waves

• waves

  • transfer energy from one place to another
  • do not move from place to place but oscillate around a particular point
  • can be reflected and refracted
• mechanical waves can propagate in a substance - they require particles
• electromagnetic waves can propagate in a vacuum.
• longitudinal waves

  • the particles oscillate parallel to the direction of energy transfer or wave travel
  • can travel through solids, liquids and gases
  • eg sound waves, P waves (earthquakes)

  
• transverse waves

  • particles oscillate perpendicular to the direction of energy transfer or wave travel
  • can travel through solids but not liquids or gases
  • eg light, EM radiation, S waves (earthquake)

  
• definitions

  • amplitude - maximum displacement from equilibrium position
  • wavelength (λ) - distance between two adjacent identical points
  • frequency (f) - number of waves/oscillations per second
  • time period (T) - length of time for one full wave in seconds

  
• the wave equation

  • wavespeed (m/s) = frequency (Hz) x wavelength (m)
  • time period (s) =1 / frequency (Hz)
• required practical to find the wavespeed (method 1 - ripple tank)

  • set up apparatus to produce waves in water
  • count number of waves passing through a point in, eg, 10 seconds
  • divide this number by 10 to find frequency
  • place a ruler next to the waves and measure across, eg, 10 waves
  • divide this number by 10 to find wavelength
  • calculate wavespeed = frequency x wavelength
  • problem - waves continuously move to hard to measure, so take a video (frequency) or photo (wavelength) to observe measurements

  A) light source

  B) water

  C) screen

  D) wooden bar supported by elastic bands

  
• required practical to find the wavespeed (method 2 - cord and signal generator)

  • set frequency so one complete wave forms
  • read off frequency from signal generator
  • measure length of cord with rule to find wavelength
  • calculate wavespeed = frequency x wavelength
  • adjustments - move bridge to alter wavelength, frequency will need to be adjusted (wavespeed is constant). change tension to alter wavespeed, frequency will need to be adjusted (wavespeed is constant)

  A) pulley

  B) masses

  C) string

  D) movable wooden bridge

  E) vibration generator

  F) signal generator

  
• sound

  • occurs as a result of the vibration of object or medium
  • travels as a result of collisions between particles in a medium
  • the pitch of a sound depends on its frequency
  • the volume of a sound depends on its amplitude
  • normal range of human hearing is 20Hz - 20,000 Hz
  • sounds that humans can't hear is called ultrasound, generally >20,000Hz
• sound waves travel best in solids because of more frequent collisions between particles as they are closer together.
• • waves can be used for detection and exploration
  • waves are partially reflected at the boundary between substances
  • it ultrasound waves are sent out, the time taken for them to travel there and back can be measured
  • if the speed of sound in the substance is known, you can find the distance to the point using the equation distance = speed x time
• detections of earthquakes

  • earthquakes produce seismic waves (P/S waves) that travel through the earth's interior
  • P waves (longitudinal) can propagate in solids, liquids and gases. they have a shadow zone which shows there is a boundary between the core and the mantle as there is change in density causing the waves to be deflected.
  • S waves (transverse) can only propagate in solids, therefore they have a larger shadow zone as they cannot go through the core, showing that it must be liquid.

  
• the electromagnetic spectrum

  • the group of all the types of electromagnetic radiation
  • all electromagnetic waves transfer energy, travel at the same speed (300,000,000 m/s in a vacuum), and can all be reflected, refracted, emitted or absorbed

  A) long

  B) short

  C) high

  D) low

  E) lower

  F) higher

  G) radio waves

  H) microwaves

  I) infrared

  J) visible light

  K) ultraviolet

  L) x rays

  M) gamma rays

  
• gamma rays, x-rays and ultraviolet have the most energy and are called ionising radiation. they can cause genetic mutations such as cancer.
• radiowaves are used for long-wave communication as they are reflected long distances by Earth's atmosphere
• how are radiowaves produced?

  • waves travel through space and can be absorbed by electrons on earth which also oscillate and can produce an alternating current
• microwaves uses

  • communication as they are absorbed by water molecules so can pass through the earth's atmosphere
  • microwave ovens - absorbed by water molecules that vibrate more and conduct energy by conduction or convection
• infrared uses

  • infrared camera - can see which areas are hot and cold, therefore animals will appear brighter as they emit lots of infrared radiation
  • cooking - ovens and grills emit lots of infrared radiation that heat our food by transferring heat energy
• infrared/visible light is used for communication - optical fibres. they transmit data quickly in long distances as pulses of light.
• ultraviolet uses

  • fluorescent lights - generates UV radiation absorbed by a layer of phosphorous and re emits energy as visible light
  • tanning salons - UV rays cause skin to tan
  • water sterilisation - destroys micro organisms
• x rays are used to view the internal structure of objects by firing x rays through a person's body and records the ones that get through.
• gamma rays

  • sterilising equipment/food as they kill microorganisms but pass through the packaging
  • detecting cancer - passes through the body to be detected
• microwaves are good for mobile phone communications as they are not ionising radiation so are not harmful. good for communication and navigation.
• UV radiation to get a tan is good for boosting confidence, supplies vitamin D and makes people happier. however it is ionising so can cause mutations which lead to cancer.
• x rays to measure foot size or for ante-natal scanning are bad as they are ionising so can cause mutations which lead to cancer, and there are also alternative methods.
• blackbody radiation

  • dark matt surfaces are best emitters/absorbers of radiation but worst reflectors
  • light shiny surfaces are the worse emitters/absorbers of radiation but are the best reflectors
  • blackbody radiation is the electromagnetic radiation emitted by an object as a result of its temperature
  • all objects with a temperature above absolute 0 (-273°C) will emit blackbody radiation
  • when an object absorbs radiation, it gets hotter and emits more radiation
  • for an object to be in thermal equilibrium, it emits energy at the same rate as it is absorbed, its temperature would stay constant
• blackbody radiator

  • a hollow object with a very small opening and black interior surface
  • any radiation entering through the opening cannot escape again and will eventually be absorbed by the black surface, causing it to heat up
  • the colour observed when looking through the opening in only due to the energy of the radiation emitted, and therefore the surface temperature
• greenhouse effect

  • the sun has charged particles that are vibrating to emit energy as radiation
  • the earth absorbs the radiation which heats it up
  • the sub emits short wave length radiation and is absorbed by the greenhouse gases and is re-emitted in all directions, warming the earth
• reflection

  • when a wave changes direction at a boundary between substances, but remains in the original substance
  • the law of reflection - angle of incidence = angle of reflection
  • the normal is an imaginary line perpendicular to the surface
• specular reflection is when light rays all reflect off in the same direction, eg from a mirror
• diffuse reflection is when light rays reflect off in different directions from a rough surface.
• law of reflection practical

  • set up apparatus as shown and mark on a normal line on the mirror's surface (perpendicular)
  • shine the light ray at the intersection between the mirror and the normal line
  • mark on where the light rays are (use crosses and join them up)
  • measure the angles of incidence and reflection from the normal
  • repeat with different angles of incidence
  • compare the angles - if angles of incidence = angle of reflection, the law is true for the angles tried

  
• in a convex mirror, the image is the right way up and smaller
• in a concave mirror, when the object is behind the focal point, the image is smaller and inverted because the light rays cross over.
• in a concave mirror, when the object is closer than the focal point is, the image is magnified and the right way up
• reflection, light and colour

  • white light is the combination of all the colours (wavelengths) of the visible spectrum
  • each colour we see corresponds to a different wavelength of electromagnetic radiation
  • the three primary colours of light are red, green and blue
  • the three secondary colours of light are yellow, cyan and magenta
  • the colours that we see objects as depends on what colours the object will reflect and absorb, and what colours of light are present
• primary and secondary colours

  • red and green make yellow
  • green and blue make cyan
  • red and blue make magenta
• refraction

  • when waves change direction at a boundary between substances, and enter a different substance, eg light passing through water
  • refraction occurs because waves travel at different speeds in different substances
  • the change in speed is often caused by the different refractive indexes of different substances
  • the wavelength also changes but the frequency remains constant
• the higher the refractive index, the slower the wave travels and the greater amount of refraction that occurs.