Physics AS 2

wave

a transfer of energy from one place to another due to a disturbance

Transverse waves

the direction of vibration is perpendicular to the direction of wave travel

Transverse waves example

water, light, electromagnetic spectrum

Longitudinal waves

the direction of vibration is parallel to the direction of wave travel

Longitudinal waves example

sound, seismic, sonar and ultrasonic waves

Amplitude

the maxiumum distance from the equilibrium position

Periodic time

the time taken to produce one complete wave or oscillation

Wavelength

the distance between two successive points which are in phase

Frequence

The number of complete waves or oscillations produced every second

phase

the fraction of a wave from some fixed point on the wave to another point on a wave which leads/ lags each other

What does light consist of

a transverse wave consisting of a varying electric field and magnetic field perpendicular to each other and the propagation of the wave

unpolarised wave

direction of vibration is in all directions

plane polarised

the direction of vibration is confined to one plane only which contains the direction of propagation of the wave

Why can longitudinal waves not be polarised

you cannot restrict the vibration to one plane only, which is what a polarised wave is

equation for phase difference

θ = x/λ x 360 (or x/T x 2π)

polarisation experiment

rotate through 360, if the light is polarised it will disappear twice

Effects of polarisation with light

sunglasses, stresses in materials, LCD displays on calculators, polarisation of sunlight due to rain droplets and air

EM Spectrum and wavelengths from high f to low f

Gamma 1 x 10-14,
X-rays 1 x 10-12,
UV 300 x 10-9,
Visible light 400-700 x 10-9
IR 1 x 10-4,
microwaves 3-100cm,
Radio/TV 1m-1km

Properties of electromagnetic waves

transverse, can travel through a vacuum, all have an E and B field, all do refraction reflection diffraction and interference

principle of superposition

when two waves meet, the resultant displacement at any point is found by the algebraic vector sum of the separate displacements of the two waves at that point

requirements for superposition

waves must meet and be of the same medium

coherent

constant phase difference

stationary waves

two progressive waves of the same amplitude and frequency and moving in opposite directions and must meet

Stationary waves on a string

individual waves move but resultant is stationary, nodes are at each end

Producing stationary waves in a laboratory

by adjusting the frequence on the signal generator, standing waves can be seen on the string

Finding the speed of sound using a resonance tube

hold a vibrating tuning fork of high frequency above the resonance tube,
slowly raise the inner tube until the first position of resonance occurs,
note the length L,
repeat for tuning forks of lower frequency,
plot a graph of L against 1/f,
repeat for reliability

stationary waves in a resonance tube

closed end has a node, open end has an antinode

Conditions for observable interference

sources must be close together, have comparable amplitude, and be coherent, there must be a path difference, must be in a dark room

interference

the superposition of two waves from two different wave fronts

equations for wavelength from Young's slit experiement

λ = ay/d

diffraction

the spreading of a wave into the geometric shadow region of an obstacle

diffraction grating

an opaque piece of glass with rulings or lines that are parallel and equally spaced marked on it

equation for wavelength using diffraction grating

nλ = dsinθ (d = 1/lines per m)

Using a laser and diffraction grating to find the wavelength of monochromatic light

-set up apparatus as shown with the diffraction grating placed 1-2m from the screen
-mark each bright point on the screen to the left and right of the central mark
-measure the dist between n=1 on each side and halve to get X
-calculate θ and sinθ for each order
-plot a graph of n/sinθ and find the x using λ = d/gradient
-repeat for reliability using a different grading

Refraction

change in direction of a wave when it travels through different mediums caused by change in wave speed

Snell's law practical

trace around a rectangular glass block
-draw a normal at 90 to the bottom surface using a protractor
-measure angles of incidence at 10, 20, 30, 40 50 from the normal
-use a ray box and narrow slit to shine a ray of light along the first angle of incidence
-mark a few points on the emergent ray, remove the block and join the dots.
-form the refracted ray by joining this to the incident ray with a straight line
-measure the angle of refraction and repeat for the next incident angle
-plot of a graph of sini/sinr. gradient is the refractive index

Snell's law

for a given pair of media, the sine of the incident agle is directly proportional to the sine of the refracted angle

Total internal reflection experiment

-set up apparatus as shown below
-mark the normal using a protractor and mark outline of block
-cause the ray to travel along a radius so it is undeflected
-arrange the ray so the angle of refraction is 90
-mark the position o the rays on the paper
-measure the angle c with a proctractor

Conditions required for TIR

ray must be incident in the more dense substance with i > c

Critical angle

the angle of incidence which produces a refracted ray at 90 to the normal

Step index fibre

an optical fibre consisting of a transparent core surrounded by a cladding. the core and cladding have a constant refractive index throughout, there is a sudden reduction in the refractive index as one moves from the core to the cladding

Axial mode

when light travels along the centre of the core in a straight line

Highest order mode

when light repeatedly meets the core/cladding boundary at the critical angle

Modal dispersion

the difference between the time taken for light to travel along the core in axial mode and highest order mode

Flexible endoscope

non-coherent fibre sends light along the endoscope to illuminate what is being observed
light reflects off the objects and travels along the coherent fibres in the endoscope
this insures the image is not distorted and is in the correct orientation

focal point (convex)

the point through which all rays travelling parallel and close to the principal axis pass after being reflected by a convex lens

focal point (concave)

te point through which all rays travel parallel and close to the principal axis appear to have come from after they diverge from passing through a concave lens

focal length

distance from optical centre to focal point

optical centre

physical centre of the lens

real image

can be produced on a screen

virtual image

cannot be produced on a screen

principal axis

an imaginary line running through the optical centre at 90 to the lens' faces and bisecting the lens

properties of concave lens images

virtual diminished erect

when is f +ve and -ve

+ve convex, -ve concave

when is v +ve and -ve

-ve virtual +ve real (u always positive)

Experiment to find focal length of a lens

-place a lens between an illuminated object and a screen
-place the lens at a set distance from the object
-move the screen until a focused image is produced on it, it will have sharp edges and appear darker
-measure the distance v
-repeat for different values of u
-plot a graph of 1/u//1uv

advantages of graphs

-anomalies can be spotted
-identifies trends and patterns
-graph can sometimes be extrapolated to find values not taken
-uses several results so line of best fit is an average

magnification

the ratio of the height of an image to the height of the object

cornea

most refraction happens here, largest change in n

lens

alters shape to change f. thick to focus nearby, thin to focus far away

retina

contains light sensitive cells and forms image of object

near point

the nearest point which can be seen clearly by the unaided eye (25cm)

far point

the furthest point which can be seen clearly by the unaided eye (infinity)

Myopia causes

unable to see distant objects sharply
eyeball too long
cornea too curved
lens too powerful

Hypermetropia

unable to see near objects clearly
lens too weak
cornea not curved enough
eyeball too short

photoelectric effect

emission of electrons from a metal surface when the surface is illuminated by light of frequency greater than a minimum value known as the threshold frequency

photon

a discrete packet of electromagnetic energy

work function

the minimum amount of energy needed to liberate an electron from a metal surface

electron volt

the energy an electron gains or loses when it passes through a potential difference of 1V

Equation for kinetic energy of photoelectrons

E = Work function + KE

Explanation for kinetic energy of photoelectrons

When a metal is illuminated with monochromatic light the kinetic energy of the photoelectrons emitted ranges from zero to some maximum value

Factors affecting KE of photoelectrons

energy of incident photons,
position of electron inside metal

Factors affecting photoelectric emission (frequency and intensity)

increasing frequency of radiation increases energy of incident photon and KE of photoelectron,
increasing intensity of the radiation increases photons arriving and photoelectrons emitted per second

Excitation

electron absorbs a photon with just enough energy to move to a higher energy level

Relaxation

electron emit a photons with exactly enough energy to move to a lower energy level

Lyman series

transitions which start or end at the ground state

Balmer series

Transitions which start or end with the first excited state

Ground state

the lowest possible energy level in which an electron can orbit

Energy levels

a position of certain radius from the nucleus where an electron will orbit with a discrete amount of energy

Why do atoms have unique line spectra?

each atom has its own unique energy levels,
electrons fall between these and emit photons with different wavelengths which produce different lines on the spectrum

Why do all energy levels have negative values

work has to be done on the electron to overcome the electrostatic attraction by the nucleus and free it,
stationary free electrons have an energy value of 0

How does line spectra prove existence of energy levels

when electrons fall back to lower energy levels they emit photons with specific wavelengths corresponding to energy lost,
only certain wavelengths of energy are produced,

equations for E photoelectric effect

E = hf E = hc/λ

properties of laser light

coherent, monochromatic and collimated (parallel rays of light)

5 steps of producing lasers

optical pumping, population inversion, metastable state, spontaneous emission, stimulated emission

Properties of X-Rays

high frequency and energy electromagnetic rays,
undetectable by humans,
very penetrating,
causes low localised ionisation,
generated when high energy electrons strike a metal target

Producing X-rays

high energy electrons are emitted from the cathode and accelerated towards a Tungsten target,
incident electron decelerates and hits target electron in tungsten,
electron in tungsten is ionised from inner shell,
outer shell electron falls into vacated shell releasing a photon of x-ray frequency

Safety of X-rays

99% of energy from incident electrons is converted to heat energy,
tungsten target rotates to reduce localised heating,
copper forms heat sink to take heat away from tungsten,
emerging X-rays are passed through a 3mm aluminium filter to absorb low energy radiation

Radiography

fully exposed film is black, fat and muscle is grey, dense objects are white

Pros and cons of Radiography

less expensive, less time to complete, 2D shadow picture

Pros and cons of CT scans

(computerised tomography) 3D cross section, more expensive, takes longer, unsuitable for pregnant women + people with soft tissue damage

CT scan method

narrow beam of X-rays,
X-ray generator rotated around patient,
detectors arranged outside the path + register transmitted intensity from opposite the generator,
detectors connect to computer which produces cross-sectional image

What can wave theory not explain

no time delay, threshold frequency, intensity affects rate of photoelectrons emitted but not their K.E, frequency affects rate of emission and K.E.

De Broglie Experiment

if atoms are arranged in a regular structure such as crystals, the rows of atoms will act as a diffraction grating and produce interference patterns,
-they have spacing comparable to x-rays or fast electrons

De Broglie equation

λ = h/p

light acting as particle

photoelectric emission

matter acting as a wave

electron beam diffraction

light exhibiting wave like behaviour

young's double slits, polarisation and superposition

Doppler shift

change in wavelength due to the relative speed between the observer and the source

red shift and blue shift

red shift moving away λ increases +z,
blue shift moving towards λ decreases -z