Particles and Waves Key Terms

Question 1

atom

Answer 1

The basic units of matter, made up of positively charged protons, negatively
charged electrons and neutral neutrons.
It is overall neutral.

Question 2

radioactive decay

Answer 2

When unstable nuclei emit nuclear radiation in the form of an alpha particle,
beta particle or gamma ray in an attempt to become more stable.

Question 3

isotope

Answer 3

Atoms of the same element with different numbers of neutrons in the nucleus.
OR
Atoms with the same atomic number but different mass number.

Question 4

mass number (A)

Answer 4

Used alongside atomic number to identify a particular isotope.
Gives the total number of protons and neutrons in the nucleus.

Question 5

atomic number (Z)

Answer 5

Used alongside mass number to identify a particular isotope.

Gives the number of protons in the nucleus.

Question 6

alpha radiation

Answer 6

An alpha particle consists of 2 protons and 2 neutrons. It is the same as a
helium nucleus.
It has a relatively large mass and short range in air (3-5 cm).
It is positively charged and is absorbed by a single sheet of paper.

Question 7

beta radiation

Answer 7

A beta particle is a fast moving electron.
It has a relatively small mass and longer range in air (15 cm).
It is negatively charged and is absorbed by 2-3 mm of aluminium.

Question 8

gamma radiation

Answer 8

A gamma ray is a high energy electromagnetic wave.
It has no mass and a very long range in air (hundreds of metres).
It has no charge and is absorbed by 2-3 cm of lead.

Question 9

nuclear equation

Answer 9

Used with isotope symbols to describe radioactive decays.

In all nuclear equations, both mass number and atomic number are conserved.

Question 10

parent nucleus

Answer 10

The original nucleus before a reaction takes place.

Question 11

daughter nucleus

Answer 11

The product produced in a nuclear reaction.

Question 12

alpha decay

Answer 12

When an alpha particle is emitted from the nucleus of an atom.
For an original parent nucleus undergoing alpha decay, its:
• mass number (A) will decrease by 4
• atomic number (Z) will decrease by 2

Question 13

beta decay

Answer 13

When a neutron decays into a proton and electron (and anti-neutrino).
The electron is fired out of the nucleus as a beta particle, whilst the proton
remains within the nucleus.
For an original parent nucleus undergoing beta decay, its:
• mass number (A) will remain unchanged
• atomic number (Z) will increase by 1

Question 14

gamma decay

Answer 14

When a high-energy electromagnetic wave is emitted from the nucleus of an
atom in an attempt for it to become more stable.
For an original parent nucleus undergoing gamma decay, its:
• mass number (A) will remain unchanged
• atomic number (Z) will remain unchanged

Question 15

lost mass

Answer 15

Describes the mass difference which arises due to sum of the masses of the
particles produced by a reaction being slightly less than the sum of the masses
of the particles before the reaction.

Question 16

Einstein’s Mass-Energy Equivalence Principle

Answer 16

According to this principle, lost mass can be turned into energy, but energy can
also be turned into mass.

Question 17

nuclear fission

Answer 17

The process in which an unstable, heavy atomic nucleus splits into two or more
lighter nuclei (called fission fragments), with energy being released.

Question 18

spontaneous fission

Answer 18

A type of nuclear fission that occurs when the nucleus randomly decays.

Question 19

induced fission

Answer 19

A type of nuclear fission that occurs when the nucleus is bombarded by a
neutron, causing it to split.

Question 20

chain reaction

Answer 20

When a nucleus undergoes induced fission, the released neutrons can go on to
hit other nuclei, causing further fission reactions, and the cycle repeats.
The process may be controlled (nuclear power) or uncontrolled (nuclear
weapons).

Question 21

nuclear fission reactor

Answer 21

In nuclear power stations, nuclear fission is used to produce heat energy within
nuclear fission reactors. This is used to turn water into steam, which drives
turbines to generate electricity.

Question 22

moderator

Answer 22

Used in nuclear fission reactors to slow down the fast moving neutrons
released in fission reactions.

Question 23

nuclear fusion

Answer 23

The process of small nuclei joining together to form a larger nucleus, with
energy being released.

Question 24

nuclear fusion reactor

Answer 24

Where nuclear fusion is used to produce energy.
Special conditions of high temperatures and pressures are required to create
the plasma in which the fusion reactions can take place.

Question 25

plasma

Answer 25

A state of matter in which nuclear fusion reactions can take place.

Question 26

irradiance

Answer 26

The power per unit area of electromagnetic radiation incident on a surface.

Question 27

point source

Answer 27

A small, compact source that emits radiation uniformly in all directions.
E.g. we can assume that a small bulb is a point source

Question 28

inverse square law

Answer 28

Irradiance is directly proportional to the inverse of the distance squared for a
point source of light.

Question 29

light

Answer 29

An electromagnetic wave that travels at 3x108 ms-1
.
It reflects, refracts and diffracts.

Question 30

wave-particle duality

Answer 30

The idea that light can act both like a wave and like a particle without
contradiction.

Question 31

photoelectric effect

Answer 31

Provides evidence for the particle nature of light.
Photoelectrons are ejected from the surface of a metal when photons of light
with sufficient energy are incident on the surface.
For photoemission to occur:
• the energy of the photons must be greater than the work function of the
metal
• the frequency of the photons must be greater than the threshold
frequency.

Question 32

photoelectron

Answer 32

An electron emitted from the surface of a material due to a photon incident on
the material.

Question 33

photoemission

Answer 33

The emission of photoelectrons from the surface of a material in the
photoelectric effect.

Question 34

photon

Answer 34

A particle of light that can also be thought of as a short burst of wave energy.

Question 35

electroscope

Answer 35

Consists of a metal plate, metal rod and gold leaf.
The deflection of the gold leaf can be used to investigate the photoelectric
effect.

Question 36

threshold frequency

Answer 36

The minimum frequency of a photon required to cause the emission of
photoelectrons from a metal surface.

Question 37

work function

Answer 37

The minimum energy of a photon required to cause the emission of
photoelectrons from a metal surface.

Question 38

photoelectric current

Answer 38

The number of photoelectrons emitted each second from the surface of a
material.

Question 39

interference

Answer 39

• provides evidence for the wave nature of light.

- observed when coherent waves overlap.

Question 40

coherence

Answer 40

When two or more waves have the same frequency, wavelength and speed.
Waves produced from the same source are likely to be coherent.
Waves are said to be coherent if they have a constant phase difference.

Question 41

phase

Answer 41

Describes how ‘in tune’ two waves are when overlapping.
Two waves are completely in phase when the crest of one matches up with the
crest of the other.
Two waves are completely out of phase when the crest of one matches up
with the trough of the other.

Question 42

constructive interference

Answer 42

When two waves of equal amplitude meet in phase, they combine to form a
wave of twice the amplitude.

Question 43

destructive interference

Answer 43

When two waves of equal amplitude meet out of phase, they combine to form
a wave of zero amplitude.

Question 44

monochromatic light

Answer 44

light of a single frequency (or wavelength)

Question 45

interference fringes

Answer 45

A series of bright and dark bands corresponding to regions of constructive and
destructive interference respectively.

Question 46

path difference

Answer 46

The difference in distance travelled by waves from different coherent sources.

Question 47

maximum/maxima

Answer 47

a point/points of constructive interference

Question 48

minimum/minima

Answer 48

a point/points of destructive interference

Question 49

order number (m)

Answer 49

an integer (whole number) value which describes the point on the interference pattern (screen) you are dealing with.

Question 50

grating

Answer 50

Consists of many equally spaced slits placed very close together.
Light diffracts through each slit.
Lets much more light through compared to a double slit, so the fringes
produced are brighter and sharper.