waves and particle nature of light

What is amplitude

A waves maximum displacement from the equilibrium position

What is frequency

The number of complete oscillations passing through a point per second

What is a period

The time taken for one full oscillation

What is a Wave speed

The distance travelled by the wave per unit time

What is wavelength

The length of one whole oscillation

What sis the equation of a wave

Speed = frequency x wavelength

What are longitudinal waves

Waves where the oscillation of particles is parallel to the direction of energy transfer. They are made up of compressions and rarefactions and can’t travel in a vacuum

Describe the displacement of particles during rarefactions vs compressions

At rarefactions neighbouring particles move away from each other, at compressions neighbouring particles move towards a point.

What are transverse waves

Waves where the oscillations of particles are perpendicular to the direction of energy transfer

What are the two types of graphs that can be used to represent waves

Displacement-distance graphs and displacement-time graphs

What do displacement-distance graphs show

How the displacement of a particle varies with the distance of wave travel and can be used to measure wavelength

Phase

The position of a certain point on a wave cycle

Phase difference

When waves have the same frequency but oscillate differently to each other

Path difference

The difference in the distance travelled by two waves

Superposition

Where the displacements of two waves are combined as they pass each other the resultant displacement is the vector sum of each waves displacement

Coherence

A coherent light source has the same frequency and wavelength and a fixed phase difference

Wavefront

A wavefront is a surface which if used to represent the points of a wave which have the same phase

Constructive interference

When two waves are in phase and so their displacements are added.

Destructive interference

This occurs when the waves are completely out of phase

Two waves are in phase…

…if they are both at the same point of the wave cycle, phase difference is an integer multiple of 360 degrees

Two waves are completely out of phase…

…when they have the same frequency and wavelength and their phase difference is an odd integer multiple of 180 degrees

Stationary wave

Made by two travelling waves in opposite directions with the same frequency

What is a node

Region s of no displacement

What is an antinode

Regions of maximum displacement

Intensity

The power (energy transfer per unit time) per unit area

Refractive index

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

What does it mean if a material is more optically dense

It has a higher refractive index

When does refraction occur

When waves enter a different medium causing it to change direction, either towards or away from the normal depending on materials refractive index

What is snells law

n1 sin01 = n2 sin02

As the light moves across the boundary of two materials…

…it’s speed changes, which causes its direction to change

When does a ray reach the 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

When does total internal reflection occur

When the angle of incidence is greater than the critical angle and the incident refractive index (n1) is greater than the refractive index of the material at the boundary (n2)

How many planes does a polarised wave oscillate in

One

What type of waves can be polarised

Transverse

How do polarised sunglasses reduce glare

By blocking partially polarised light reflected from water and tarmac as they only allow oscillations in the plane of the filter to pass through making it easier to see

Converging lenses

Curved outwards on both sides and cause parallel light rays to move closer together/ converge at a point

Diverging lenses

Curved inwards on both sides and cause parallel light rays to move apart/ diverge

What is the principal focus in a converging lens

The point at which the light rays which are parallel to the principal axis are focused

What is the principal focus in a diverging lens

The point from which the light rays appear to come from

What is the focal length

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

What is the power

The measure of a lens ability to bend light

Is the value for power positive or negative for converging lenses

Positive

Is the value for power positive or negative for diverging lenses

Negative

What is a thin lens

A lens with a thickness which allows rays of light to refract but not experience dispersion or aberrations

What do thin lenses used in combination act as

A single lens, with a power equal to the sum of the powers of the individual lenses

What is a real image

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

What is a virtual image

An optical image that cannot be projected onto a screen

Equation for power using distances u and v

1/u + 1/v = 1/f = power

U= distance between object and the lens axis

V= distance between the lens axis and image

f= focal length

Equation for magnification using u and v

M = image height/ object height = v/u

What are the uses for a convex/converging lens

Telescopes, camera lens, microscopes, magnifying glasses, eye glasses

What are the uses of a concave/diverging lens

Binoculars, telescopes, flash light, eye glasses

Why are things lenses used in series

To get a higher magnification and clarity of images. Used in devices like microscopes or telescopes

What is the power of a lens measured in

Diopters, D

What is the equation for photon energy

E=hf

What is the photoelectric effect

When photoelectrons are emitted from the surface of a metal after light above a certain frequency is shone on it. The certain frequency is shone on it.

What is a threshold frequency

The minimum frequency of light required to emit photoelectrons

What is the work function of a metal

The minimum energy required for electrons to be emitted from the surface of a metal

How to convert Joules to ElectronVolts

Divide by 1.6×10^-19

How to convert ElectronVolts to Joules

Multiply by 1.6×10^-19

Why couldn’t the photoelectric effect be explained by wave theory

•Photoelectric effect is immediate, which contradicts wave theory which suggests time is needed for the energy to be supplied to the electrons to reach the work function.

•wave theory suggests that any frequency of light should be able to cause photoelectric emission as energy absorbed by each electron will gradually increase with each incoming wave- can’t explain threshold frequency

•increasing intensity of light doesn’t increase speed of photoelectric emission as suggested by wave theory instead increases the number of photoelectrons released per second

•photoelectrons released with a range of kinetic energies

What does photon model of EM radiation suggest

That EM waves are released in discrete packets called photons which have particle like interactions

Explain the photon model of EM radiation

One photon interacts with one electron. Each photon has energy proportional to the frequency (E=hf), the electron is emitted instantly only if the energy of the photon is greater than the work function of the metal. Any photon energy over and above the work function is gained by the electron as kinetic energy

Explain how/why the wave model doesn’t explain the photoelectric effect

According to the wave model it is expected the energy of the electron would build up over time and eventually be emitted.

The kinetic energy of the emitted electrons would depend on the intensity of the wave (and not the frequency)

What happens if the intensity of EM radiation is increased

Intensity is equal to number of photons released per second, if this is increased the number of photoelectrons emitted increases because more photons interact with electrons per second

What happens to electrons deeper in the metal in the photoelectric effect

All electrons receive the same amount of energy from a photon light but electrons deeper in the metal will lose energy through collisions when leaving the metal so will have a lower kinetic energy

What is excitation (energy)

Occurs when an electron gains enough energy to move up energy levels

What is de-excitation

When an excited electron returns to its original energy level and releases the energy it gained in the form of a photon light

Explain why different elements produce spectra containing different specific wavelengths of light

Electrons move to higher energy levels/ get excited they then move to lower energy levels and release that energy change out as a photon

How does pulse echo technique provide information about the position of an object (feotal scanning)

Short pulse ultrasound waves are transmitted into the target using a transducer , pulse travels inside the body until it reaches a boundary between two materials, the amount of reflection depends on the difference in densities the greater the difference the greater the reflection, reflected waves are detected as they leave the target, the intensities of the reflected waves are used to determine the structure of target and speed of reflected wave and time taken for reflected waves to return determines position of objects in target (using s=vt)

How does the pulse echo technique work

Ultrasound sends out a pulse and detect the time to receive an echo off a boundary material. They then determine the distance with s=d/t

What are the two main pieces of information pulse echo technique gives you

Depth-time between transmission and receipt of the pulse (time delay), nature- amount of transmitted energy received

What are the uses of echo pulse technique

Sonar, prenatal scanning, scanning tunneling microscope (STM) and tunnelling electron microscope (TEM)

What happens if the duration of pulses in echo pulse technique is too long

Pulses will overlap so amount of information obtained (resolution of image) will decrease

What happens if the wavelength of the waves used increases

Less fine details can be resolved, meaning amount of information obtained will decrease