Waves

Question 1

Mechanical waves

Answer 1

Waves that move through a medium.

Question 2

Electromagnetic waves

Answer 2

Oscillating electric and magnetic fields that progress through space without the need for a substance.

Question 3

Longitudinal waves

Answer 3

Waves in which the direction of vibration of the particles is parallel to the direction in which the wave travels.

Question 4

Transverse waves

Answer 4

Waves in which the direction of vibration is perpendicular to the direction in which the wave travels.

Question 5

Plane-polarised

Answer 5

The vibrations stay in one plane only.

Question 6

Displacement of a vibrating particle

Answer 6

Its distance and direction from its equilibrium position.

Question 7

Amplitude of a wave

Answer 7

The maximum displacement of a vibrating particle. For a transverse wave this is the height of a wave crest or the depth of a wave trough from its equilibrium position.

Question 8

Wavelength

Answer 8

The least distance between two adjacent vibrating particles with the same displacement and velocity at the same time (distance between adjacent crests).

Question 9

Cycle of a wave

Answer 9

From one wave peak to the next.

Question 10

Period of a wave

Answer 10

The time for one complete wave to pass a fixed point.

Question 11

Frequency of a wave

Answer 11

The number of cycles of vibrations of a particle per second. Measured in Hertz (Hz). f = 1 / T.

Question 12

Wave speed, c

Answer 12

c = f λ

Question 13

Phase difference

Answer 13

The fraction of a cycle between the vibrations of two particles vibrating at the same frequency, measured in degrees or radians.

Question 14

Phase difference in radians

Answer 14

2πd / λ

Question 15

Wavefronts

Answer 15

Lines of constant phase (e.g. crests).

Question 16

Refraction

Answer 16

The effect that occurs when waves pass across a boundary at which the wave speed changes, the wavelength also changes.

Question 17

Diffraction

Answer 17

Occurs when waves spread out after passing through a gap or round an obstacle. The narrower the gap, the more the waves spread out. The longer the wavelength, the more the waves spread out.

Question 18

Principle of superposition

Answer 18

When two waves meet, the total displacement at a point is equal to the sum of the individual displacements at that point.

Question 19

Nodes

Answer 19

Points of zero displacement on a wave.

Question 20

Standing waves

Answer 20

Set up as a result of the superposition of two waves with the same amplitude and frequency, travelling at the same speed but in opposite directions.

Question 21

Coherent

Answer 21

Waves with a constant phase difference.

Question 22

First harmonic

Answer 22

Simplest stationary wave pattern consisiting of two nodes at either end.

Question 23

Phase difference between two particles on a stationary wave

Answer 23

Zero if the two particles are between adjacent nodes or separated by an even number of nodes. 180 degrees or π radians if the two particles are seperated by an odd number of nodes.

Question 24

The normal

Answer 24

An imaginary line perpendicular to a boundary between two materials or surfaces.

Question 25

If the incident ray is along the normal

Answer 25

No refraction occurs.

Question 26

If a light ray passes into a more dense substance

Answer 26

Bends towards the normal.

Question 27

If a light ray passes into a less dense substance

Answer 27

Bends away from the normal.

Question 28

Refractive index of a substance, n

Answer 28

n = sin i / sin r = c / cs

Question 29

Law of refraction

Answer 29

n1 sin i = n2 sin r

Question 30

Refractive index of air

Answer 30

1

Question 31

Refractive index of a vacuum

Answer 31

1

Question 32

Refractive index of water

Answer 32

1.333

Question 33

Critical angle

Answer 33

The minimum angle of incidence beyond which total internal reflection occurs for light travelling from a medium of higher to one of lower refractive index.

Question 34

Requirements for total internal reflection

Answer 34

The incident substance has a larger refractive index than the other substance and the angle of incidence exceeds the critical angle.

Question 35

Total internal reflection

Answer 35

Complete reflection of a ray of light within a medium.

Question 36

Critical angle equation

Answer 36

sin 𝜃c = n2 / n1 for n1 > n2

Question 37

Fringe separation

Answer 37

The distance from the centre of a bright fringe to the centre of the next bright fringe.

Question 38

Fringe separation, w equation

Answer 38

w = λD / s where D is the distance from the slits to the screen and s is the distance between the centres of the slits.

Question 39

Diffraction grating equation

Answer 39

d sin 𝜃 = n λ