GCSE Physics P6: Waves

period

frequency

amplitude

• time taken for a whole wave to comepletely pass a single point

• the number of waves that pass a single point per second

• maximum displacement of a point on a wave away from its undisturbed position

velocity equation

velocity = frequency x wavelength

relationships

• increase frequency, velocity increases

• wavelength increases, velocity increases

• period is inversly proportional to frequency

• smaller period, higher frequency, greater velocity

transverse and longitudinal waves

• Transverse:
  light, em wave
  has peaks and troughs
  waves oscillate perpindicular to the direction of travel of energy transfer

• Longitudinal:
  sound waves, compressions, rarefraction
  waves oscillate parallel to the direction of travel of energy transfer
  

• they dont transfer matter but they transfer energy

• the wave travels, not the water ripples or the air

measuring velocity of the sound in air

• Make a noise at ~50m from a solid wall, and record time for the echo to be heard, then use speed = distance/time

• Have two microphones connected to a datalogger at a large distance apart, and record the time difference between a sound passing one to the other – then use speed = distance/time

measuring velocity of the ripples on water surface

• Use a stroboscope, which has the same frequency as the water waves, then measure distance between the ‘fixed’ ripples and use v = f x lamda

Where v is the wave speed in metres per second m/s, f is the frequency in hertz Hz and λ is the wavelength in metres m.

• Move a pencil along the paper at the same speed as a wavefront, and measure the time taken to draw this line – then use speed = distance/time

Reflection

• Waves will reflect off a flat surface

• The smoother the surface, the stronger the reflected wave is

• Rough surfaces scatter the light in all directions, so they appear matt and not reflective.

• The angle of incidence = angle of reflection

• Light will reflect if the object is opaque and is not absorbed by the material

• The electrons will absorb the light energy, then reemit it as a reflected wave

Transmission

Waves will pass through a transparent material

The more transparent, the more light will pass through the material

It can still refract, but the process of passing through the material and still emerging is transmission

Ray Diagrams

absorption

• if the frequency of light matches the energy levels of the electrons

• the light will be absorbed by the electrons and not reemitted

• they will be absorbed and then reemitted over time as heat

• so that particular frequency has been absorbed

• if a material appears green, only green light has been reflected and the rest of the frequencies in visible light have been absorbed

Sound waves

• sound enters the ear canal

• causes eardrum vibrates

• these vibrations are amplified by ossicles and passed into the cochlea

• inside the cochlea, hair cells detect the vibrations and convert them into electrical impulses

• this is sent to the brain and interpreted as sound

• the conversion of sound waves to vibrations of solid works over limited frequency range

• this restricts limits of human hearing

limitations

• humans cannot hear below 20Hz above 20kHz

• in the cochlea, the hairs attuned to the higher frequencies die or get damaged

• can be due to constant loud noise damaging these hairs over the years

• or can be due to the changes in the inner ear as you grow older

• smoking, chemotherapy, diabetes are also causes

• so high frequencies cannot be heard as we get older

• we have evolved to hear this range of frequencies as it gives us the greatest survival advantage

• we cannot hear ultrasound as we do not use sonar to hunt. we have accurate vision instead

Ultrasound

• frequency higher than the upper limit of hearing for humans

• partially reflected back when they meet boundary between two different media and the remaining waves continue and pass through

• the time taken for the reflections to reach a detector can be used to determine how far away the boundary is

• allowing ultrasound waves to be used for both medical and industrial imagine

Infrasound (seismic waves)

• a sound wave with frequency lower than 20Hz (seismic waves)

• used to explore the earth’s core

• P-waves:
  longitudinal
  can pass through solids and liquids

• S-waves:
  transverse
  only passing through solids
  move slower than p-waves

• on the opposite side of the earth to an earthwuake, only p-waves are detected suggesting the core of the earth is liquid hence no s-waves can penetrate it
  

Sonar

• pulse of ultrasound sent below a ship and the time taken for it to reflect and reach the ship can be used to calculate depth

• this is used to work out whether there is a shoal of fish below the ship

• or how far the seabed is below the ship

Electromagnetic Waves

• when frequency increases, wavelength decreases it goes in this order:
  Radio, microwave, infared, visible, ultraviolet, x-ray, gamma ray

• they are transverse waves

• do not need particles to move

• in space or air (vacuums) all waves have the same velocity (speed of light)

• they can transfer energy from a source to absorber

examples to illustrate the transfer of energy by em waves

• microwave source to food (heating food)

• sum emits energy to the earth

How does heating up food works

• Microwaves are a type of electromagnetic waves that transfer energy to food by radiation. Inside the microwave oven, a magnetron produces microwaves which reflect off the metal walls and pass into the food.

• The microwaves are absorbed mainly by water molecules in the food. These molecules are polar, so they rotate and vibrate when exposed to the microwaves. This increases their kinetic energy.

• As the molecules move faster, they collide with other particles in the food, transferring energy. This increases the thermal energy of the food, causing its temperature to rise and the food to heat up.

• Heating is not always even because microwaves can form standing wave patterns, creating hot and cold spots, and because different parts of the food contain different amounts of water.

explain why white light is dispersed when travelling through the prisms

When white light travels through a prism, the red light refracts less than the blue light and so the white light is spreads out or splits into a spectrum. This is because there is a different refractive index for each colour, meaning they travel at different speeds.

relationships in waves

• as speed is constant for all EM waves

• as wavelength increases, frequency must increase

• as frequency increases, energy of the wave increases

eyes

• our retina can only detect visible light, a small part of the em spectrum

Refraction

• if entering a denser material, it bends towards the normal

• if entering a less dense material it bends away from normal

• substances will asborb, transmit, refract or reflect certain em waves depending on wavelength

• e.g glass will transmit/refract visible light

• absorb UV radiation

• reflect IR radiation

• the material interacts differently for different parts of EM spectrum because the wavelengths and frequencies are different

• some effects are due to differences in velocity

• when light enters a denser medium, it slows down

• shorter wavelengths slow down more than longer wavelengths

radio waves

• produced by oscillations in electrical circuts.

• when radio waves are absorbed they may create an alternating current with the same frequency as the radio wave itself

• so radio waves can ,themselves, induce oscillations in an electrical circut

Atoms and em radiation

• if an electron gains enough energy it can leave the atom to form an ion so gamma rays originate from changes in the nucleus of an atom as when electrons change orbit higher/lower they absorb or emit em radiation

• changes in atoms and the nucleu of atoms can result in em waves being generated or absorbed over a wide frequency range.

• gamma rays originate from changes in the nucleus of an atom

Hazards of EM waves

UV, X-ray and gamma have hazadous effects on human body tissure

• effects depend on type of radiation and size of dose

• radiation dose: how much exposure leads to harm for a person

• UV: skin ages prematurely, increases risk of skin cancer
  sunscream prevents over-exposure in summer

• x-ray and gamma are ionising radiation that can cause the mutation of genes causing cancer
  minimal exposure should be ensured

how do em waves cause cancer

• Electromagnetic waves can cause cancer if they are ionising (e.g. gamma rays and X-rays). These waves have high energy and remove electrons from atoms, causing ionisation.

• Ionisation can damage DNA in cells, leading to mutations. If mutations occur in genes controlling cell division, this can cause uncontrolled cell growth and tumour formation (cancer).

• Non-ionising waves (like microwaves) do not cause ionisation but can heat body tissue at high exposure.

• Therefore, higher frequency EM waves are more dangerous because they are ionising and can directly damage DNA.

uses of EM waves

• radio: tv and radio
  long wavelength, can travel far without losing quality

• microwave: satellite communication, cooking food
  can penetrate atmosphere to reach satellites

• IR: cooking food, infared cameras
  transfers thermal energy

• visible: fibre optics
  best reflection/scattering in glass (other have too long/short wavelengths)

• UV: sun tanning, energy efficient lamps
  radiates the least heat but more energy

• x-ray: medical imaging and treatment (and gamma)
  very high in energy and can penetrate material easily

• gamma: sterylise medical equipment

Lenses

• forms an image by refracting light

• if light passes through centre of lens, it does not change direction

Concave Lenses

• spreads light outwards: to correct short sightedness as light is focused in front of the retina so needs to be spread out slightly to be able to be focused onto retina

• only have virtual images

• light comes from focal point

• represented with arrows pointing inwards

convex lens

• focus light inwards

• have virtual or real images (appear to be on same or opposite side as the real object)

• used for magnifying glasses, binoculars

• used to correct long sightedness as it focuses the rays closer

• represented with arrows pointing outwards

magnification

image height/ object heigh

visible light

• each colour within the visible light spectrum has its own narrow band of wavelength and frequency

• blue has a shortwer wavelength and high frequency than red

• sunlight is a mix of all colours and this mix appears white

types of reflection

specular - smooth surface gives a single reflection

diffuse - reflection off a rough surface causes scattering

colour filters

work by only absorbing certain wavelengths through and transmitting other wavelengths as the filter absorb every other colour

opaque colours

• if it has a colour, determined by the strength of reflection for different wavelengths

• wavelengths which are not reflected are absorbed

• if all wavelengths reflect equally it is white in colour

• if all wavelengths are absorbed it is black

• the wavelength which is absorbed = colour which it appears

• objects that transmit light are ether transparent or translucent (scatter most light and let some through)

colours lighting

RGB mixing

black body and space

• all objects, no matter what temperature, emit and absorb IR radiation. the hotter the body:
  the more IR it radiates in a given time
  the greater amount of shorter wavelength radiation released (waves with more energy e.g. x-rays)

• the intensity and wavelength distribution of any emission depends on the temp of the body

black body

• does not reflect or transmit any radiation and absorbs all the radiation it receives
  - as a good absorber is also a good emitter, a perfect black body would be the best possible emitter

body at diff temps

• at constant temp it is still absorbing radiation at the same time as it is emitting radiation

• the temp of a body increases when the body absorbs radiation faster than it emits radiation

• when it cools down energy is released at a greater rate than it absorbs

Earth

• temp of the earth depends on: rate of absorption and emission of radiation, reflection of radiation into space

• sun’s energy is mostly absorbed by the earth’s atmosphere and some is reflected

• the amount of energy re-radiated and absorbed leads to the earth’s temp

Greenhouse effect

• The greenhouse effect is the warming of Earth caused by greenhouse gases in the atmosphere, such as carbon dioxide and methane.

• Short-wave radiation from the Sun passes through the atmosphere and is absorbed by the Earth’s surface. The surface then emits long-wave infrared radiation.

• Greenhouse gases absorb this infrared radiation and re-emit it in all directions, including back towards Earth, trapping heat in the atmosphere.

• This increases the Earth’s average temperature. Human activities (like burning fossil fuels) increase greenhouse gas levels, enhancing the greenhouse effect and causing global warming.

leslie’s cube

A Leslie cube is a hollow metal box made of steel and emission of radiation. The cube has different surface finishes on four of the faces. Typically these are:  

• Matt black surface 

• Shiny silver surface 

• White surface 

• Shiny black surface 

The cube is filled with hot water. 

metal cube is filled with boiling water from the kettle, so it is all at the same temperature​

a radiant heat thermometer is pointed towards the different surfaces making sure it is at the same distance from each - this works by detecting the infra-red radiation emitted from each surface​