Waves And Particle Nature Of Light Flashcards
Describe what is meant by plane polarised light
Oscillations/ vibrations are in one plane only
Plane includes the direction of energy transfer
Only transverse waves can be polarised
What happens to the amplitude of vibrations of the lattice ions as the temperature of a metallic conductor increases
increases
Stationary wave
Superposition of two progressive waves with the same wavelength moving in opposite directions in the same plane
Same frequency
Same amplitude
No energy is transmitted
Constructive interference
Path difference n x wavelength
Whole number of wavelengths
Destructive interference
Path difference of (n + 1/2) x wavelength
Path difference is an odd number of wavelengths
Velocity of wave
Root of tension divided by mass per unit length of string
Phase
A measurement of the position of a certain point on a wave cycle in degrees or radians
Phase difference
How much a particle/ wave lags behind another in radians or degrees
Path difference
Difference in distance travelled by two waves
Superposition
When the displacements of two waves are combined as they pass each other, the resultant displacement is the vector sum of each waves displacement
Coherence
Same frequency and wavelength and a fixed phase difference
Wavefront
Surface which is used to represent the points of a wave which have the same phase
Antinodes
Regions of maximum displacement
Refractive index (n)
Property of a material
Measures how much it slows down light passing through it
Higher refractive index= more optically dense
Total internal reflection
Angle of incidence greater than critical angle and n1 is greater than n2
Measuring refractive index of a solid material
Draw around material on paper
Protractor to draw normal
Protractor to draw lines leaving the normal at 10 degree intervals (incident rays)
Replace block onto outline and shine light through it with ray box
Mark the line the light leaves the block
Join this line to the incident ray
Protractor to measure the angle between this line and the normal
Repeat for all incident angles
Averages
Graph of sine of incident angles (sin i) against sine of refractive angles (sin r)
Gradient of line of best fit is refractive index of the material
u
Distance between the object and the lens
v
Distance between the lens and the image
Positive if real
Negative if virtual
Power
Positive in converging lenses
Negative in diverging lenses
Lens ability to bend light
Focal length
Distance from the centre of the lens to the principle focus
+ converging
- diverging
Principle focus in converging lens
Point at which the light rays which are parallel to the principle axis are focussed
Principle focus in diverging lens
Point from which the light rays appear to come from
Distant object
Real image
Can be projected into a screen
Object further than focal length (converging lenses)
Magnification
Image height divided by object height
v/u
Magnification in terms of v and u
v divided by u
Diffraction
Spreading out of waves when they pass through or around a gap
dsin 0 = n wavelength
distance between slits
Angle to the normal made by the maximum
Order
Wavelength
N=1/d
N is the number of slits in the grating
Per metre (convert if necessary)
Diffraction of electrons to show particles can behave like waves
If electrons had a particle nature, the pattern would look like a single point
However, the electrons diffract
Photon model
EM waves travel in discrete packages called photons which have an energy directly proportional to their frequency
If one photon is absorbed by one free electron the electron will gain enough energy equal to hf
Threshold frequency
Minimum frequency required of light/ a photon to cause the emission of a photoelectron
Work function
Minimum energy required for electrons to be emitted from the surface of the metal
hf
Value depends on the metal
Wave theory
Can’t explain existence of a threshold frequency
Suggests that increasing intensity of light will increase the energy of the photoelectrons
For a particular frequency of light the energy carried is proportional to the intensity of the beam.
The energy carried by the light would be spread evenly over the wavefront
Each free electron on the surface of the metal would gain a bit of energy from each incoming wave
Suggests any frequency of light should be able to cause photoelectric emission as the energy absorbed by each electron will gradually increase with each incoming wave (gradually each electron would gain enough energy to leave the metal)
Time is needed for the energy supplied to the electrons to reach the work function
Increasing intensity
Does not increase the speed of photoelectric emission
It increases the number of photoelectric released per second
Photon model of EM radiation
EM waves released in discrete packets called photons
An electron can only interact with a single photon
Intensity is equal to the number of photons released per second so if this is increased the number of photoelectrons emitted increases because more photons interact with electrons per second
Atomic line spectra as evidence electrons exist in discrete energy levels
Each line in the spectrum represents a different wavelength of light emitted
Discrete values of wavelength
Difference between two energy levels is equal to a specific photon energy emitted
Photon frequency from difference in energy levels
f= E1-E2 divided by h
Wavelength
Length of one whole wave cycle
(Eg Trough to trough)
Determine speed of sound in air using a 2- beam oscilloscope, signal generator, loud speaker and a microphone
- Set frequency on oscilloscope
- Change distance between loudspeaker and microphone so that the two traces (waves on the oscilloscope) are antiphase to each other. Use a metre ruler to measure this distance
- Calculate frequency of the wave
- Move the microphone away from the speaker until one wavelength has occurred and measure this new distance
- Difference between these two distances is the wavelength
- Calculate mean wavelength and use wave speed equation
Transverse waves
Consist of vibrating electric and magnetic fields
Right angles to each other
How a standing wave is formed
2 waves travelling in opposite directions
They interfere/ superpose
Antinodes occur at maximum displacement/amplitude where constructibe interference occurs. n x wavelength
Nodes occur due to destructive interference- zero displacement. (n+0.5)wavelength
In phase
Point in phase have the same displacement and velocity
Phase difference of 0 or a multiple of 360
Antiphase
Phase difference of odd number multiples of 180
Investigate the effects of length, tension and mass per unit length on the frequency of a vibrating string or wire
- Calculate mass per unit length of the string by measuring mass with mass balance and length with a metre ruler
- Calculate tension using T=mg where m is total mass of the masses
- Turn on signal generator and vary the frequency until you find the first harmonic
- Wavelength of wave is 2xlength for the fundamental frequency
Varying length: by moving the vibration transducer towards or away from the pulley
Varying tension: adding or removing masses
Varying mass per unit length: using different string samples
Find first harmonic again and record f against l/T/u
Effects of the 3 factors in resonant frequency
Longer string, lower frequency
Heavier string, lower frequency
Looser string, lower frequency as waves travel more slowly
Refractive index
Ratio between the speed of light in a vacuum and the speed of light in that material
Noticeable diffraction
Slit width same size as the wavelength
Determine the wavelength of light from a laser or other light source using a diffraction grating
- Position laser (monochromatic light) in front of a diffraction grating so that interference pattern appears in wall/ flat surface. Measure the distance, D, between the diffraction grating and the wall
- Measure the distance between the zero order maximum and the 1st order maximum, x, for both sides and take an average of the two readings
- Calculate the angle the 1st order fringe makes using x/D (using tan trig)
- Calculate wavelength of light using dsin0=n wavelength
- Repeat the measurements for more order lines to find an average wavelength
- Repeat the experiment for a diffraction grating that has a different distance between the slits (d)
Effect of larger wavelength on the spread of the interference pattern
Pattern more spread out
Effect of increasing d
Angle smaller
Pattern less spread out
Rule for sin0
Less than 1
Electron volt
The kinetic energy carried by an electron after it has been accelerated through a potential difference of 1V
1eV is equal to the kinetic energy of an electron accelerated across a potential difference of 1V or 1.6x10^-19J
Energy of photon equations
E=hf=hc/ wavelength
Conclusion of photoelectric effect about kinetic energy
Photoelectrons emitted with a variety of kinetic energies ranging from zero to a maximum value
Value for maximum kinetic energy increases with frequency of the radiation and is unaffected by the intensity of the radiation
Conclusion of photoelectric effect about intensity and electrons
The number of photoelectrons emitted over second is proportional to the intensity of the radiation
More wave theory
Higher intensity of wave the more energy it should transfer to each electron
The kinetic energy should increase with intensity
Electrons should be emitted eventually no matter what frequency is
Photon model explains maximum kinetic energy
Energy transferred to electron is hf
Kinetic energy when it leaves is hf minus energy lost in escaping
Minimum amount if energy it can lose is the work function
hf= work function + 1/2mv max^2
Kinetic energy independent of intensity as they can only absorb one photon at a time
1:1
Photon of light discrete packet of energy which interacts with an electron in a one to one way
All the energy in the photon is given to one electron
De broglie and electron diffraction
Smaller acceleration voltage/ slower electrons gives widely spaced rings
Momentum higher then wavelength shorter and spread of lines smaller
You only get diffraction if a particle interacts with an object of about the same size as its de broglie wavelength
Reflection/ transmission at an interface
Reflection occurs more if the densities of the materials involved are different
Ultrasound imaging
Transducer directs ultrasound into body
Gel to get rid of air between skin and transducer because air has a very different density to skin so lots of ultrasound is reflected
At an interface in the body some ultrasound is reflected back to the transducer and a computer calculates how far the boundary is by timing this
Short pulses of ultrasound produce clearer images
Transducers cannot transmit and receive at the same time
So pulses of ultrasound must be short so that the reflections from nearby interfaces don’t reach the transducer before the pulse has ended
Gap between pulses must be long so that all reflected waves from one pulse return to the transducer before the next pulse is transmitted
Shorter wavelengths produce clearer images
Shorter wavelengths diffract much less
So less the waves spread out
More precise location of interfaces can be mapped
In order for an object to be resolved, the wavelength of the ultrasound must be of a similar size to the width of the object being resolved
Phrases for polarising filter questions
Rotate filter and brightness of screen varies
Screen appears brightest when the plane of the polarising filter is parallel to the plane of polarised light
As polarised light from the screen is transmitted by the filter
Screen appears dark when the plane of polarisation of the polarising filter is perpendicular to the plane of polarised light
As polarised light from screen is absorbed by filter
Describe how light is transmitted as a transverse wave
Electromagnetic wave/ oscillations of electric and magnetic fields
Oscillations perpendicular to the direction of energy transfer
Use huygens construction to describe what happens to light waves after passing through a narrow gap
Light waves spread out
Each point on the wave acts as a source of secondary wavelets
That interfere/ superpose
Explain how atoms absorbing the same amount of energy results in the atoms emitting radiation of a particular frequency
Atoms contain energy in discrete energy levels
Atom loses energy and falls back down energy levels emitting a photon
With energy equal to the difference in energy levels
Energy if photon directly proportional to frequency
So emitted frequency of radiation corresponds to difference in energy levels of a particular atom
Explain why monochromatic light source is important in diffraction experiments
Emits very small range of wavelengths
So smaller variation at each diffraction angle
Producing a clearer/sharper interference pattern
EM waves characteristics
Travel in a vacuum at 300000000ms-1 and at slower speeds in other media
Transverse waves consisting of vibrating electric and magnetic fields
Can be refracted, reflected, diffracted and can undergo interference
Carry energy
Can be polarised as they are transverse
Equation for power of a lens
P=1/f
Huygens construction
Every point on a wavefront may be considered to be a point source of secondary wavelets that spread out in the forward direction at the speed of the wave
The new wavefront is the surface that is tangential to all of these secondary wavelets
Wave and particle theory over time
Light believed to be composed of tiny particles (explained refraction and reflection of light)
Diffraction experiments proved light to act as a wave
Photoelectric effect proved light to acts as a particle
Wave particle duality
Joules to electron volts conversion.
Divide by 1.6x10^-19
If the distance from object to lens is less than a certain value, no image is produced on the screen
Why
Only a real image will be produced on a screen
The object cannot be closer than f for a real image
Because light diverges after passing through the lens (virtual image)
Which quantities have a + or - value if it’s converging or diverging?
Power
Focal length
V (distance between image and lens)
How can you tell the direction of movement (up or down) of a particle on a wave in a wave diagram?
Draw a dot to the left on the particle the questions has drawn. If this dot is above the particle, then the particle is moving up
If the dot is below the particle, it is moving down
