How has knowledge and understanding of EM radiation changed over time
End of 17th century - 2 theories published
Newton - light is composed of ‘corpuscles’ (tiny particles)
Huygens - light is composed of waves
Young’s double slit provided evidence for light as waves
Then later Einstein provided evidence for light as particles
Conclusion - it’s both
Applications of diffraction gratings
Used by chemists and astronomers to identify elements - more accurate than prisms
E.g. X-Ray crystallography and analysing the elements present in stars - split up light from a star to form a line absorption spectra
X-Ray crystallography
X-Rays are diffracted by the atoms of crystalline solids
Forms a diffraction grating
Atoms act as a diffraction grating
Used to identify elements
Used in the discovery of DNA structure
Out of phase
Destructive interference
Phase difference of pi radians or 180 degrees
Resonance, displacement and periodic force
Phase difference of 1/2 pi radians between displacement and periodic force
Differences between stationary waves and progressive waves
In a stationary wave points of a particular phase remain in the same location as time passes, but in progressive waves the points of the given phase travel forwards
In a stationary wave, no energy is transferred along the wave (from one point to another) it instead stores energy in ‘pockets’ whereas a progressive wave transfers energy
Stationary waves, nodes and antinodes for open ends and fixed (closed) ends
Open end -> antinode
Closed end -> node
SHM amplitude and displacement definitions
Displacement is the distance from equilibrium
Amplitude is maximum displacement
Brownian motion conclusions
Movement of smoke particles caused by collisions by randomly moving air molecules
Smoke particles are visible but air molecules are not hence air molecules must be very small
Small movement of smoke particles is due to the large number of air molecules from all sides
Boyle’s law definition
Pressure is inversely proportional to volume for a fixed mass of gas at a constant temperature
Single slit diffraction pattern
Wider central maximum surrounded by other lower intensity maxima with dark minima between maxima
Impact of increasing wavelength on diffraction pattern
Larger fringe spacing
Wider central maxima
Impact of using a light source which emits a range of wavelengths on the interference pattern
Central maxima unchanged in width (depending on the range)
Broader maxima and maxima get even broader as order increases
Equation for calculations involving mol
N = nNa
N is number of atoms
n is number of moles
Na is Avagadro’s constant
Widths of central maximum
Increasing slit width decreases the amount of diffraction - decreases width of central maximum (gets narrower) and increases its intensity
Increasing wavelength increases diffraction - central maximum is wider and intensity is lower
Phase difference for two points on the same wave
The phase difference of two points on a wave is the difference in their positions in the wave’s cycle
Resonant frequencies on a string - stationary waves
A frequency at which a stationary wave is formed because an exact number of waves are produced in the time it takes for a wave to get to the end of the string and back again
An exact number of half wavelengths fit on the string
The string vibrates at maximum amplitude
Observing stationary waves using sound waves and microwaves
Use a loudspeaker to direct sound waves into a glass tube with a flat end to reflect them, put powder in the tube and it will collect at the nodes
Use a metal plate to reflect microwaves and a probe to observe the nodes and antinodes
Why does f decrease when L increases
The harmonic frequencies correspond to the number of half wavelengths on a string
If the length is increased, the wavelength of the resonant frequency would also increase
Speed of the wave must remain constant so f must increase
Can also use the formula for the first harmonic to reason this
Why is a laser better than white light for producing a clear diffraction pattern?
Laser light is monochromatic - all the same wavelength, all diffract by the same amount giving sharp, bright fringes
White light is made up of a range of wavelengths which diffract by different amounts
How to create coherent sources of sound waves and light
Connect two loudspeakers to a single amplifier
Direct monochromatic light into a double slit
Investigating the interference and diffraction pattern of sound waves and microwaves
Attach two loudspeakers to an amplifier and create sound waves at a set frequency, walk along a straight line parallel to the line of the speakers and mark the minima and maxima
Same for microwaves except using microwave emitters and a probe
Change in direction at material boundaries
If light is passing into a more optically dense material then it moves towards the normal
If light is moving into a less optically dense material then it moves away from the normal
Requirements for TIR
Light is passing from a more optically dense medium into a less optically dense medium
The incident angle of the light exceeds the critical angle
Relative refractive index
The ratio of the speed of light in material 1 to the speed of light in material 2
= C1 / C2 = n2 / n1
Absolute refractive index
The ratio between the speed of light in a vacuum, c, and the speed of light in the material, cs
n = c/cs
Advantages of optical fibres over old system (copper cables)
The signal can carry more information because light has a high frequency
The light doesn’t heat up the fibre - almost no energy is lost as heat
No electrical interference
Fibre-optic fibres are much cheaper to produce
The signal can travel a long way, very quickly and with minimal signal loss
Energy transfers in an oscillating spring SHM
Kinetic energy - minimum at A and max at equilibrium
GPE - Increases as mass rises from highest to lowest point
Elastic energy - decreases as mass rises from highest to lowest point
Unless its theoretical then for a pendulum or a spring energy is transferred to the surroundings due to air resistance
When temperature increases:
When a substance is changing state:
KE increases and PE increases (But mainly KE)
KE remains constant and PE increases
Extra things to know about conservation of momentum
Conserved when no external forces act and when the mass of the system remains constant
Definition of wave speed
Speed at which energy is transferred
Speed of the wavefront
The distance travelled by the wave in a given time
What affects Crms of molecules?
The ONLY thing that will affect Crms is temperature
Effect of light intensity on LDR?
Effect of increasing temperature on a thermistor?
Resistance decreases
Resistance decreases
Couple
Moment of a couple
Pair of equal and opposite coplanar forces
Defined as force x perpendicular distance between the lines of action of the forces
Why does a photoelectron emitted from deeper within the metal have a lower kinetic energy than one emitted from the surface?
The electron has to do work to reach the surface
Cloud chamber
Can be used to identify ionising radiation based off the length and shape of the path followed
What happens to the frequency when using large tensions for the stationary waves on a string experiment?
Diameter of the string is reduced due to the extension of the string
This results in a lower mass per unit length
So frequencies would be higher than expected
Double slit interference experiment, but one of the slits is then closed, what is the effect on the resulting interference pattern?
Fewer maxima are observed
A wire has young modulus, E
A second wire made from identical material has 3 times the length and half the diameter, what is it’s young modulus in terms of E?
E
Remember Young modulus is a constant property of a material!!!
Conditions for the formation of stationary waves
Two progressive waves moving in opposite directions
Waves must have the same frequency
Waves must have the same/or similar amplitude
What are the differences between a diffraction pattern formed by white light compared to one formed by red light?
Central maximum would be white
The maxima would be continuous spectra with violet closest to the central maxima
The fringe spacing would be smaller due to the average wavlength of white light being less than that of red light
The bright fringes (maxima) are wider and the dark fringes are narrower
Difference between emf and potential difference
EMF is the energy transferred per unit charge passing through the source whereas pd is the energy transferred per unit charge between two points
How do springs used as dampeners reduce the oscillations of a bridge? Why must they be critically damped?
Springs absorb energy from the bridge
Springs oscillate with the natural frequency of the bridge
There is energy transfer from the bridge to the springs
Springs must be critically damped so the don’t return any energy back to the bridge - the damping ensures the energy is dissipated
Why might the spacing between the first orders and the zeroth order be uneven?
The screen may not be parallel to the grating
The incident light may not be normal to the grating
Why does having a rotating turntable on a microwave reduce uneven heating of the food?
The food rotates moving through nodes and anti nodes such that overtime all parts of the food receive a similar amount of energy
Displacement of a wave
How far a point on a wave has moved from the its undisturbed position
When is the normal contact force maximum and minimum when considering circular motion in a vertical plane?
ask mr simpson
Which is better for determining wavelength: young’s double slits or diffraction grating?
Diffraction grating:
As the fringes are sharper and likely to be wider spaced resulting in a lesser percentage uncertainty
How can you experimentally determine whether or not two points on a stationary wave are in phase or out of phase?
Use a video camera to record the stationary wave when it is in motion
If in they are in phase then the two waves will move up and down together
If they are out of phase then one wave will be moving up whilst the other wave moves down
What happens to the diffraction pattern if you increase the amplitude of the waves incident on the double slits?
No change to dark fringes
No change in fringe separation or fringe width
Bright fringes get brighter/more intense
How does the electron diffraction experiment support the idea that electrons behave as waves?
Particles would scatter randomly
Graphite acts as a diffraction grating
Diffraction is a property of waves
Bright fringes represent in phase/ constructive interference
Dark fringes represent out of phase/destructive interference
How does electron diffraction experiment show electrons behaving as particles?
Electrons collide with the screen causing emission of photons
Electrons can’t be behaving like particles as emission is instantaneous and it would take time to absorb waves
Stationary waves
Superposition of two waves travelling in opposite directions
Constructive interference forms nodes
Destructive interference forms antinodes
Fixed boundaries are nodes
Waves have same frequency and similar amplitude
When does light move towards or away from the normal?
Moving into a more optically dense medium - refracts towards the normal
Moving into a less optically dense medium - refracts away from the normal
Role of the cladding of an optical fibre
protects from scratches
avoids cross talk - keeps the core away from adjacent fibres
helps reduce signal loss
needed for TIR as it has a lower refractive index than the core
Explain why the resistance of a NTC thermistor decreases as temperature rises
Increased temperature releases electrons - they have sufficient energy to leave the atom
Current therefore increases as the number of charge carriers increases
The effect of increased charge carriers has more effect than the effect of more collisions with the lattice
