3d: Light and sound

is visible light longitudinal or transverse

transverse

can light be refracted

yes

can light be reflected

yes

sound waves

vibration of air molecules

are sound waves longitudinal or transverse

longitudinal

can sound waves be refracted

yes

can sound waves be reflected

yes

what are reflected sound waves called

echo

angle of incidence

angle of the wave approaching the boundary

angle of reflection

angle of wave leaving the boundary

law of reflection

angle of incidence = angle of reflection (i=r)

how are angles measured

between the wave direction and a line at 90 degrees to the boundary

where does the arrow of the incident ray point

towards the boundary

where does the arrow of the reflected ray point

away from the boundary

snell's law

n = sin i / sin r

what happens to light when it enters a denser medium

it bends towards the normal

which factor affects how much the light bends

density of the material

less dense to more dense medium

light bends towards the normal

more dense to less dense medium

light bends away from the normal

angle of incidence of light (notation)

i

angle of reflection of light (notation)

r

refractive index (notation)

n

relationship between refractive index, angle of incidence and angle of refraction

n = sin i / sin r

total internal reflection

when light is reflected when moving from a denser medium towards a less dense one

when does total internal reflection occur

when the angle of incidence is greater than the critical angle and the incident material is denser than the second material

two conditions for total internal reflection

• angle of incidence > critical angle

• incident material is denser than second material

where is total internal reflection utilised

• optical fibres

• prisms

total internal reflection - optical fibres

light travelling down an optical fibre is totally internally reflected each time it hits the edge of the fibre

total internal reflection - where is it used along optical fibres

• communications

• endoscopes

• decorative lamps

• safety reflectors

total internal reflection - prisms

the light totally internally reflects in both prisms

total internal reflection - where is it used along prisms

• periscopes

• binoculars

• telescopes

• cameras

periscope

a device that can be used to see over tall objects consisting of two right-angled prisms

what happens to the angle of incidence once the angle of refraction is exactly 90

it's known as the critical angle

angle of incidence larger than critical angle

refracted ray is reflected

relationship between critical angle and refractive index

sin c = 1/nl

large refractive index of a material

small critical angle

frequency range for human hearing

20 - 20000 Hz

what can be observed by an oscilloscope

changing signals like sound waves and alternating current

what happens to the longitudinal sound wave when a microphone is connected to an oscilloscope

it is displayed as though a transverse wave on the screen

what does the time base measure

time period of the wavew

what does the height of the wave measured from the centre of the screen tell us

amplitude

what does the number of waves on the screen tell us

frequency

more waves displayed

increased frequency

less waves displayed

decreased frequency

high pitch

high frequency of vibrationl

low pitch

low frequency of vibration

loud sound

large amplitude

soft sound

small amplitude

investigating refraction - variables

independent variable: shape of block
dependent variable: direction of refraction
control variables: width and frequency of light

investigating refraction - equipment

• ray box

• protractor

• paper

• pencil

• ruler

• perspex blocks

investigating refraction - method

• place glass box on a sheet of paper

• draw around rectangular perspex

• direct a beam of light at the face of the box using the ray box

• mark the paper

• draw a dashed line at right angles to the outline of the block where the points are

• remove block and join marked points

• replace block with outline and repeat at a different angle

• repeat for each shape of perspex block

investigating refraction - marking the paper

• point on the ray

• point where the ray enters

• point where the ray exits

• point on the exit light ray

investigating refraction - refraction patterns for different blocks

investigating refraction - analysis

i and r are measured from normal

investigating refraction - systematic errors

incorrectly drawn lines

investigating refraction - random errors

• inaccurately marked points

• protactor resolution

investigating refraction - safety considerations

• ray box light could cause burns

• light might damage eye

• keep all liquids away

investigating refractive index - variables

independent variable: angle of incidence
dependent variable: angle of refraction
control variables: use of perspex block, width and frequency of light beam

investigating refractive index - method

same as investigating refraction

investigating refractive index - method

same as investigating refraction

investigating refractive index - analysis

n = sin i / sin r

investigating refractive index - systematic errors, safety consideration

same as investigating refraction

investigating the speed of sound - equipment

• trundle wheel

• wooden blocks

• stopwatch

• oscilloscope

• microphones

• tape measure

investigating the speed of sound - measuring the speed of sound between two points (variables)

independent variable: distance
dependent variable: time
control variable: same location

investigating the speed of sound - measuring the speed of sound between two points (method)

• measure distance between 2 people

• 1 person should hold two wooden blocks to bang together above their head

• the other should hold a stopwatch which they start when they see the blocks bang together and stop when they hear it

• repeat for average

• repeat with various distances

investigating the speed of sound - measuring the speed of sound between two points (analysis)

speed = distance / time

investigating the speed of sound - measuring the speed of sound with oscilloscopes (variables)

independent variable: distance
dependent variable: time
control variables: same location, same set of microphones

investigating the speed of sound - measuring the speed of sound with oscilloscopes (method)

• connect two microphones to an oscilloscope and place them 2 m

• set up the oscilloscope so that it triggers when the first microphone detects a sound and adjust the time base so that the sound arriving at both microphones can be seen on the screen

• make a large clap using the two wooden blocks next to the first microphone and use the oscilloscope to determine the time at which the clap reaches each microphone and the time difference between them

• repeat at several distances

investigating the speed of sound - measuring the speed of sound with oscilloscopes (analysis)

speed = distance / time

using an oscilloscope - variables

independent variables: tuning forks of different frequencies
dependent variable: time period

using an oscilloscope - equipment

• tuning forks

• microphone

• oscilloscope

• wires

using an oscilloscope - method

• connect the microphone to the oscilloscope and test it

• adjust the time base of the oscilloscope until the signal fits on the screen

• strike the tuning fork on the edge of a hard surface to generate sound waves of a pure frequency

• hold the tuning fork near to the microphone and observe the sound wave on the oscilloscope screen

• freeze the image on the oscilloscope screen

• measure and record the time period of the wave signal on the screen

• repeat for varying tuning forks

using an oscilloscope - analysis

frequency = 1 / time period