waves and the particle nature of light Flashcards
wave velocity
v=λf
longitudinal waves
oscillations are parallel to the direction of energy transfer, made up of compressions and rarefactions. sound is the best example
rarefaction stage of longitudinal wave
pressure is decreased, neighbouring particles move away from each other
compression stage of transverse wave
pressure is increased, neighbouring particles move towards a point
interference
occurs during superposition
constructive interference
occurs when two waves are in phase and so their displacements are added
destructive interference
occurs when two waves are completely out of phase and so displacements are subtracted
phase difference
two waves are in phase if they are both at the same point in the wave cycle, so they have the same frequency, wavelength and their phase difference is an integer of 360°. the waves do not need the same amplitude
path difference equation
∆x=∆ φx λ/2π
stationary waves
formed by the superposition of two waves, travelling in opposite directions in the same plane, with the same frequency, wavelength and amplitude. no energy is transferred in a stationary wave
stationary wave in phase
constructive interference, where an antinode forms at a point of maximum displacement
stationary wave out if phase
destructive interference, a side forms at a point of minimum displacements
speed of transverse wave on string equation
v=rootT/μ
refractive index
a property of a material which measures how much it slows down loath passing through it, n=c/v. higher refractive index means the material is more optically dense
refraction
when a wave enters a different medium, causing it tot change direction, either towards or away from the normal depending on the materials refractive index. as the light moves across the boundary, its speed changes which causes its direction to change.
n1sinθ1=n2sinθ2
n2>n1, which way does light bend
towards the normal
critical angle
as the angle of incidence is increased, the angle of refraction also increases until it gets closer to 90°. when the angle of refraction is exactly 90° and the light is refracted along the boundary, the angle of incidence has reached the critical angle (C)
critical angle equation
if n2 is air: sinC=1/n
if otherwise: sinC=n2/n1
total internal reflection
occurs when the angle of incidence is greater than the critical angle, and the incident refractive index (n1) is greater than the refractive index material at the boundary (n2)
converging lenses
cause parallel rays to move closer
diverging lens
cause parallel rays to move apart
3 features of lenses
-principle focus:
converging-where light rays focus
diverging- where rays appear to come from
-focal length: distance from lens to principle focus
-power: measure of lens’ ability to bend light. in a converging lens it is positive. diverging it is negative
power of a lens
the reciprocal of the focal length
P=1/f
power of several thin lessons in combination
acts as a single lens with a power equal to the sum of the powers of the individual lenses
P=P1+P2+P3
real vs virtual image
real image can be projected onto a screen, a virtual image cannot
magnification
the ratio of the size of the image it creates with respect to the size of the object
image height/object height
plane polarisation
allowing oscillation of waves in one plane. a polarised wave only oscillates in one plane. only transverse waves can be polarised.
why does light diffract less through a door than sound
the wavelength of light is so much smaller, and the greatest amount of diffraction occurs when the gap is the same size as the wavelength
diffraction grating
a slide containing many equally spaced slits very close together. when light is passed through, it forms an interference pattern composed of light and dark fringes
diffraction grating equation
nλ=dsinθ
d= distance between slits
what is the pattern produced when blue and red light pass through a diffraction grating
lines seen where constructive interference occurs. red wavelength longer than blue so diffracted greater and so blue lines on the inside and red lines on the outside of the light fringes.
electron diffraction
using an electron gun to accelerate electrons through a vacuum towards a crystal lattice, they interact with the small gaps between atoms and form an interference pattern on a fluorescent screen behind the crystal. the pattern looks like concentric rings
deBroglie hypothesis
λ=h/p
this shows all particles have a wave nature
what does electron diffraction show
if electrons had particle nature, the pattern would be a singular point. this is nit the case as the electrons are diffracted which proves evidence of their wave nature
wave behaviour at an interface
transmitted- pass into next material
reflected- bounce off surface
pulse echo technique using ultra sound to form an image
1) short pulse of ultra sound waves are transmitted into the target
2) pulse travels until it reaches a boundary between two mediums where some of the pulse is reflected back. the amount of reflection depends on the difference in densities. the greater the difference the greater the reflection
3) reflected waves are deflected as they leave
4) the intensities of the reflected waves are used to determine the structure and the time taken is used to determine the position
issues with pulse echo technique
-if the duration is too long, they will likely overlap, meaning the amount of information you obtain will decrease
-the larger the wavelength, the less fine details can be resolved, meaning less information
wave model of light
EM radiation can be described as a transverse wave
the work function
minimum energy required for electrons to be emitted from the surface of a metal
photon model of light
EM waves travel in discrete packets called photons, which have energy directly proportional to their frequency (E=hf)
photoelectric effect as evidence of the particle nature of EM radiation
-wave theory suggests any frequency of light should be able to cause photoelectric emission
-photoelectric effect is immediate
-increasing the intensity does nit increase the speed of emission, but increases the amount of electrons emitted
-electrons released with a range of kinetic energies
photoelectric effects
photoelectrons are emitted from the surface of a metal after light above a certain frequency (threshold frequency) is shine on it.
photoelectrons are emitted because electrons near the surface of the metal absorb a photon and gain enough energy to leave the surface
why must light be a particle, not a wave?
-all photon energy is transferred to the electron, one photon interacts with one electron
-energy transfer ins immediate
-intensity is equal to the number of photons released per second, an increase would increase the number of interactions with electrons per second
-all electrons receive the same amount of energy, but those deeper electrons loose energy through collisions and so have less kinetic energy
evidence that electrons in atoms can only transition between discrete wavelengths
inside a fluorescent tube, electrons are accelerated, causing gas atoms to become excited and then de-excite, releasing a photon. by passing the light through a diffraction grating or prism, you get a line spectrum. Each line in the spectrum represents a different wavelength of light emitted. spectrum contains discrete values of wavelength , the photon energies correspond to these wavelengths.
the difference in energy levels
it is equal to a specific photon energy emitted or absorbed, therefore the energy of emitted photon:
∆E=E1-E2
hf=E1-E2
f=(E1-E2)/h
atomic line spectra
electrons in atoms exist in discrete energy levels, if an electron gains enough energy it can move up in energy level, however t will quickly return to its original level and release the energy it gained in the form of a photon of light
superposition definition
when two waves meet, their resultant displacement is the sum of their individual displacements
visible light wavelengths
400-750nm
intensity relation to amplitude
I is proportional to A^2, and so if intensity is doubled, the amplitude is multiplied by four
phase difference calculation
∆d/λ= ∆t/T then multiply by 2π or 360
how to calculate how many order will be visible
using dsinx=nλ, make x=90 and then you should find a value for n e.g 4.7 and so 4 is the maximum order found and 9 is how many light fringes will be seen
order in relation to wavelength
as d is proportional to nλ, λ and n are inversely proportional and so a smaller wavelength means more visible orders
number of slits
you will be told how many slits per mm which should be converted into metres and then find the reciprocal to fid the distance between slits which =d
