Module 4.2 - Waves ~✓

Question 1

What is Displacement? (waves)

Answer 1

Displacement - how far a point has moved from it’s equilibrium position

Question 2

What is Amplitude? (waves)

Answer 2

Amplitude - the maximum magnitude of the displacement

Question 3

What is wavelength? (waves)

Answer 3

Wavelength - the length of one whole wave cycle, from crest to crest

Question 4

What is period? (waves)

Answer 4

Period - the time taken for one whole wave cycle to complete

Question 5

What is phase? (waves)

Answer 5

Phase - a measurement of the position of a certain point along the wave cycle

Question 6

What is phase difference? (waves)

Answer 6

Phase difference - the amount one wave lags behind another

Question 7

What is the wave equation

Answer 7

wave speed = frequency * wavelength

Question 8

Which way are the vibrations in transverse/longitudinal waves?

Answer 8

Transverse - Perpendicular to the direction of travel

Longitudinal - parallel to the direction of travel

Question 9

What is intensity? (waves)

Answer 9

Intensity is the power transfer per unit area at right angles to the direction of travel of a wave

Question 10

What is the equation for intensity?

Answer 10

Intensity = Power/Area

Question 11

State three properties of EM waves

Answer 11

• Travel in a vacuum at 3*10^8 ms-1 (3sf)
• They are transverse waves consisting of vibrating electric and magnetic fields at right angles to each other
• They carry energy (like all progressive waves)
• They can be polarised
• They can be refracted, reflected and diffracted

Question 12

Name the types of EM wave in order of wavelength (large > small)

Answer 12

Radio, Microwave, Infrared, Visible light, Ultraviolet, X-rays, Gamma rays

Question 13

What is polarisation?

Answer 13

Polarisation is the filtering out of vibrations of a wave in other directions

Question 14

How does a polarising filter work?

Answer 14

A polarising filter only transmits vibrations in one direction

Question 15

Define diffraction

Answer 15

Diffraction is the spreading out of the wave fronts of a wave when passing through a gap or around objects

Question 16

What is the law of reflection?

Answer 16

Angle of incidence = angle of reflection

Question 17

Define refraction

Answer 17

Refraction is the changing of the direction of a wave as it enters a different medium

Question 18

What is the absolute refractive index?

Answer 18

The absolute refractive index of a material is the ratio between the speed of light in a vacuum and the speed of light in the material

Question 19

What is the equation for absolute refractive index?

Answer 19

n = c/v
n > absolute refractive index
c > speed of light in vacuum
v > speed of light in material

Question 20

What is the Snell’s law equation? (for refraction)

Answer 20

n1 sin()1 = n2 sin()2

Question 21

What transition in optical densities is needed for total internal reflection?

Answer 21

For total internal reflection to occur light must travel from more optically dense material to a less optically dense one

Question 22

What is total internal reflection?

Answer 22

Total internal reflection is when all light is reflected back into the material at a boundary between two materials - happens at angles of incidence greater than C

Question 23

What happens at the critical angle of incidence?

Answer 23

At the critical angle C, the angle of refraction is 90 degrees, so the light gets refracted along the boundary between materials.

Question 24

What is Snell’s law equation for light hitting a material to air boundary?

Answer 24

sin(C) = 1/n

Question 25

State the principle of superposition

Answer 25

When two or more waves cross, the resultant displacement equals the vector sum of the individual displacements

Question 26

How do you know if two points on a wave are in phase?

Answer 26

They should have the same displacement and velocity

Question 27

What does it mean for two sources to be coherent?

Answer 27

Two wave sources are coherent if they have the same wavelength and frequency, and a fixed phase difference between them

Question 28

What is path difference?

Answer 28

Path difference is the amount by which the path travelled by one wave is longer than the path travelled by another

Question 29

At what path difference does constructive,destructive interference occur?

Answer 29

Constructive > whole number of wavelengths | Destructive > half number of wavelengths

Question 30

State young's double slit formula

Answer 30

Fringe Spacing x = (λ*D)/a λ > wavelength D > distance of slits to screen a > spacing between slits

Question 31

Why was young's double slit experiment so important?

Answer 31

Young's double slit experiment showed that light could diffract and interfere, providing evidence for the wave nature of light

Question 32

What is the naming scheme for diffraction grating patterns?

Answer 32

the central spot is the zero order line, the next one out from each side is the 1st order, then 2nd and so on

Question 33

What is the equation for finding the wavelength of light from a diffraction grating?

Answer 33

``` d sin(x/D) = nλ d > slit separation x > fringe width D > dist form screen to laser n > order of maximum observed ``` ``` d sin(θ) = nλ (x/D = θ) > using small angle approximations ```

Question 34

Why does shining white light through a diffraction grating produce spectra?

Answer 34

White light is a mixture of colours. If diffracted, the different wavelengths of light are diffracted by different amounts, so each order in the pattern becomes a spectrum (the zero order stays wight tho)

Question 35

What is a stationary wave?

Answer 35

A stationary wave is the superposition of two progressive waves with the same wavelength, travelling in opposite directions.

Question 36

What is the difference between progressive and stationary waves?

Answer 36

- Progressive waves transfer energy however stationary waves store it - Progressive waves have a fixed amplitude throughout whereas stationary waves have different amplitudes along them

Question 37

What is a Node?

Answer 37

Nodes are points on a stationary wave with an amplitude of zero

Question 38

What is an Antinode?

Answer 38

Antinodes are points on stationary waves with maximum amplitude

Question 39

What are "resonant frequencies"?

Answer 39

resonant frequencies are frequencies at which a stationary wave forms, at these an exact number of half wavelengths fit on a string

Question 40

What is the fundamental mode of vibration?

Answer 40

The first resonant frequency/first harmonic. On a string it has a wavelength of ½

Question 41

at what lengths do stationary waves form in closed instruments? Where are the nodes/anti-nodes located?

Answer 41

1/4 λ, 3/4 λ, 5/4 λ, etc... A node always forms at the closed end and an antinode at the open end, more can form in between.

Question 42

At what lengths do stationary waves form in open ended instruments? Where are the nodes/anti-nodes?

Answer 42

½λ, 1λ, 3/2λ, etc... Anti-nodes will always form at the open ends, nodes in between

Question 43

Where does intensity ∝ (amplitude)^2 come form?

Answer 43

Intensity is proportional to energy and the energy of a wave depends on the square of its amplitude, therefore: Where does intensity ∝ (amplitude)^2 come form?

Question 44

Why do polarising filters not work on microwaves?

Answer 44

Their wavelength is too long, metal grilles must be used instead

Question 45

How can you tell if a light ray is slowing down or speeding up when refracted?

Answer 45

If the ray bends towards the normal it is being slowed down, if it bends away it is speeding up

Question 46

What is an optically dense material?

Answer 46

A more optically dense material is one where light slows down more when travelling through it

Question 47

Define interference

Answer 47

Interference is the combining of the displacements of two or more superposed waves

Question 48

What general conclusions can be drawn from d sin(θ) = nλ?

Answer 48

- The larger the wavelength, the larger the spacing | - The coarser the grating the less spread out the pattern (d bigger > sin theta smaller)

Question 49

Why does a stationary wave form when vibrating one end of a fixed string?

Answer 49

1) The wave generated by the vibrations at one end moves down the string 2) This wave is reflected at the fixed end of the string 3) If an exact number of waves is produced in the time it takes for a wave to travel to the end and back the waves superpose and form a stationary wave This happens at resonant frequencies

Question 50

A string has a mechanical oscillator at one end and is tensioned using massed and a pulley at the other. Explain how stationary waves form in the string?

Answer 50

1) The oscillator sends a progressive wave down the string 2) This wave gets reflected at the end of the string to produce an equal progressive wave moving in the opposite direction 3) The two progressive waves interfere to produce nodes and anti-nodes producing a stationary wave