physics unit 1 working with waves

periodic time

time it takes for one full oscillation of the wave to occur

oscillation

a repeated back-and-forth movement or vibration around a central position - sound waves air molecules oscillate back and fourth

speed

how fast the wave travels through a medium - speed - frequency (units m/s)

wavelength

distance between 2 identical points on a wave (crest to crest) - units (m)

amplitude

maximum displacement from the waves rest central position

types of waves

• transverse waves - the oscillations are perpendicular to the direction the wave travels

• sometimes needs medium ( light can travel in vacum)

• examples - light waves , water waves , em waves , seismic waves

longitudinal waves

• the oscillations are parallel to the direction the wave travels

• have compressions and rarefractions

• examples - sound waves , ultrasound

• always needs a medium to travel

interferance waves

• constructive interference - in phase - the waves add together and the resultant amplitude is bigger

• coherent waves - both producing waves of the same frequency, similar amplitudes, and they are in phase

• destructive interference - antiphase

• waves add together and cancel each other out

• path difference - difference in how far they have travelled

• if the path difference at a point - 0 or a multiple of wavelengths, then the waves will arrive in phase at that point

• constructive interference will happen

• if the waves arrive in phase, then constructive interference will occur. This happens when the path difference is 0 or a multiple of wavelengths.

displacement in a wave

distance a point on the wave has moved from its rest position at a given moment in time

diffraction gratings

• many slits that close together - spreading of waves when passing through a gap

• when light passes through a diffraction grating, it disperses into its component wavelengths, creating a spectrum.

• pattern caused by constructive and destructive interferance of diffracted light waves

• the position of the bright fringes depends on the wave length and the speacing between between the slits

emission spectra

• unique to each spectrum

• when atoms or molecules are excited through heat or electrical energy they can absorb energy and promote electrons to higher energy levels - when these electrons return to their lower energy states - they release energy in the form of light - forms of light have specific wavelengths which result in emission spectrum that consist of distinct lines or bands of colour - bright lines on a dark background.

• when light encounters this surface it encounters diffraction - these bending and spreading of waves when they encounter the opening - light waves pass through or reflect off small slits and spreads - leads to a formation of various angles at which light waves interfere with each other.

• This links to constructive and deconstructive - specific angles - the light waves from different grooves will constructively interfere, enhancing certain colours or wavelengths, while at others they may destructively interfere - this cancels it and eliminates the light waves.

• by passing a sample of gas through a spectrometer with a diffraction grating - the system spreads the emission or absorption spectrum - if the observed spectrum matches known patterns ( hydrogen, oxygen , chlorine) the gas can be identified quickly and accurately.

identifying gases

• diffraction gratings are used in spectrometers

• detect which gases are present

• measure concentration

• monitor for toxic or flammable gases

examples of industrial use

• astronomy - Identify elements in stars from emission spectra

• forensics/labs - analyse unknown samples by looking at spectral lines

• environmental monitoring - measure air pollution or greenhouse gases

the wave equation

speed = frequency x wavelength

stationary wave and resonance

wave that doesn’t appear to travel

Stationary waves form when 2 waves of the same frequency and amplitude move in opposite directions and interfere with each other

node - point where no movement happens - displacement - 0

antinode - point where a wave vibrates the most (maximum displacement)

resonance - when an object is made to vibrate at its natural frequency, if the driving frequency (external forces matches object’s natural frequency amplitude of vibration increases - plucking a guitar string

musical instruments

• musical instruments create sound using vibrations, causing stationary waves to form either on strings or inside air columns when these waves match the natural frequency of string resonance occurs - makes sound louder and clearer.

in stringed instruments - guitars and violins

• pluck or bow a string - vibrates and reflects at the end

• creates a stationary wave with nodes at each fixed end

• only certain wavelengths fit onto the string - called harmonics or resonant frequencies

• changing the length , tension or mass of the string changes the pitch

• shorter string - higher pitch

• looser string - lower pitch

in wind instruments (flute, trumpet)

• blowing into instruments causes air columns to vibrate, creating stationary waves. The length of the air column, which can be altered by opening or closing holes - end of the tube act like nodes or antinodes depeding on if they are open/closed - determines the pitch of the sound produced. Resonance occurs when the frequency of the blowing matches the natural frequency of the air column.

• just like stringed ones - only certain wave lengths resonate producing musical notes

summary

stringed instruments - resonance on strings

wind instruments - resonance on air columns

used to control pitch , volume and tone

wave speed equation

wave speed =sqaure root - tension on the string divided by mass per unit length

speed of the waves depends on

tension - higher tension - faster wave

mass per length - heavier string - slower wave

refractive index - measures how much a material slows down

n=c/v sin i = n x sin r

• when light enters glass at an angle it changes direction

• if we know the angles we can work out the refractive index

• example - light eneters glass and angle of 45 degrees to the normal as the angle of refraction is 32 degrees celcius calculate the refractive index of the glass - sin45/sin32

example - light enters glass an angle of 60 degrees the refractive index of the glass is 1.4 calculate the angle of refraction - 60/1.4= 0.62 r= 0.62 sin-1 (shift) - 38.3

total internal reflection

• light can be trapped in glass

• inner surface of glass acts like a mirror and the light reflects back inside

• this happens if the angle to the normal is bigger than a certain critical angle

sin c= 1/n - example - glass has a ri of 1.4 calculate the critical angle for glass - 1/1.4 - 0.71 = sin-1 0.71 =45.2

the critical angle or glass is 42 degrees calculate its refractive index = n=1/sin(42)=1.5

refraction and total internal reflection - speed of light in air is 3×108

• when a wave enters a denser medium at an angle to the normal it bends towards the normal - the wave slows down

• when a wave enters a less dense medium at an angle to the normal - it bends away from the normal and the wave speeds up

glass fibre

• light can travel through glass fibres it reflects off the inside of the fibres by total internal reflection

• fibre optic cables can be used for communication they can carry digital signals in the form of light

• a bundle of fibres can carry an image - endoscope

advantages and disadvantages of modern fibre optics for broadband

advantages of optical fibres:

• Signals can carry more data because band width is greater can carry more channels

• more secure because more difficult to hack

• better quality signals received because its not affected by noise

• signals can be regenerated so no loss of quality of signal

• rate of transfer of data faster because of higher frequency

• digital signals used by computer so no need to convert

  disadvantages

• brittle and fragile - can easily be broken

• jointing , repair is difficult so needs specialist equipment , technician is expensive

• poor joints leads to loss of signal strength so signal is poor

analogue and digital signals

• the digital signal could be a voltage that can either be low or high - low vol represents 0, and high represents 1

• differences between analogue and digital

• value of analogue signal at a certain time can have any value within a range - it is continuous

• value of a digital signal at any time can only have 1 of 2 values - we call these 1 and 0

• how can a digital signal carry information?

• analogue signal - analogue to digital signal - digital signal - (transmission) or (storage in computer)

• reception and storage - digital signal digital to analouge converter - analouge signal

why are digital signals better

• Reason 1 - A digital signal can carry much more information than an analogue signal

• a fibre optic communication signal like this can carry a broadband signal - lots of different signals can be transmitted at the same time with high speed internet and access

• reason 2 - when are signals are transmitted they often pick up noise due to external interference - information in digital os not lost - can get rid of noise easily as you can recognise noise more easily

• Analoge not good - you could lose signals and cant easily remove or recognise it

electromagnetic waves

all em waves travel at the same speed in a vacum - 3×10 to the power of 8

radiowaves

microwaves

infrared

visible light

ultraviolet

xrays

gamma rays

• radio waves

longest wavelength and shortest frequency

• travel easily through the atmosphere, so there used for most wireless communication, TV signals , Wi-Fi , Bluetooth and cell phones

• microwaves

• astronomy

microwaves

• can penetrate clouds , dust and rain so they are sued in weather radar and satellite communication

• cooking , heating

• radar - detecting planes and ships

• satellite communications

infrared

• thermal imaging

• remote control

• night vision

• medical

visible light

• human vision

• photography

• lighting - LED , artificial light

ultraviolet light

• sterilization - uv light kills bacteria and viruses

• tanning - uv rays are used in tanning beds

x - rays

• medical imaging

• diagnosing broken bones

• dental problems

gamma rays

• cancer treatment

• used in radiotherapy to target and destroy cancerous cells and other health problems

communication satellites

• receives signals from ground stations

• transmits hem over a wide area - advantage

• used for microwaves

mobile phones

• has its own transceiver that receives and sends signals

• they are all networked together and the network covers the whole country

• nearby cells use slightly different frequencies to avoid interferance

• each cell sends and receives different frequencies to avoid interferance

network

• a group of devices linked together

• devices in a wifi network connect to a local hub or router that allows communication and data exchange.

Bluetooth

• infrared devices do not need to connect to a local hub they can communicate with each other

• range of the BT devices is limited and small

• blutooth power needs are low

• cant carry high qaulity videos or sounds

• made of short wavelengths cant travel longer distances - loss of signal

infrared devices

• tv remote

• have a range of just a few metres

• info sent as pulses like turning a light on and off

hand shaking

• before devices can send info to each other they need to pair

• involves sending a few short messages

• allows them to recognise each other and decide how they are going to communicate

• stops information from getting lost and jumbled

• many security features - passwords and encryption to protect data

• how optical fibres in an endoscope produce an image

light travels in - one set of optical fibres, light from an external source (computer screen into body lights up the area being examined

Image travels out

Another set of optical fibres collects the reflected light from inside the body and carries it back out these fibres transmit the image by balancing the light along their length using total internal reflection

the light that comes out of the endoscope forms an image which is captured by a camera at the end- the camera converts the signal into an electrical signal that is sent to the computer screen so doctors can see whats going on inside bodies

structure of fibre optics

very thin fibres of class - It has a high refractive index to keep the light within the core through total internal reflection.

each one has cladding - It surrounds the core and causes the light to reflect back into the core, maintaining the signal within the fibre.

outer jacket - Protects the fibre from damage (