact diffract and interfere but it was difficult to observe due to the small wavelength of the waves.
This provided conclusive evidence that light is in fact a wave.

Foucault

1/1/1850 - 2/1/1850

Observes that light travels faster in air than in water, thus disproving Newton's hypothesis

Maxwell

1/1/1864 - 2/1/1864

Suggested that a magnetic field would be created by a changing electric field. Therefore, when a m.f. is changing in such a way as to produce a changing e.f., then this in turn will create a changing m.f. and so on. He predicted that this would form a electromagnetic wave.
He was able to calculate the speed of the waves, proving their existance

Hertz

1/1/1887 - 2/1/1887

Used a spark-gap transmitter to produce radio waves.
Also, demonstrated that the waves were transmitted by insulators but reflected by metals

Einstein

1/1/1905 - 2/1/1905

Explained the Photoelectric Effect.
Stated that light consists of tiny packets of energy called photons, each with an energy given by E=hf. Each photon can make a direct hit on a single electron, giving it all of its energy, If the energy of the photon is greater than the work function, then the electron will have enough energy to escape.
This explained the threshold frequency

Michelson

1/1/1926 - 2/1/1926

Measured the speed of light using a rotating octagonal steel prism.