Waves and Quantum physics

Question 1

Define progressive waves

Answer 1

a wave that transmits from a region of high energy to a region of low energy

Question 2

Define longitudinal wave

Answer 2

when particles oscillations parallel to the direction of energy transfer

Question 3

Define transverse waves

Answer 3

when particles oscillations perpendicular to the direction of energy transfer

Question 4

define displacement

Answer 4

how far a part of a wave is from its equilibrium position

Question 5

Define amplitude

Answer 5

the maximum displacement from the equilibrium to the antinode

Question 6

Define wavelength

Answer 6

the distance between 2 peaks on a displacement/distance graph

Question 7

Define time-period

Answer 7

the time taken for 1 full wave to pass through a point

(the distance between 2 peaks on a displacement vs time graph)

Question 8

define phase difference

Answer 8

the difference in displacement of parts of a wave expressed as an angle

Question 9

define frequency

Answer 9

the number of waves per second passing through a point

Question 10

wave speed

Answer 10

the distance which a wave transmits energy per second

Question 11

describe how you would setup the oscilloscope

Answer 11

connect the source of the waves to the y-plates of the oscilloscope

adjust the y-gain and the time base until 1 full waveform is seen

Question 12

describe how you would determine the frequency

Answer 12

measure the number of divisions for 1 full waveform

multiply the number of divisions by the x-gain to get time period

take the reciprocal of that answer to get the frequency

Question 13

convert between time-period and frequency

Answer 13

f = 1/t

Question 14

state the wave equation

Answer 14

v = fλ

Question 15

describe the key feature of reflection

Answer 15

angle of incidence = angle of reflection

Question 16

describe a diagram showing a light ray passing from air into a glass block and then back into air again

Answer 16

before hitting block: bends towards the normal

leaving the block: bends away from the normal

Question 17

define polarisation

Answer 17

the process of restricting the oscillations of a transverse wave into only one direction

Question 18

identify the tools used to make the wave-fronts easier to use

Answer 18

place white paper below the ripple tank

use a strobe light to illuminate the wave fronts

Question 19

define intensity

Answer 19

intensity is the energy delivered per second per unit area by radiation (power per unit area)

Question 20

relationship between intensity and amplitude

Answer 20

I ∝ A^2

Question 21

describe the key features of electromagnetic waves

Answer 21

they require no particles to transfer energy

they are instead propagated by vibrations in electric and magnetic fields

they travel at 3 * 10^8 in a vacuum

Question 22

state the order of magnitude of the wavelengths of the 7 parts of the electromagnetic spectrum

Answer 22

Gamma - 1 pm
X-ray - 0.1 nm
UV - 10 nm
Visible - 500 nm
Infrared - 0.01 mm
Microwave - 1 cm
Radio - 10^3 m

Question 23

describe how to practically demonstrate the polarisation of electromagnetic waves

Answer 23

the microwave emitter and detector are setup to detect vertically polarised microwaves

when the aluminium gratings are parallel to the plane of polarisation, the electrons in the gratings absorb the microwaves and re-radiate them in all directions so very few are detected

if the aluminium gratings are perpendicular to the plane of polarisation, the electrons cannot absorb the microwaves and it is detected strongly

Question 24

define refractive index

Answer 24

the ratio of the speed of light in a vacuum to the speed of light in a medium

Question 25

define snells law

Answer 25

As a wave crosses a boundary, the product of the refractive index and the sine of the angle in that same medium to the normal is constant

n1 x sin α1 = n2 x sin α2

Question 26

describe experiments to determine the refractive index of a material using Snell’s law

Answer 26

n1 x sin α1 = n2 x sin α2

Shine a monochromatic laser into a glass block at an angle and mark the points that the laser is emitted, enters the block and exits the block. Then calculate the angles of incidence and refraction using trigonometry

repeat this process for 7 different angles of incidence

the initial material is air so n1 = 1 so plot a graph of sin(i) against sin(r) and measure the gradient which will be the refractive index of the material

using a laser poses the risk of retina damage so care should be taken to ensure that the laser points away from the eyes of anyone around the equipment

to enable the laser to be seen and marked accurately, this experiment should take place in a room in darkness by turning off the lights and blocking external light sources

Question 27

define critical angle

Answer 27

the angle of incidence at which the refracted ray travels along the boundary so sin(r) = 1

Question 28

critical angle equation

Answer 28

n1 x sin α1 = n2
=> α = sin^-1(n2/n1)

Question 29

state the principle of superposition

Answer 29

at a point where 2 waves meet, the resultant displacement will be the sum of the displacements of the 2 individual waves

Question 30

describe how you could use 2 sound wave generators to demonstrate the principle of superposition

Answer 30

set up 2 speaker pointing towards each other

as you move between the speakers, the loudness of the sound will change depending on whether the sound waves added together(loud) or cancelled each other out (quiet)

Question 31

define interference

Answer 31

when 2 or more waves superpose to form a resultant wave in which the displacement is either maximum or zero

Question 32

define coherence

Answer 32

when 2 sources produce waves with a fixed phase difference

Question 33

define path difference

Answer 33

the difference is the distance travelled by 2 waves from their source to a point where they meet

Question 34

define phase difference

Answer 34

the difference in displacement of 2 waves expressed as an angle

Question 35

describe constructive and destructive interference in terms of path difference or phase differnece

Answer 35

constructive interference is when 2 waves meet in phase (phase difference of 0°)

destructive interference is when 2 waves meet in anti-phase (phase difference of 180°)

Question 36

describe how an interference pattern can be produced from 2 sound wave generators

Answer 36

2 speakers produce sound waves with a fixed phase difference

where 2 maximum meet, constructive interference occurs which is loud

where a maximum meets a minimum, destructive interference occurs which is quiet

Question 37

describe the impact of young double slit experiment on the scientific communities understanding of light

Answer 37

before the double slit experiment, the scientific community was divided over whether light was a particle (as Newton’s said) or a wave (as Huygens said)

Interference is a purely wave phenomenon providing evidence to contradict newtons hypothesis

Question 38

state the approximate formula for wavelength from an interference pattern

Answer 38

λ = ax / D

a is the distance between the sources
x is the distance between maxima in the interference pattern
d is the distance between the sources and the interference pattern

Question 39

describe experiments to determine the wavelength of a source of electromagnetic waves using a double slit and a diffraction grating

Answer 39

shine light from a highly monochromatic laser onto the double slit. as lasers produce a parallel beam of light, the 2 slits will be always in phase with each other and so are coherent

measure the distance between the slits

change the distance from the slits to the screen from 1m to 5m to get 7 readings and measure the fringe separation by measuring multiple separations and divided by the number of separations.

plot a graph of fringe spacing against the distance, determine the gradient and then multiply by the slit separation to calculate the wavelength

Question 40

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