Page 23 - Topic 3
P. 23
CONCEPT CHECK – WAVE-PARTICLE high-intensity peaks at particular
DUALITY frequencies (known as characteristic
X-rays).
DO YOU KNOW?
The intensity of X-rays is decreased
(i.e. attenuated) as they pass through
Concept While Before matter by scattering and absorption.
studying the
test
In interacting with matter, light Particles exhibit wave behaviour with
behaves like particles (called a wavelength that depends on the
‘photons’), with energy given by momentum of the particle. This de
E = hf and momentum given by Broglie wavelength can be
h determined using the formula
p = , where h is Planck’s constant,
λ λ = h , where h is Planck’s constant
f is the frequency of the light, and λ p
is its wavelength. and p is the momentum of the
particles.
When light of sufficiently high
frequency is incident on matter, it The wave behaviour of particles can
may be absorbed by the matter, be demonstrated using a double-slit
from which electrons are then experiment and the Davisson–
emitted. This is called the Germer experiment.
‘photoelectric effect’.
CAN YOU?
The intensity of the incident light
affects the number, but not the
energy, of emitted electrons. Application While Before
Studying the test
• Describe an experimental
method for investigating the
The minimum frequency, f , at relationship between the
0
which electrons are emitted varies maximum kinetic energy of
with the type of material and is called the emitted electrons,
the ‘threshold frequency’. calculated from the measured
stopping voltage using
E K max = eV and the
s
The work function, W , of a surface is frequency of the light incident
the minimum energy required to on a metal surface.
remove an electron from it.
• Describe how Einstein used
the concept of photons and
the conservation of energy to
The work function is related to the
threshold frequency by W = hf . explain the experimental
0
observations of the
photoelectric effect.
• Deduce the formula
X-ray photons can be produced E K = hf − W where E K
when electrons that have been max max
accelerated to high speed interact is the maximum kinetic energy
with a target. This is done in a simple of the emitted electrons.
X-ray tube. • Plot experimental values of
maximum kinetic energy vs
frequency, and relate the
slope and axes intercepts to
The three main features of the
spectrum of the X-rays produced in the formula: E K max = hf − W .
this way are: • Solve problems that require
− a continuous range of the use of E = hf − W .
frequencies (bremsstrahlung) K max
− a maximum frequency given by • Describe the purpose of the
e V following features of a simple
∆
f max = h where V∆ is the X-ray tube: filament, target,
potential difference across the X- high-voltage supply,
evacuated tube, and a means
ray tube
of cooling the target.
