A-Level Physics: Waves and Particle Nature of Light: Overview

what is amplitude?

a wave’s maximum displacement from the equilibrium position

what is frequency?

the number of complete oscillations passing through a point per second

what is period?

time taken for one full oscillation

what is wave speed?

the distance travelled by a wave per unit time

what is the equation for wave speed?

frequency x wavelength

what is wavelength?

the length of one whole oscillation

what is a longitudinal wave?

wave where oscillation of particles is parallel to the direction of energy transfer

what is an example of a longitudinal wave?

sound waves

can longitudinal waves travel in a vacuum?

no

what happens to rarefactions and compressions in a longitudinal wave when pressure is applied?

rarefactions are decreased while compressions are increased

what is a transverse wave?

a wave in which oscillations of particles are perpendicular to the direction of energy transfer

what are examples of transverse waves?

electromagnetic waves

what two types of graphs can be used to show waves?

displacement-time graphs and displacement-distance graphs

what do displacement-distance graphs show?

how the displacement of a particle varies with the distance of wave travel

what does a transverse wave look like on a displacement-distance graph?

very similar to the actual wave

what can displacement-distance graphs be used for?

can be used to measure wavelength

what do displacement-time graphs show?

how displacement of a particle varies with time

what can displacement-time graphs be used for?

can be used to measure a period of a wave

what is phase?

the position of a point on a wave cycle

what is phase difference?

how much a particle or wave lags behind another particle or wave

what is path difference?

difference in distance between two waves

what is superposition?

where the displacements of two waves are combined as they pass each other, creating a resultant displacement that is the vector sum of each wave’s displacement

what is coherence?

where a series of waves has the same frequency and wavelength, as well as a fixed phase difference

what is wavefront?

surface used to represent points on a wave with the same phase

what is an example of a wavefront?

ripples around a rock dropped in water

what two types of interference can occur during superposition?

constructive interference and destructive interference

what is constructive interference?

where two waves interfere when they are in phase and their displacements are added

what is destructive interference?

where waves interfere when completely out of phase so their displacements are subtracted

when are waves in phase?

when they are at the same point of the wave cycle

what does it mean if two waves are in phase?

they are coherent, and their phase difference is an integer multiple of 360 degrees or 2 pi radians

when are waves completely out of phase?

when they are coherent and their phase difference is an odd integer multiple of 180 degrees or pi radians

what is the formula relating phase difference (in radians) and path difference?

path difference = (wavelength /2 pi) x phase difference

what is a stationary wave?

a wave formed by the superposition of 2 identical progressive waves travelling in opposite directions on the same plane

does a stationary wave transfer energy?

no

what occurs at points of constructive interference in a stationary wave?

antipodes (regions of maximum displacement) are formed

what occurs at points of destructive interference in a stationary wave?

nodes (regions of minimum or no displacement) are formed

how can a stationary wave be formed as an experiment?

a string is fixed at one end, and fixed to an oscillator at the other end

what will occur during the experiment forming a stationary wave?

the wave formed by an oscillator will be reflected at the fixed end of the string, and travel back along the string where the waves will superimpose

by what formula can the speed of a transverse wave on a string be calculated?

square root of tension/mass per unit length of string

what is intensity?

the power (energy transferred per unit time) per unit area

what is the equation for intensity?

power/area

what is the refractive index?

property of material which measures how much it slows down light passing through it

what is the formula for refractive index?

speed of light in a vacuum (3×10^8) / speed of light in that material

what is a material with a higher refractive index known as being?

more optically dense

when does refraction occur?

when a wave enters a different medium, causing it to change direction towards or away from the normal (depending on refractive index)

what law is used for calculations involving the refraction of light?

snell’s law

what is snell’s law?

(refractive index 1) x sin (angle of incidence 1) = (refractive index 2) x sin (angle of incidence 2)

what causes refraction?

as light moves across the boundary of 2 materials, the speed changes which causes the direction of the wave to change

when does the angle of incidence reach the critical angle?

as the angle of incidence increases, the angle of refraction increases until it reaches 90 degrees, so the wave is refracted across the boundary as the angle of incidence reaches the critical angle

what is the formula for the critical angle is one of the materials in air?

sin C = 1/n

what is the angle of incidence?

angle between the wave and the surface’s ‘normal’

when can total internal reflection occur?

when the angle of incidence is greater than the critical angle and the refractive index is greater than that of the material at the boundary

what is the procedure for finding the refractive index of a solid material?

-draw around a block of the material, then draw a line perpendicular to the outline to create a normal line.

-draw lines at 10 degree intervals from the normal then put the material back on top.

-using a ray box, shine a ray along a 10 degree interval, and mark where the light leaves the material, join this point to the normal.

-plot graph of the sin of the incident angle against the sin of the refracted angle, the gradient is equal to the refractive index

what are the two types of lenses?

converging and diverging

what is a converging lens?

lens that in curved outwards on both sides and causes parallel light rays to move closer together at a point

what is a diverging lens?

lens that is curved inward on both sides and causes parallel waves to move apart

what is the principal focus in a converging lens?

the point at which parallel light rays are focused

what is the principal focus in a diverging lens?

point from which light rays appear to come from

what is the focal length of a lens?

the distance from the centre of the lens to the principal focus

what is the power of a lens?

a measure of a lens’ ability to bend light

what is the value of power in a converging lens?

a positive value

what is the value of power in a diverging lens?

a negative value

what can a ray diagram be used for?

to map where an image will appear after passing through a lens

how can a ray diagram be drawn?

draw a line from the top of the object which passes through the centre of the lens, and another from the top of the object in a perpendicular line to the lens, then through the principal point

how can the power of a lens be found?

1/ focal length

what do thin lenses do?

allow rays to refract but not be dispersed

what is the equation for a combination of thin lenses?

resultant power= power of lens 1 + power of lens 2 + power of lens 3 + ….

what is a real image?

an image which can be projected onto a screen as light rays reach image location

what is a virtual image?

an image which cannot be projected onto a screen

how can distance be used to find power?

(1/distance a) + (1/distance b) = 1/ focal length = power

what is magnification of a lens?

ratio of the size of the image it creates with respect to the size of the object

how can magnification of a lens be found?

image height/object height

what is a polarised wave?

a wave is filtered so that oscillates in only one plane

what type of wave can be polarised?

transverse

what is an application of polarisation?

polarised sunglasses

how do polarising sunglasses work?

they block partially polarised light reflected from surfaces, as they only allow oscillations on a certain plane through

what is diffraction?

spreading out of waves when they pass through or around a gap

what does Huygen’s construction state?

every point on a wavefront is a point source to secondary wavelets which spread out to form the next wavefront

what can Huygen’s construction be used for?

to explain the diffraction of light when it meets an obstacle or passes through a gap

what is a diffraction grating?

a slide containing many equally spaced slits very close together

what is the zero order line?

ray of light that passes through the centre of a diffraction grating

what is the diffraction grating equation?

distance between slits x sin of angle to the normal = order x wavelength

what happens during electron diffraction?

electrons are accelerated through a vacuum towards a crystal lattice where they interact with small gaps between atoms and form an interference pattern in a fluorescent screen behind the crystal

what is the interference pattern created by the electron diffraction experiment?

concentric ring pattern

how does the electron diffraction experiment prove wave nature of electrons?

if electrons only had particle nature, they would look like a single point where the electron beam passes through the lattice. however instead it undergoes diffraction which can only be experienced by waves

what does the de Broglie hypothesis state?

all particles have wave nature and particle nature

what is the de Broglie equation used to find wavelengths of particles?

planck constant / momentum of a particle

what is an interface?

boundary between two materials

what are the two things that can occur at an interface?

waves can be transmitted or reflected

what happens when waves are transmitted at an interface?

waves are transmitted into the meat materials and may experience refraction if the materials have different refractive indices

what happens when waves are reflected at an interface?

waves bounce off the interface without passing into the next material

what is the pulse-echo technique?

technique in which ultrasound waves are released in short pulses and transmitted into a target, where it reaches a boundary between two mediums and the pulse is reflected back

what happens if the duration of the pulses in the pulse-echo technique is too long?

pulses are likely to overlap so the amount of information obtained will decrease

what does the photon model state?

electromagnetic waves travel in discrete packets called photons, which have an energy directly proportional to their frequency

how can electromagnetic radiation be describes in terms of the wave model?

as a transverse wave

what was light initially believed to be composed of?

tiny particles as this could explain the reflection and refraction of light

why was light later believed to be formed of waves?

as it was later proved to act as a wave through diffraction experiments

what is the current attitude towards the behaviour of light? why?

photoelectricity was discovered and the attitude towards composition of light changed again and led to the development of the photon model of light and wave-particle duality

what does the equation E=hf describe?

describes that photons have an energy that is directly proportional to their frequency

what is the photoelectric effect?

where photoelectrons are emitted from the surface if metal after monochromatic light above a certain frequency is shone on it