Physics - Waves

Intro
 Waves transfer energy without transferring any matter
 When waves travel through a medium, particles of the medium oscillate
& transfer energy between each other
 Amplitude – maximum displacement from undisturbed position
 Wavelength – distance between the same point on two adjacent waves
 Frequency (Hz) – number of complete waves passing a point per second

Transverse & Longitudinal Waves

 Transverse:
Oscillations are perpendicular to the direction of energy transfer
Ripples on a water surface
Electromagnetic waves
S waves
 Longitudinal:
Oscillations are parallel to direction of energy transfer
Sound Waves
P Waves

Measuring Speed of Sound Waves

 Attach signal generator to speaker - sounds with specific frequency
 Use two microphones & a oscilloscope to find wavelength
 Move 1 microphone away from the speaker until the display on the
oscilloscope has the waves exactly one wavelength apart
 Measure the distance between the microphones
 Use the equation to calculate speed of sound – should be 330m/s
 Physics - Waves

Required Practical – Speed of Water Ripples

 Use signal generator attached to dipper – water waves with set frequency
 Use a lamp to see the wave’s shadows on a screen below the tank
 Measure the total distance between 10 shadow lines
 Divide this value by 10 to find the average wavelength
 Use the equation to calculate speed of the water waves

Required Practical – Speed of Waves in a Solid

 Connect signal generator to vibration transducer
 Connect to taut string – use masses on a pulley
 Adjust frequency on signal generator for clear waves
 Find length of 4 half wavelengths & divide to get mean half wavelength
 Double to get wavelength – use equation to calculate speed of wave

Waves at a Boundary of Two Different Materials

 Absorbed – transfers energy to material’s energy stores - microwave
 Transmitted – wave continues through material – refraction
 Reflected
Depends on wavelength & properties of materials involved

Reflection

 Specular Reflection – reflected in single direction by a smooth surface

 Diffuse Reflection – Rough surfaces scatters reflected waves
Surface appears matte (not shiny)

Refraction
 Physics - Waves

 Wavelength changes but frequency stays the same

 If wave travels along the normal, changes speed but not direction
 Optical density – measure of how fast light travels through a material
 Refractive index – measure of much waves refract through a material

Required Practical – Refraction

 Place transparent rectangular block on a piece of paper and trace around it
 Using a laser, shine a ray of light with an angle of incidence of 30°
 Measure angle of refraction
 Use different material blocks & measure angle of refraction for each one
 Keep angle of incidence the same

Required Practical - Reflection

 Place glass block on paper, draw around with pencil, remove block
 Draw the normal
 Use a protractor to measure & then draw line at 10° to the normal
 Replace glass block & use ray box to shine ray of light down the line
 Draw the reflected ray, remove block, &measure angle of refraction
 Repeat, increasing the angle of incidence by 10° each time
 Repeat experiment 10 times, remove anomalies, calculate mean
 Possible source of inaccuracy:
width of light ray makes it difficult to judge where the centre is
Sound Waves
 Can travel through solids causing vibrations through the material
 Cause ear drum to vibrate – creates sensation of sound
 Vibrations passed to bones called ossicles, through semi-circular canals
 Cochlea turns vibrations into electric signals to allow you to sense

 Conversion of sound to vibrations works over limited frequency range

 Restricts limit of human hearing between 20Hz & 20kHz

 Reflected by hard flat surfaces – echoes

 Refract as they enter different media
 Physics - Waves

Ultrasound
 Sound with frequencies higher than 20kHz
 Ultrasound waves get partially reflected at boundaries
 Passed through body – when reaches a boundary (baby) it is partially
reflected – Not as dangerous as X-rays
 Used to find flaws in pipes or wood or metal
 Used by boats to find depth of water/locate objects
 Medical & Industrial Imaging

Seismic Waves
 P Waves – Longitudinal – travel different speeds through solids & liquids
 S Waves – Transverse – cannot travel through a liquid
 Provide evidence for the structure & size of Earth’s core

Electromagnetic Waves
 Transverse waves that transfer energy from source to an absorber
 Forms continuous spectrum - all types travel same velocity through
vacuum or air
 Waves that form the spectrum are grouped in wavelength & frequency

 Rude - Monkeys - Invented - Very - Unhealthy - X rated - Games

 Lowest frequency ---------------------------------------- Highest Frequency
 Longest wavelength -------------------------------------Shortest Wavelength

Properties of EM Waves
 Physics - Waves

 Different substances may absorb, transmit, refract, or reflect EM waves in

ways that vary with wavelength

 Radio Waves – Produced by oscillations in electrical circuits – a.c

When absorbed they create a.c with same frequency as radio wave itself
– radio waves induce oscillations in electrical circuits

 Infrared – skin burns

 Ultraviolet, X, & gamma rays have hazardous effects on human tissue.

 Ultraviolet - cause skin to age prematurely & increase risk of skin cancer
 X & Gamma rays - ionising radiation - cause mutation of genes & cancer

 Changes in atoms & nuclei 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
 Radiation dose (in sieverts) - measure of risk of harm from exposure

Uses of EM Waves
 Radio Waves – Television & radio
Long microwaves diffract (bend) around curved surface of the Earth
 Micro Waves – Satellite Communication & cooking food
 Infrared Waves – Electrical heaters, cooking food, & infrared cameras
 Visible Light – Fibre Optic Communications
Light isn’t easily absorbed or scattered when travelling along a fibre
 Ultraviolet Waves – Energy efficient lamps, sun tanning
 X rays & Gamma rays – Medical imaging & treatment
Passes easily through flesh

Lenses
 Physics - Waves

 Convex – rays converge

To decrease Concave Convex

focal length, (Magnifies until F – (Diminishes)
make lens then diminishes)
more extreme (Moves away until F -
then gets closer to lens)
<F Virtual, Upright, Magnified Virtual, Upright, Diminished
@F No image – parallel waves Virtual, Upright Diminished
<2F Real, Inverted, Magnified Virtual, Upright Diminished
@2F Real, Inverted, Same size Virtual, Upright Diminished
>2F Real, Inverted, Diminished Virtual, Upright Diminished

Visible Light
 Each colour has narrow band of wavelength & frequency
 Colours filters absorb certain wavelengths & transmit others
 Colour of opaque object - which wavelengths are more strongly reflected
 White – all wavelengths are equally reflected
 Black – all wavelengths absorbed
 Objects that transmit light are either transparent or translucent

Black Body Radiation

 All objects emit & absorb infrared radiation
 Hotter the body, the more infrared radiation it radiates
 Perfect black body – object that doesn’t reflect or transmit any radiation
 Perfect black body – best possible emitter & absorber
 Intensity – power/area
How much energy is transferred to an area in a certain time
 As temperature increases, intensity of every emitted wavelength increases
 Intensity increases at a faster rate for shorter wavelengths
 Physics - Waves

Radiation Affects Earth’s temperature

 During day – more radiation absorbed than emitted or reflected
Increase in local temperature
 During night – less radiation absorbed than emitted or reflected
Decrease in local temperature
 Changes to atmosphere can affect overall temperature of Earth
If atmosphere starts absorbing more radiation then temp. will increase