Thursday, 9 May 2013

Turning points significance questions

Questions
1.Give one reason why Huygens’ wave theory of light did not replace Newton’s theory of light when the fringe pattern due to double slits was first observed. (1)
2.The interference fringe pattern observed by M’n’M on their interferometer did not shift when the apparatus was rotated by 90°. Explain the significance of this null observation. (1)
3.   Discuss the significance of Einstein’s explanation of the photoelectric effect. (2)
4.   Millikan found that the charge on an oil droplet was always in multiples of e. Explain the significance of this result. (1)
5.   What was the significance of Thompson’s calculations of the specific charge of an electron? (2)
Answers

1.Newton’s scientific pre-eminence/ reputation
No evidence that light travelled slower in water or glass
(as predicted by Huygens’ theory)
Huygens’ theory considered light waves as longitudinal
and therefore could not explain polarisation(1)
2.The speed of light does not depend on the motion of the light source  or of the observer(1)
3.the photon is the quantum of em radiation/light (1)
classical wave theory could not explain threshold frequency (1)
classical wave theory was replaced by the photon theory (1)
[or photons can behave as waves or particles]
4.Shows that charge is quantised – comes in “packets” of 1.6 x 10-19C
5.Bigger than any specific charge measured before; 1840 x bigger than that of H nucleus; therefore electron is very small (or has huge charge)

Turning points summary

Name
Nationality
Dates
Interesting fact
What did he do?
Significance
Albert Einstein
Austrian
1905
Published over 450 papers, 150 of them non-scientific
Published photon theory of light and used it to explain the photoelectric effect; published theory of special relativity
First to say that light consisted of packets of energy called photons, beginnings of quantum theory
Isaac Newton
British
1704 ish
Built the first telescope
Put forward theory that light was made of particles called corpuscles
Not a lot; he was basically wrong, for once.  Reputation is sometimes more important than facts
Robert A Millikan
American
1915
May have fudged some of his results to make them more convincing
First accurate measurement of the charge on an electron
Found that charge was quantised into multiples of e, 1/6 x 10-19C
Heinrich Hertz
German
1885
Discovered photoelectric emission
Discovered how to produce and detect radio waves

Showed that Maxwell was right – energy as waves can travel from source to detector

James Clark Maxwell

Scottish
1865
Died age 36
Gave mathematical proof that light consists of electromagnetic waves
Showed that light was a wave with a constant speed, also predicted existence of other EM waves outside the visible spectrum
Max Planck
German
1900
His son was executed for his part in the failed assassination attempt on Hitler
Energy of a photon is proportional to its frequency related by h, Planck’s constant
Founder of quantum theory
JJ Thompson
British
1897
Seven of his research assistants and his son won Nobel prizes
Discovered Cathode rays, measured the specific charge of an electron
Showed that e/m for an electron was 1860 times bigger than anything ever measured before
Thomas Young
British
1804
Deciphered the Rosetta Stone
Showed that light behaved as a wave with his double slit experiment
Refuted Isaac Newton’s corpuscular theory once and for all
Christiaan Huygens
Dutch
1678
Invented the pendulum clock
Put forward theory that light is a wave not a particle
He was right but didn’t have a lot of evidence
Louis de Broglie
French
1924
Got his first degree in humanities before turning to Physics
Put forward the idea of electron waves
He proposed the wave particle duality theory of matter, creating a new field in Physics
Albert Michelson and Edward Morley
American
1887
Worked on the atomic weight of oxygen for 11 years
Tried to detect the ether with an interferometer
Showed that there was no ether and also that the speed of light is constant.  Most important failed experiment in this unit