3 - Waves

Question 1

amplitude

Answer 1

max displacement from equilibrium position, in m

Question 2

Frequency

Answer 2

n° of complete oscillations through a point per second, in Hz

Question 3

Wavelength, λ

Answer 3

Length of one whole oscillation in m (distance between successive peaks)

Question 4

Speed, c

Answer 4

Distance traveled per unit time by a wave (m/s)

c = fλ

Question 5

Phase

Answer 5

Position of a certain point on a wave cycle (in radians, degrees or λ fractions)

Question 6

Phase difference

Answer 6

How much a wave lags behind another, (in radians, degrees or λ fractions)

Question 7

Period, T

Answer 7

Time taken for one full oscillation (in s’
(Frequency is 1/T)

Question 8

Points in phase

Answer 8

both at the same point of the wave cycle, same displacement and velocity and their phase difference will be a multiple of 360° / 2π
do not need the same amplitude, only the same frequency and wavelength

Question 9

Completely out of phase

Answer 9

odd integer of half cycles apart
e.g. 5 half cycles apart where one half cycle is 180° (π radians).

Question 10

Transverse waves

Answer 10

oscillation of particles is perpendicular o the direction of energy
transfer
shaking a slinky vertically
e.g: electromagnetic (EM) waves (3 x 10 8 ms -1 in a vacuum)

Question 11

Longitudinal waves

Answer 11

oscillation of particles is parallel to the direction of energy transfer
in a vacuum.
compressions and rarefactions - can’t travel in a vacuum.
e.g Sound
pushing a slinky horizontall

Question 12

Polarised waves

Answer 12

oscillates in only one plane (e.g only up and down), only transverse waves
can be polarised
(Proof of nature of transverse)
applications: Polaroid sunglasses, TV and radio signals

Question 13

Superposition of waves

Answer 13

displacements of two waves are combined as they pass each other
resultant displacement is the vector sum of each wave’s displacement

Question 14

Constructive interference

Answer 14

when 2 waves have displacement in the same direction
waves have same frequency, speed, have to be in phase

Question 15

Destructive interference

Answer 15

when one wave has positive displacement and the other
has negative displacement
equal but opposite displacements ->total destructive interference occurs

Question 16

Stationary waves

Answer 16

superposition of 2 progressive waves same λ, f and amplitude but opposite direction
No energy is transferred
● in phase , constructive interference occurs so antinodes form (regions of maximum amplitude)
● completely out of phase , destructive interference occurs and
nodes (regions of no displacement)

Question 17

Formation of stationary waves

Answer 17

String fixed at 2 ends, one fixed to a driving oscillator
wave from oscillator is reflected at the end of string,

Question 18

First harmonic

Answer 18

lowest frequency at which a stationary wave forms
forms a stationary wave with two nodes and a single antinode
The distance between adjacent nodes (or antinodes) is half a wavelength (for any harmonic)
f = 1/2L • (T/μ)^1/2

μ = mass per unit length (kg/m)
T is tension
L is length of VIBRATING string

Question 19

Other harmonics

Answer 19

Double first to get second (2 antibodies)
Triple for 3rd…

Question 20

Examples of stationary waves

Answer 20

Stationary microwaves: reflecting a microwave beam at a metal plate, to
find the nodes and antinodes use a microwave probe.

Stationary sound waves:
The distance between each node is half a wavelength, and the frequency of the signal generator to the speaker is known so by c=fλ the speed of sound in air can be found.

Question 21

Path difference

Answer 21

Difference in distance travelled by two waves, causes phase difference

Question 22

Coherent light

Answer 22

same frequency and wavelength and a fixed phase difference .
Lasers are an example of light which is coherent and monochromatic
(emit single wavelength of light - form clear interference pattern)

Question 23

Young double slit

Answer 23

Filter & single slit to make light monochromatic and coherent

light source through 2 slits (the same size as the wavelength of the
laser light so the light diffracts)
Each slit acts as a coherent point source making a pattern of light and dark fringes.
Light fringes: interferes constructively,(nλ, where n is an integer).
Dark fringes: destructive - path difference is a whole number and a half
wavelengths ((n+½)λ).

W= λD/s
w is fringe spacing
s is slit separation
D is distance between screen and slit

white light to give a wider max & less intense pttern

Question 24

Lasers (core prac)

Answer 24

Can damage eyesight
Wear laser safety goggles
● Don’t shine the laser at reflective surfaces
● Display a warning sign
● Never shine the laser at a person

Question 25

Wave nature of light proof

Answer 25

Young's double slit: diffraction and interference are wave properties, and so proved that EM radiation must act as a wave (at least some of the time).

Question 26

Diffraction

Answer 26

spreading out of waves when they pass through or around a gap. greatest diffraction = gap is the same size as the wavelength. (smaller -> reflection , greater -> not as noticeable) When a wave meets an obstacle you get diffraction round the edges, the wider the obstacle compared to the wavelength, the less diffraction

Question 27

To vary width of maximum

Answer 27

Increasing the slit width decreases the amount of diffraction so the central maximum becomes narrower and its intensity increases . Increasing the light wavelength increases the amount diffraction as the slit is closer in size to the light’s wavelength, therefore the central maximum becomes wider and its intensity decreases

Question 28

Diffraction grating

Answer 28

slide containing many equally spaced slits very close together monochromatic light is passed through a diffraction grating the interference pattern is much sharper and brighter because there are many more rays of light reinforcing the pattern. measurements of slit widths are much more accurate as they are easier to take dsinθ=nλ where d is distance between slits application: line absorption soectra to detect elements in stars, cray crystallography

Question 29

Refractive index (n)

Answer 29

property of a material which measures how much it slows down light passing through it. calculated by dividing the speed of light in a vacuum (c) by the speed of light in that substance (c s) refractive index can also be known as being more optically dense . n of air = 1 (always > 1)

Question 30

Refraction

Answer 30

wave enters a different medium, changes direction, either towards (speeds up) or away from the normal depending on the material’s refractive index n1sinθ1 (incidence) = n2sinθ2 (refraction)

Question 31

Critical angle

Answer 31

angle of refraction is exactly 90° and the light is refracted along the boundary , the angle of incidence has reached the critical angle (θ c ) sin θ c = n1/n2

Question 32

Total Internal Reflection

Answer 32

angle of incidence is greater than the critical angle and the incident refractive index (n 1 ) is greater than the refractive index of the material at the boundary (n 2 ) optical fibres