Waves Flashcards
What is Displacement
The distance and direction from equilibrium position of a particle on a wave
What is wavelength?
Least distance between 2 adjacent vibrating particles with same phase
What is a complete cycle?
The max displacement to next max displacement
What is time period?
Time for 1 wave to pass a fixed point (time for 1 cycle)
What is frequency?
No of cycles per second
What is phase difference?
The difference by which 1 wave leads or follows another
What are progressive waves?
Waves which transfers energy from one place to another via oscillations
What is intensity?
The rate at which energy is transferred from one location to another
I=P/A
If spread out equally in all directions
I=P/4πr²
What is the relationship of energy and amplitude? ∴ what is the relationship of I and E?
E ∝ A²
∴ I ∝ A² ∵ I =P/A and P = E/T
What is interference?
Superposition of 2 or more waves
What can interference cause?
Change in amplitude of a the wave
What can all em waves do?
- travel through a vacuum
- speed of light
- transverse
- can be reflected, refracted and diffracted
- can demonstrate interference
- can be polarised
- possess a magnetic wave and an electrical wave interlocked + oscillation @right angles to each other + direction of travel
Why can’t EM from R-V ionise?
Because they do t have enough photon energy
What is the plane of an EM wave?
The plane of oscillation of the electric field.
What is a plane-polarised wave?
A wave that has oscillations/fields only in one plane only
Perpendicular to direction of E transfer
Why can’t longitudinal waves be polarised?
They don’t have oscillations at right angles to their direction of travel.
What is a polarising filter?
It produces plane-polarised light by selective absorption of one component of the incident oscillations - the filter transmits only the component of light polarised perpendicular to that direction
What is crossed polaroids?
When the analyser is at right angle to the polariser = no light will pass through
What is Malus’ law?
When a perfect polariser is put in a beam of polarised light
I=ImaxCos²θ
Describe refraction
Occurs when a wave enters from one transparent medium to another where the wave will change speed and direction (unless it’s travelling along the normal)
What happens if the angle of incidence is lower than the critical angle?
Refraction will occur and a weak reflected ray
What happens if the angle of incidence is equal to the critical angle?
Angle of refraction = 90 degrees
Reflection
What happens if the angle of incidence is greater than the critical angle?
No refraction
Total internal reflection
What is the principle of superposition?
States - When two or more waves of the same type meet, the resultant wave can be found by adding the displacements of the individual waves
What is constructive interference?
When 2 waves in phase superpose to make a wave w/ a larger displacement
What’s destructive interference?
When 2 waves in anti-phase superpose to make a wave w/ reduced displacement
What is coherence?
2 waves with a constant phase relationship w/ same wavelength and hz
What’s in phase?
Same velocity
& same hz & wavelength
What is needed for constructive interference to take place?
Path difference must be a whole number + waves be in phase
What is needed for destructive interference to take place?
Waves arrive out of phase + path difference of half a wavelength
Why is it difficult to determine the wavelength accurately w/young double slit? How is this overcome?
Diffraction fringes are quite blurred ∴ measuring fringe width difficult. Using multiple slits - diffraction grating
What does increasing the no.of slits do?
Improves brightness + sharpness + easier to measure an accurate value for wavelength and angle of maxima
What is the photoelectric effect?
When photoelectrons are released when em radiation of a particular Hz is shone on the surface of a metal
What is the threshold frequency?
The lowest frequency of radiation that will result in the emission of electrons from a particular metal surface
What is the work function?
= min energy required to release an electron from its surface, overcoming the electrostatic attraction between the electron and the positive metal ions
What happens if the incident radiation < f0 (threshold frequency)
Photons don’t have enough energy to overcome the work function
What does work function =
Hf0
What happens if the single slit is too wide?
The contrast will be lost between the dark and bright fringes
Describe how a stationary wave is produced
- a strong is tied at one end to a mechanical vibrator connected to a frequency generator
- the other end = attached to a pulley + supports a weight = tension in string
- as Hz is increased, different stationary patterns are seen on the string
- nodes and anti nodes
How can the pitch/frequency on a stationary wave be changed?
Changing the tension or length of string
Large tension/short length = increases pitch
What do you measure for the wavelength for the greatest accuracy?
From on max to the furthest one from it
What is quantum?
Small discrete unit of energy
What is photon?
Quantum associated with electromagnetic radiation
What is the total energy transferred by an EM wave = ?
The sum of energies of the photons emitted by the light source
Describe how you find Planck’s constant part (1)
- Increase pd across LED by adjusting variable resistor + record current
- Record 6 values for V + I in steps of 0.05V
- Plot a graph of I against V for each LED
- Activation voltage can be obtained by extrapolating the straight line till it reaches the x axis
- Repeat measurements several times for each LED
Describe how you find Planck’s constant part (2)
- Plot a graph of activation voltage against 1/wavelength
- eV = hc/wavelength
- Gradient = hc/e
- ∴ h = gradient * e/c
What is the wave particle duality?
The idea that objects can exhibit both wave and particle properties
Why can’t we use diffraction gratings for electrons?
The wavelength is too small
What do we use instead of diffraction gratings for electrons?
A carbon disc - they diffract as they emerge from the gaps between atomic layers
What pattern do you get in graphite atoms + why?
Circular pattern ∵ graphite atoms = not lined up in the same direction as in a diffraction grating
What does de Broglie’s equation predict?
That the wavelength of the electron can by changed by varying its velocity
How does the velocity affect the wavelength of electrons?
As velocity increased, wavelength decrease ∴ diffraction pattern = bigger
What are the uses of electron diffraction?
- used to determine atomic spacing
∵ increasing speed = decrease in wavelength -> smaller values of inter atomic spacing can be measured
What is an electronvolt?
The eK gained by an electron when accelerated through a pd of 1V
What happens when the electroscope is charged?
The leaf is repelled from the stem
How do you charge an electroscope?
- -ve charged polythene rod
* +ve charged perspex rod
What happens when you charge by contact?
Charge produced = charge on rod
What happens when you charge by induction?
Charge produced = opposite to charge on rod
What will happen if the plate is positively charged?
No light source has an effect
What will happen if the plate is negatively charged?
Visible light has no effect
UV -> leaf falls
What does the gold leaf electroscope show?
UV light causes metal plate to lose its negative charge through emission of electrons which are repelled by the negative charge on the electroscope
Why won’t the effect work if the electroscope is positively charged?
Any electrons will be attracts back to the plate
Why does the Zn have to be cleaned before the test?
Zn may be covered in ZnO ∵ reacted with air
Why should the electroscope be locked in a vacuum?
∵ ions in the air don’t cause discharge of the electroscope
How do you increase the eK of electrons emitted?
Increase frequency
What happens if the incident radiation is = f0?
Electrons will emit BUT with 0 eK
Why doesn’t wave intensity affect the eK?
It ejects more electrons
What is required for TIR?
I > C
Wave to be travelling from a material of higher refractive index to lower
Why is it better to have coherent waves interfering?
Clearer patterns of constructive/destructive can be seen
What determines interference to be constructive or destructive?
Path difference
What is the fundamental mode of vibration?
Lowest possible resonant frequency
Describe it - node on each end
What is a stationary wave?
The superposition of 2 progressive, coherent waves moving in opposite directions
Formed at resonant frequencies
How do stationary waves form generally
Incident wave is reflected
Reflected wave interferes/superpose with incident wave
Resultant wave w/nodes and antibodies
How is the work function related to the threshold frequency?
A photon < f0 cannot cause electron emission ∴ work function = hf0
Why is there a maximum value for kinetic energy of emitted electrons?
e- in the metal have a range of energy is
most require more than WF energy to escape from the surface
Why can’t we use two separate identical light sources in the double slit experiment?
Light from two sources must be coherent – only possible to produce Constant phase difference using a single source
Explain the nature of interference that occurs at the origin point in the double slit
Constructive interference due to path difference equalling 0/Waves meet in phase
Why does the amplitude of the detected signal change when the detector is moved back from 2 microwave transmitters
The path difference between signals changes causing detected signal to vary between maximum and minimum
Max when in phase and min when out of phase
Why does the reflective wave have a different amplitude?
Reflected wave is weaker/reflective wave has travelled a greater distance
State and explain the conditions necessary for light From two slits to produce a visible pattern on the screen
Light must be coherent i.e. constant state relationship and similar amplitudes
Slits must also be narrow so diffraction patterns overlap
What’s the difference between an absorption line spec and an emission line spec
In an absorption spectrum a series of dark lines appear against a bright background
In emission there’s a series of bright lines against a dark background
