Page 27 - Spotlight A+ Physics Form 4.5

Form
5 Physics Chapter 7 Quantum Physics

I V
a

hc
m = __
e

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0 V
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V
a
1
Figure 7.9 Graph of I against V Figure 7.10 Graph of V against —–
a
λ
The Characteristics of the Photoelectric Effect (d) The threshold frequency, f is the minimum
0
frequency required to produce photoelectric
Source
1. According to the classical theory, light in wave effect on a metal of light
form is a spectrum with continuous energy.
2. Photoelectric effect should be able to occur at Metal
any light wave frequency.
3. Bright light which has high energy should be Thermionic emission Metal
able to emit electrons quickly. • The emission of electrons from a metal
4. Dim light has low energy so the electrons need surface by thermionic emission may take
a longer time to absorb sufficient amount of some time.
energy to escape from the metal surface.
Source
5. However, the results of the photoelectric effect of light
experiments show that:
(a) The higher the frequency of the photon of
light, the higher the kinetic energy of the Metal
photoelectrons emitted from the metal
surface. Metal

(b) The minimum frequency of light needed for Thermionic emission
a metal to emit electrons is known as the • The emission of electrons from a
threshold frequency, f for that metal. metal surface by photoelectric effect is
0
(c) The kinetic energy of photoelectrons does instantaneous.
not depend on the intensity of light. An Try questions 3 and 4 in Formative Zone 7.2
increase in the light intensity does not 6. Threshold frequency is the minimum
produce photoelectrons with a higher frequency required to produce photoelectric
kinetic energy. effect on a metal.

7.2
1. What is photoelectric effect? C1
2. Compare the emitted number of photons from the metal surface by a bright light and a dim light of
the same frequency. C2
3. Explain four characteristics of photoelectric effect from the experiment. C2
4. Why are photoelectrons emitted instantaneously from a metal surface when it is illuminated by a
CHAP light of certain frequency? C3
7 5. Does an increasing of light intensity increase the kinetic energy of the photoelectrons? Why? C4

482 7.2.2