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Early Experiments in Quantum Physics 223

FIGURE 10.8 Albert Einstein (1879–1955) was a German physicist whose work affected scientific thought
more than any scientist since Isaac Newton. His flurry of papers in 1905 on the photoelectric effect, Brownian
2
2
motion, special relativity and mass–energy equivalence (E ¼ mc ; m ¼ E=c ) established him as a foremost
scientist of his time. In 1911, his calculations using relativity predicted that light from a distant star would be
bent by the gravity of the Sun, which was confirmed by Sir Arthur Eddington in 1919 using an eclipse of the
Sun. Other strange effects related to time dilation have more recently been confirmed during experiments in
space flight. Einstein received the 1921 Nobel Prize in Physics for his work interpreting the photoelectric effect
because his theory of special relativity was not well understood. He also received the Copley Medal from the
Royal Society in 1925, the Royal Astronomical Society Gold Medal in 1926, and the Benjamin Franklin Medal
from the Franklin Institute in 1936, as well as the Max Planck Gold Medal from the German Physical Society in
1929 shown in Figure 10.3. Einstein immigrated to the United States in 1933 when Adolf Hitler became
Chancellor in Germany and took a position at the Institute for Advance Study in Princeton where he remained
until his death at age 76 in 1955. In 1939, he sent a letter to President Franklin Roosevelt of the United States
warning of the danger of Germany developing an atomic bomb, and the U.S. Manhattan Project won the race to
build an atomic weapon under the leadership of Robert Oppenheimer while the German effort under Werner
Heisenberg suffered constant setbacks due to Partisan raids in Norway and Allied bombing of German Industry.

12398:4
DE (eV)
student will realize there is a factor of h included in the formula used as l (angstroms) ¼
so Figure 10.10 is not a surprise, but the slope in Figure 10.9 is amazing! That is a direct
measurement of e ¼ hn! The modern value of h is 6.6260693 10 34 J s so the photoelectric
effect gives the value of h graphically within the number of significant figures (3) of the eV data!
Wait a minute, isn’t that the value Planck had to use to ‘‘fudge’’ his blackbody spectrum? A lot of
credit should be given to Einstein for his interpretation of the photoelectric effect and reinforcing
Planck’s work. In fact, Albert Einstein received a Nobel Prize for this work in 1921 (in reserve),
actually in 1922, but delivered his Nobel Address in 1923. Although the concept of photons can be
mixed with classical electromagnetic waves, we should recognize that Einstein thought of the
photoelectric effect in terms of quantized chunks of light energy. We have chosen this experiment
as one of our ‘‘essential’’ topics because it confirms Planck’s hypothesis and gives a direct way to
measure the value of h.