Unit 2 AS Physics WJEC

light will pass through a sphere

in a straight line

more dense =

bend towards the normal

less dense =

bend away from the normal

angle of incidence =

angle of reflection

snells law

n 1 sin theta 1 = n 2 sin theta 2

what is n

refractive index

define n

ratio of velocity of light in a vacuum to its velocity in a specified medium

equation for n

c/vm

what happens to frequency going through materials

stays the same

what happens to speed going into a denser material

slows down

what happens to wavelength going into a more dense material

decreases

equations needed for snells law derivation

t = d/v, v = c/n

critical angles occur going from

more dense to less dense

critical angle equation =

arcsin n2/n1

optical fibres

light rays approach at larger than critical angle, total internal reflection occurs, inner core has high refractive index, outer cladding has low

uses of optical fibres

communication - immune to electromagnetic interference, can travel long distances, and endoscopes - less invasive and faster recovery

issues with optical fibres

attenuation - signal loses energy as it travels, and multi-mode dispersion, multiple paths to take so light doesn’t arrive at the same time, need to leave extra time to ensure no interference

graph for refractive index practical

plot sin theta 1 y against sin theta 2 x and gradient is refractive index

the photoelectric effect

photons of sufficient energy are absorbed by electrons in the metal causing electrons to escape

why does the surface heat up during the photoelectric effect

emission of incorrect wavelength photons

work function =

minimum energy required for electrons to escape the surface of a metal aka threshold frequency

higher frequency of light =

higher speed

higher intensity of light =

higher number of electrons released

photoelectric equation

ek max = hf - work function

photon energy equation

e = hf

wavelength equation

c = f lambda

electron volts are

the energy gained by an electron when it is accelerated through a potential difference of one volt

charge equation

energy = charge x pd

what wavelength is red light

650-750

what wavelength is violet light

380-450

arguments for light being a particle

no electrons released below threshold frequency, continuous energy transfer should eventually release, increasing intensity affects number of electrons, frequency increasing causes speed increasing not intensity

practical for photoelectric effect

if negative pd introduced, electrons are repelled, so reverse polarity, pd turned up until current is zero, this is stopping voltage, then use e=qv to find kinetic energy

modifications for photoelectric effect practical

add ammeter, add variable power supply, reverse connection on power supply

graphical analysis of photoelectric effect

planck’s constant = gradient, work function = y intercept, plot ek against f

what do waves do

transfer energy, reflect, refract, diffract, superposition

what happened when electrons were sent through a double slit

behaved as a wave

debroglie wavelength equation

lambda = h/p

what is radiation pressure

force when a photon is reflected or absorbed

radiation pressure =

(h/lambda)/change in time // area

if a photon is reflected then what happens to radiation pressure equation

h/lambda doubles because momentum is double

emission line spectrum looks like

black with coloured lines

how are emission line spectrums formed

hot gas emits photons, electrons excited then fall back

absorption line spectrums look like

colours with black lines

absorption line spectrums are formed when

specific wavelength photons are absorbed by elements

LEDs practical to find planck’s constant

plot threshold voltage for each colour bulb against 1/lambda, gradient is hc/e so find plancks constant

laser stands for

light amplification by stimulated emission of radiation

what happens during absorption

photon absorbed, electron excited

what happens during spontaneous emission

photon of energy difference given off, electron drops down due to unstable state

what happens during stimulated emission

photon enters, knocks electron down which sends another photon out activating more aka light amplification

what happens during pumping

supplying photons of exact energy difference to cause multiple absorptions, achieving population inversion where more electrons are excited than in ground state

why cant population inversion happen with only two levels

once half the electrons are excited, stimulated emission becomes just as likely as absorption

three level lasers

pumping, absorption up to top level, spontaneous emission to metastable second level, spontaneous emission which triggers stimulated emission

how are lasers different from normal light

monochromatic, plane polarised, coherent, travels in same direction

four level lasers

same as two level lasers but bottom level spontaneous emission occurs instead of amplification which occurs on second level. electrons are recycled after dropping back to ground state

laser construction

pumping into population inversion in an amplifying medium, 100% reflecting mirror one side, 99% reflecting mirror other side

facts on laser construction

photons bounce back from mirror causing stimulated emissions, low efficiency, large energy needed to cool laser

laser diodes are

newer lasers

advantages of laser diodes

cheaper, smaller, more efficient, easy to mass produce

uses of laser diodes

dvd / cd players, barcode readers, telecomms, image scanning and surgery

progressive waves =

a pattern of disturbances which transfer energy where particles in the medium oscillate about the equilibrium position

transverse waves

direction of oscillation is perpendicular to energy transfer

longitudinal waves

direction of oscillation is parallel to energy transfer

polarisation

a transverse wave is changed to only oscillate in a single plane

what happens if two polarisation filters are put in a row

if parallel, some light transmitted, if perpendicular, no light transmitted through

what will graph look like for polarising experiment

changing light intensity each quarter

in phase

doing the same thing at the same thing

antiphase

doing the opposite thing at the same time

radians

the angle subtended at the centre of a sector when the arc length equal to the radius

frequency equation

f = 1/time

wave speed equation

v = f lambda

0 radians =

in phase

pi radians =

antiphase

displacement =

vector distance of a particle from its equilibrium position at a given time

amplitude

maximum displacement from equilibrium

frequency

number of waves in one second (hz)

wavelength

distance between two consecutive points oscillating in phase

diffraction =

the spreading out of a wave as it passes through a gap

max diffraction occurs when

the gap is roughly the same size as the wavelength

interference

if two waves occupy the same region of space, the resultant displacement is the vector sum of individual displacement. in phase = constructive interference. antiphase = destructive interference

path difference =

difference in distance that two waves travel to meet at the same point expressed in terms of wavelength

if waves arrive in phase

path difference n lambda, maxima

if waves arrive in antiphase

path difference n lambda + 1/2, minima

path difference at second maxima

1 lambda

double slit equation

lambda = ay/D

double slit equation words

wavelength = distance between slits x distance between bright fringes / distance from slits to screen

diffraction grating equation

d sin theta = n lambda

diffraction grating equation words

distance between slits x angle between central axis and maxima = order number x wavelength

first order maxima

n = 1

second order maxima

n = 2

n max =

slit distance / wavelength

coherent sources

constant phase difference and same frequency

stationary waves have

no net transfer of energy

how are stationary waves formed

a progressive wave is reflected off a boundary and meets itself, interferes to form nodes and antinodes

what happens to amplitude on a stationary wave

varies

fundamental (harmonics) =

lowest frequency at which a stationary wave is formed

harmonics equation

lambda = 2l / number of loops

speed of sound practical

place tube into water, tune fork of known frequency above, raise tube until fundamental note heard, measure length of tube above water, repeat until note heard again and find average using equation v = f lambda

current =

rate of flow of charge

conventional current goes from

positive to negative

current equation

q = it