WAVES reflection &
refraction
wave
properties
EM waves
RE FLE CTI ON
TY PES OF WA VE
EX PER IME NTS
TR ANS FER OF
EN ERG Y
IN VES TIG ATING
LI GHT
uses &
dangers of
EM waves
US ES
DA NGE RS
- PART 1 -
- When waves travel through a
medium, the particles of the
medium oscillate and transfer
energy between each other
- Overall the particles stay in the
same place, only energy is
transferred
- Amplitude: maximum displacement of point on a wave from its undisturbed position
- Wavelength: distance between same point on two adjacent waves (eg between two troughs)
- Frequency: number of complete waves passing a certain point per second, 1Hz = 1 wave/sec
- Period of a wave is how long it takes for a full cycle of a wave
- Wave speed: speed at which energy is being transferred
Transverse:
- Oscillations are perpendicular to direction of energy transfer
- Most waves are transverse
- EM waves, ripples & waves in water, wave on string
T = 1/F
PERIOD(S) = 1/FREQUENCY(HZ)
Longitudinal:
- Oscillations are parallel to direction of energy transfer
- Sound waves in air eg ultrasound, shock waves eg seismic wavs
V = F
WAVE SPEED(M/S) = FREQUENCY(HZ) X
WAVELENGTH(M)
λ
Measuring speed of sound:
1 Set up oscilloscopes so detected waves at
microphones are shown as separate waves
2 Start with both microphones next to speaker,
then slowly move one away until the two waves
are aligned on display but are one wavelength
apart
3 Measure distance between microphones to find
one wavelength
4 Use wave speed formula to find speed of
sound waves (frequency is whatever you set
signal generator to, eg 1kHz)
5 Speed of sound in air is around 330m/s
-
Measuring speed of water ripples:
1 Signal generator makes rod create water waves at a set frequency
2 Strobe light used to see wave crests on paper underneath tank
3 Increase frequency of strobe light until wave pattern on paper
'freezes' -> strobe light frequency = wave frequency
4 Measure distance between shadow lines that are 10 wavelengths
apart then divide by 10 to find average wavelength
5 Use wave speed formula to calculate speed
Measuring waves on strings:
1. Set up, turn on signal generator & vibration transducer
2. Adjust frequency until there is a clear wave on string
3. Measure 4 or 5 half wavelengths then divide by 4/5 for mean
half-wavelength then double to get full-wavelength 4. Use wave speed formula to find speed
- All EM waves are transverse & transfer energy from a
source to an absorber
- All EM waves travel at same speed
through air or a vacuum (space)
- They form a continuous spectrum over a range of frequencies, and
they are grouped into 7 basic types based on wavelength & frequency
- There is a large range of frequencies because they are generated by a
variety of changes in atoms & their nuclei
When waves arrive at boundary between two
different materials three things can happen:
- Waves absorbed by material which transfers
energy to material's energy stores
- Waves transmitted through material which
often leads to refraction
- Waves reflected
- Angle of incidence: angle between incoming wave and normal
- Angle of reflection: angle between reflected wave and normal
- Normal: imaginary line that is perpendicular to surface at point of
incidence, normally shown as dotted line
ANGLE OF
INCIDENCE =
ANGLE OF
REFLECTION
- Reflection can be specular or diffuse
- Waves are reflected at different boundaries in
different ways
- Specular reflection happens when wave is
reflected in a single direction by a smooth
surface
eg mirror
- Diffuse reflection happens when wave is
reflected in lots of different directions by a
rough surface eg paper
Investigating refraction with
transparent materials:
1 Place transparent rectangular block
on paper & use ray box/laser to
shine ray at middle of side of block
2 Trace incident ray & mark emerging light ray
3 Remove block & join up incident ray with
emerging point to show path of refracted ray
4 Draw normal at point of incidence & use
protractor to measure angle of
incidence and angle of refraction
5 Repeat using blocks of
different materials, keep incident angle the same
Angle of refraction should change for different
materials as they have different optical densities
Investigating materials that reflect
light by different amounts:
1 Draw straight line on paper & place
object so one side lines up with it
2 Shine ray of light at object & trace
incoming & reflected light beams
3 Draw normal at point of incidence
& use protractor to measure angle of
incidence & angle of reflection
4 Repeat with different objects,
make note of width & brightness of
reflected light ray
Smooth surfaces = thin & bright ray
- When wave crosses boundary between
materials at an angle it
changes direction -> it is refracted
- How much it is refracted by depends on how
much the wave's speed changes, which depends
on the density of the two materials
- Higher density = slower speed of wave
- If wave slows down = bends towards normal
- If wave speeds up = bends away from normal
- The wavelength
changes when a
wave is refracted but
frequency stays same
- If wave is travelling
along normal it changes
speed but is
not refracted
- Optical density: measure of
how quickly light travels through
a material
- Higher optical density = slower
speed of light rays
Constructing ray diagram for refracted light ray:
1 Draw boundary between materials & normal which is 90 to
boundary, then draw incident ray meeting normal (angle
between them is angle of incidence)
2 Draw refracted ray on other side of boundary. If second
material is optically denser than first, refracted ray bends
towards the normal so angle of refraction is smaller than angle
of incidence. If second material is less optically dense, angle of
refraction is larger than angle of incidence
°
Microwaves:
- Satellite communication
- Satellite TV -> signal transmitted into space &
picked up by satellite receiver dish orbiting above
Earth which transmits signal back to Earth in different
direction & is received by satellite dish on ground
- There is slight time delay due to long distance
Infrared Radiation:
- Increase/monitor temperature
- Infrared cameras detect IR & turns it into electrical signal
which is displayed on screen, hotter = brighter it appears
- Food cooked by IR -> absorbing IR increases temperature
- Electric heaters contain wire that heats up when current
flows which emits IR which is absorbed by objects & air, energy
transferred to thermal energy stores which increases temperature
- Microwave ovens -> different
wavelength used in satellite communication
- Microwaves penetrate few cms into food,
are absorbed, transfer energy they are
carrying to water molecules in food which
heats it up, water molecules transfer
energy to rest of food by heating
Light:
- Optical fibres (thin glass/plastic fibres that carry data
over long distance as pulses of visible light)
- Light rays reflected back & forth until they reach end
Ultraviolet Radiation:
- Fluorescence -> UVR absorbed
& visible light emitted
- Fluorescent lights generate UVR
which is absorbed & re-emitted
as light by layer of phosphorus
- Security pens (invisible ink)
- UV produced by Sun -> UV
lamps used to give suntans
X-Rays & Gamma Rays:
- Radiographers take X-Ray 'photographs' to look for broken
bones -> X-rays pass easily through flesh but not through bones
- Radiotherapy used to treat cancer as high doses of x-rays &
gamma rays kills living cells
- Gamma radiation used as medical tracers
