4.4 WAVES

Question 1

what is a progressive wave?

Answer 1

It transfers energy from one point to another through a medium without any net movement of the medium itself. This movement of energy is characterized by oscillations or vibrations that travel through the medium, allowing the wave to progress or move forward.

Question 2

what are the two types of waves?

Answer 2

transverse and longitudinal

Question 3

define longitudinal waves

Answer 3

waves have oscillations that are parallel to the direction of energy transfer

Question 4

define transverse waves

Answer 4

waves which have oscillations perpendicular to the direction of the energy transfer

Question 5

define displacement

Answer 5

Distance from the equilibrium position in a particular direction

Question 6

define amplitude

Answer 6

the maximum displacement from the equilibrium position

Question 7

define wavelength

Answer 7

minimum distance between two points in phase on adjacent waves, for example, the distance from one peak to the next or from one compression to the next

Question 8

define period

Answer 8

the time it takes for one complete oscillation to occur at any point

Question 9

define phase difference

Answer 9

The measure of how much one wave is out of step (or phase) with another wave. It is expressed in degrees (from 0° to 360°) or radians (from 0 to 2π), where 360° or 2π radians represent a full cycle.

Question 10

When are waves in phase?

Answer 10

A phase difference of 0° or 0 radians means the waves are in phase

Question 11

When are waves out of phase?

Answer 11

a phase difference of 180° or π radians means the waves are in complete opposition, or out of phase.

For example when one particle reaches its maximum displacement at the same time as the other reaches its maximum negative displacement

Question 12

define frequency

Answer 12

the number of complete oscillations that pass a point per unit time

Question 13

define wave speed

Answer 13

the distance the wave travels per unit time

Question 14

what is the equation for frequency?

Answer 14

f = 1/T
(T = period)

Question 15

what is the phase difference formula

Answer 15

Phase difference = x (distance between two points) / wave length x 360° (or 2π)

Question 16

Examples of transverse waves

Answer 16

Waves on the surface of water
all EM waves
Waves on a stretched string
S - waves produced in earthquakes

Question 17

Examples of longitudinal waves

Answer 17

P - waves from earthquakes
Sound waves

Question 18

outline an experiment using an oscilloscope to determine the frequency of a wave

Answer 18

An oscilloscope is used to determine the frequency of a wave

use a microphone to generate a graph of p.d. against time (microphone converts sound waves into electrical signals)

-voltage on y axis (controlled by GAIN DIAL, volts per division), time on the x axis (controlled by TIMEBASE DIAL, seconds per division)

-each horizontal square represents a certain time interval. This is called timebase

-use frequency = 1/t (t=period of wave in seconds) to calculate frequency

(note if the time base is turned off, the wave no longer moves across the screen making it easy to look at the intensity of the wave)

Question 19

in a graphical representation of a longitudinal wave, what parts are the peaks and troughs?

Answer 19

rarefaction = trough (where the lines far apart)
compression = peak (where the lines bunch up)

Question 20

what is reflection and what is the key rule?

Answer 20

Reflection occurs when a wave changes direction at a boundary between two different media, remaining in the original equilibrium.

the angle of incidence is ALWAYS equal to the angle of reflection

Question 21

what is refraction?

Answer 21

refraction occurs when a wave changes speed and direction as it travels through a different medium, this depends on how optically dense the medium is (bending of a wave)

Question 22

what is diffraction?

Answer 22

the spreading of a wave through a small gap or around an obstacle, the effect is most significant when the gap width is equal to the wavelength, noticeable effects when the gap width is several wavelengths wide

Question 23

when is diffraction effect the most significant and what happens when the wavelength is smaller than the gap size

Answer 23

when the size of the gap is about the same size as the wavelength of the wave

Small wave length will not diffract

Question 24

when waves reflect, refract and diffract what happens in terms of speed, wave length and frequency

Answer 24

reflect: frequency and wavelength do not change

Refract: has affect on wavelength, but not frequency

Diffract: speed, wavelength and f do not change

Question 25

what is plane polarisation? what types of waves can be polarised?

Answer 25

plane polarisation is when a wave is restricted so that it only oscillates in one direction, only TRANSVERSE waves can be polarised

Question 26

what is the wave speed equation?

Answer 26

v = fλ

with questions concerning EM radiation, c=fλ as all EM waves travel at speed of light

Question 27

outline an experiment using a ripple tank to investigate wave effects (reflection, diffraction and refraction)

Answer 27

Set up the apparatus as shown (look it up) and fill the ripple tank with water to a depth of no more than 1 cm

Turn on the power supply and the light source to produce a wave pattern on the screen

The wavelength of the waves can be determined by using a ruler to measure the length of the screen and dividing this distance by the number of wavefronts

The frequency can be determined by timing how long it takes for a given number of waves to pass a particular point and dividing the number of wavefronts by the time taken

Record the frequency and wavelength in a table and repeat the measurements

Speed => v=fλ

Question 28

what is Malus’ law and what does it tell you?

Answer 28

I = Io x cos^2θ
I = final intensity after passing through second filter
Io = initial max intensity
θ = angle between the first and second filter

-it tells you the intensity of plane polarised light that passes through a filter

Question 29

outline an experiment using polarising filters to polarise visible light

Answer 29

place a light source in front of two polarising filters (unpolarised light is in all directions), keep the first filter in a fixed position and rotate the second to change the intensity from maximum light to no light, rises and falls as the angle is changed remember MALUS’ LAW

Question 30

outline an experiment to observe polarisation of microwaves

Answer 30

place a metal grille in between a microwave transmitter and receiver (opposite sides of grille), connect the receiver to an ammeter

the microwave transmitter produces vertically plane-polarised radiation

the metal grille absorbs radiation of the same plane as the radiation meaning when the metal grille is horizontal very few of the microwaves will be absorbed so the ammeter will show a high output, when the metal grille is vertical all of the microwaves will be absorbed meaning the ammeter will show no output

Question 31

what is the formula for intensity of a progressive wave and what is the relationship between intensity and amplitude?

Answer 31

intensity = power/area

intensity is directly proportional to amplitude^2

this comes from the fact that intensity is proportional to energy and the energy of a wave depends on the square of the amplitude

Question 32

what are the main properties of EM radiation?

Answer 32

they all travel at c (3x10^8m/s)
they are transverse
they consist of an electric and magnetic filed that are at right angles to each other and the direction of wave travel
they can be refracted at a boundary, reflected at a surface, diffracted through a gap, polarised and can undergo interference

Question 33

What is interference?

Answer 33

Interference occurs when two or more waves overlap and combine to form a new wave pattern. This can result in areas of increased amplitude (constructive interference) when the waves are in phase, or decreased amplitude (destructive interference) when the waves are out of phase.

Question 34

what is the order of the EM spectrum?

Answer 34

Radio, Micro, Infrared, Visible, Ultraviolet, X-Ray, Gamma

Question 35

order of magnitude for radio (m)

Answer 35

10^6 - 10^-1 m

Question 36

order of magnitude for micro (m)

Answer 36

10^-1 - 10^-3 m

Question 37

order of magnitude for infrared (m)

Answer 37

10^-3 - 7x10^-7 m

Question 38

order of magnitude for visible (m)

Answer 38

7x10^-7 - 4x10^-7 m

Question 39

order of magnitude for ultraviolet (m)

Answer 39

4x10^-7 - 10^-8 m

Question 40

order of magnitude for x-rays (m)

Answer 40

10^-8 - 10^-12 m

Question 41

order of magnitude for gamma (m)

Answer 41

10^-12m - 10^-16 m

Question 42

frequency of radio

Answer 42

<(3 × 10⁹ Hz)

Question 43

frequency of micro

Answer 43

(3 × 10⁹ Hz) to (3 × 10¹⁰ Hz)

Question 44

frequency of infrared

Answer 44

(3 × 10¹⁰ Hz) to (4.3 × 10¹⁴ Hz)

Question 45

frequency of visible

Answer 45

(4.3 × 10¹⁴ Hz) to (7.5 × 10¹⁴ Hz)

Question 46

frequency of UV

Answer 46

(7.5 × 10¹⁴ Hz) to (3 × 10¹⁶ Hz)

Question 47

frequency of X-rays

Answer 47

(3 × 10¹⁶ Hz) to (3 × 10¹⁹ Hz)

Question 48

frequency of gamma

Answer 48

> (3x10¹⁹ Hz)

Question 49

what is the formula for refractive index?

Answer 49

n = c/v
c = speed of light in a vacuum
v = speed of light in the medium
where

Question 50

what does the refractive index tell us?

Answer 50

it tells us the ratio of the speed of light in the medium compared to the speed of light in a vacuum (measures how much the material slows down light)

Question 51

if something has a high refractive index what does that mean about the speed of light in the medium?

Answer 51

slow speed

Question 52

if something has a low refractive index what does that mean about the speed of light in the medium?

Answer 52

high speed

Question 53

how does light bend when it moves into a medium with a high refractive index (slows it down)?

Answer 53

light bends TOWARDS the normal

Question 54

how does light bend when it moves into a medium with a low refractive index (speeds it up)?

Answer 54

light bends AWAY from the normal

Question 55

what is snell’s law?

Answer 55

n1sinθ1 = n2sinθ2
n1 = refractive index of first medium
θ1 = angle of incidence
n2 = refractive index of first medium
θ2 = angle of refraction

nsinθ = CONSTANT at a boundary where θ is the angle to the normal

Question 56

what is total internal reflection and when does it occur?

Answer 56

when the light is reflected back into the material and it occurs when the angle of refraction is above 90 degrees and therefore the angle of incidence is more than CRITICAL (when it is equal to critical is will travel along the boundary of the material at 90 degrees)

Question 57

what is the critical angle equation?

Answer 57

sinC = 1 / n

(n = refractive index of the material)

this comes from snells law as n2 = 1 because its in air and θ2 is 90 degrees (angle of refraction) making sin90 = 1

Question 58

outline an experiment to investigate total internal reflection

Answer 58

shine a light ray into the curved face of a semi-circular glass block so that it always enters at right angles to the edge (this means the ray won’t refract as it enters the block, just when it leaves from the the straight edge)

vary the angle of incidence until the light beam refracts so much that it exits the block along the straight edge, this angle of incidence is critical

if you increase the angle of incidence so its greater than C, you’ll find the ray is reflected from the straight edge of the block (total internal reflection occurs)