Waves

Wave

An oscillation that transfers energy WITHOUT transferring any matter by making particles of the substance oscillate.

Transverse Wave

• Waves with oscillations that are perpendicular to the direction of travel/energy transfer.

• Frequency of a wave is number of time it oscillates in one second

• 1Hz - 1 oscillation per second

fREQUENCY = 1.5hZ
Oscillates 1.5 times in 1 second

• Amplitude

• Maximum displacement from equilibrium position

Wavelength is the length of one oscillation

• measured in meters

• distance from a point on one wave to the equivalent point on the adjacent wave

Crests

high points on a wave (peak)

Throughs

low points on a wave

Rest Position

The position where the wave is at rest, half way between the crest and trough.

frequency

The number of waves passing a given point in a second.

Period of a wave

The amount of time taken for a full cycle of a wave to be comepleted./ entire oscillation of wave

Time , Period Formula

P=1/frequency(hz) or 1/T (s)

Time period

Wave Speed

v=fλ velocity(m/s)=wavelength(m)*frequency(hz)

Example of transverse waves

• EM Waves

• Water Waves

• Waves on a string

• Seismic S-Waves

Longitudinal Wave

• Waves with oscillations that are parallel to the direction of travel/energy transfer.

• The show areas of compression and refraction

Examples of Longitudinal Waves

• Sound Waves

• Seismic P Waves

What are the two types of waves?

• Longitudinal Waves

• Transverse Waves

Differences between longitudinal/transverse

Property	Transverse waves	Longitudinal waves
Structure	Peaks and troughs	Compressions and rarefactions
Vibration	Right angles to the direction of energy transfer	Parallel to the direction of energy transfer
Vacuum	Can travel in a vacuum (electromagnetic waves only)	Cannot travel in a vacuum
Material	Can move in solids and on the surface of liquids	Can move in solids, liquids, and gases
Density	Density is constant	Density is not constant
Pressure	Pressure is constant	Pressure is not constant
Speed of wave	Depends on the material it is travelling through (fastest in a vacuum)	Depends on the material it is travelling through (fastest in a solid)

Wavefront Description

• Both transverse and longitudinal waves can be represented as wavefronts

  • This is where the waves are viewed from above

• For a transverse wave:

  • One line represents either a peak or a trough

• For a longitudinal wave:

  • One line represents either a compression or a rarefaction

• The arrow shows the direction the wave is moving and is sometimes called a ray

• The space between the lines represents the wavelength

  • When the lines are close together, this is a wave with a short wavelength

  • When the lines are far apart, this is a wave with a long wavelength

Explain why the water doesnt travel but the water in ripples?

• Ripples on water surface cause floating objects

• To bob up and down

• They don’t move object across the water to the edge

• This means that the wave travels but no water (no movement of matter)

Describe evidence that waves transfer energy, not the medium.

• Particles in the medium vibrate/oscillate about a fixed position

• No net movement of the medium

• Wave transfers energy, not matter

• Example: cork on water moves up and down, not along

• Example: air particles vibrate back and forth in sound waves

• Shows wave travels through medium, medium stays in place

What is wave speed?

speed at which the energy is transferred (or the wave moves) through the medium

How to measure the speed of sound in air?

• need to find out the

  • frequency and wavelength of sound wave

• you can do this by generating a sound wave with specific frequency

• by attaching a signal generator to a speaker

• sound wave then detected by microphones

• which convert it to a trace on an oscilloscope

Methods to measure speed of sound on air

• set up both microphones next to the speaker

• detected wave at each microscope seen as separate wave on oscilloscope

• move one microphone away from speaker

  • wave will shift sideways on oscilloscope

  • keep moving it until the two waves on the oscilloscope are aligned once more

  • at this point microphones will be exactly one wavelength apart so measure distance between them

  • you can then use wave speed formula to find speed

  • typically speed out sound is 330m/s

Waves at a boundary (between two different materials) can

- Absorption (by material)

- Reflection

- Transmission (travel through material while undergoing refraction)

How to draw ray diagram for reflection

1. Draw normal perpendicular to the boundary at the point of incidence.

2.Measure angle of incidence and repeat on the other side

3.Then draw reflected ray on other side

Angle of incidence

Angle between incoming wave and normal

Angle of reflection

Angle between reflected wave and normal

Law of reflection

angle of incidence = angle of reflection

What are two types of reflection?

• specular reflection

• diffuse reflection

Specular Reflection (Smooth boundary)

when waves are reflected in a single direction by a smooth surface (gives clear reflection)

Diffuse Reflection (Bumpy Boundary)

When waves are reflected by rough surfaces and so are scattered in different directions as the normal for each bump are different.

• creates an unclear image/no image at all

A - Incident Ray

B - Normal

C - Reflected Ray

D - Angle of Incidence

E - Angle of reflection

F - Point of Incidence

Reflection

A wave hits a boundary between two media and does not pass through, but instead stays in the original medium

Transmisson

A wave passes through a substance

Describe the effects of reflection, transmission and absorption at material interfaces.

• Reflection: wave bounces off boundary

• Transmission: wave passes through material

• Absorption: wave energy taken in by material

• Reflected waves change direction

• Transmitted waves may change speed/wavelength

• Absorbed energy often converted to heat

• Depends on properties of the materials at the boundary

How to draw a ray diagram

1. Draw the boundary between the materials and the normal at the point of incidence (perpendicular to boundary)

2. Draw incident ray that meets the normal at the point

3. Draw refracted ray, if the second material is denser than the first then the refracted ray will bend towards the normal

4. If the material is less dense that the ray will bend away from the normal.

Absorption

Energy is transferred from the wave into the particles of a substance

• waves can be partially or completely absorbed

• Light will be absorbed if the frequency of light matches the energy levels of the electrons

Refraction through medians

• Wave travels faster in some materials than other speed of wave can change as it crosses a boundary

• If waves slow down at boundary it bends towards normal

• If wave speed up at boundary it bends away from normal

• If wave enter more optically dense material waves move more slowly bends towards normal angle of refraction smaller than angle of incidence

• If wave enter less optically dense material waves speed up so it bends towards away from the normal angle of refraction greater than angle of incidence

What is optical density of a material?

• how quickly light travels through it

• higher optical density - slower light travels

Refraction

• refers to change in direction of a wave

• as it passes from one medium to another

  • due to change in speed/density of medium it is passes through changes

What happens to light as it passes through a denser median?

• wavespeed of light will decrease (more slowly)

• it will bend towards the normal

• frequency of light will stay the same

• wavelength of light will decrease

• white light contains all the different wavelengths of visible light (ROYGBIV)

• as light passes through prism → waves of different wavelength slow down and get refract to different degrees

• which causes them to effectively spread out

• allowing you to see the different colours

A - Incidence Ray
B - Angle of Incidence
C - Point of Incidence
D - Angle of Refraction

E - Refracted Ray

F - Normal

G - Emergent Ray

EM Spectrum Order

• radio waves - Long Wavelength/Short Frequency

• micro waves

• IR

• Visible Light

• Ultraviolet Rays

• X Rays

• Gamma Rays - Short Wavelength/Long Frequency

EM Spectrum

• transverse waves that transfer energy from the source of the waves to an absorber

• continuous spectrum of all types of electromagnetic wave travel at the same velocity through a vacuum (space) or air.

EM Waves Speed

• Travel at same speed in a vaccum (3*10^8) and same speed in air (slower than vaccumn)

• Transfer energy from a source to absorber

What EM waves can our eyes detect?

• our eyes can only detect visible light

• so they can only detect a limited range of electromagnetic waves

Examples of how EM WAVES TRANSFER ENERGY

• microwaves → water molecules absorb certain wavelengths of microwave radiation

  • microwave oven transfers energy by radiation to the thermal store of food placed inside it

• infared → energy is transferred by radiation to thermal store of object/surroundings

• sun

  • visible light

  • IR waves heat up earth

  • UV waves cause suntans

  • via radiation

Diffract

Diffraction is the spreading out of waves as they pass through an aperture or around objects

Dispersion

Dispersion is defined as the spreading of white light into its full spectrum of wavelengths.

Properties of electromagnetic waves when they meet a boundary

• refract/reflect electromagnetic waves in ways that vary with wavelength

• some example are due to difference in velocity of waves in different substances

How are radiowaves produced?

• produced by oscillation in electrical circuits/magnetic fields

• when radio waves are absorbed they create a alternating current → with the same frequency as radio wave itself

• so radio waves can induce oscillation in an electrical circuit themselves

How to produce radiowaves part 2?

• you can produce radio waves using ac current in an electrical circuit

• object which charges electron to oscillate to create radio waves is called a transmitter

• when transmitted radiowaves reach a reciever the radiowaves are abosrbed

• energy carried by waves is transfered to electrons of material in reciever

• energy causes electrons to oscillate → complete electrical circiut (then ac current generated)

• current has same frequency as the radio wave that generated it

Radio waves Applications

• Long wavelength radio - 1-10km diffract bend around curved surface of the earth

  • so signals can be received even if it isn’t in line of transmitter

• Short wavelengfth radio - 10m-100m reflected between ionosphere (electrically charged layer) to get to places

  • can be recieved long distances from transmitter

• Medium wave - reflect off ionsphere depnding on atmospheric conditons

• TV signals Fm radio - short wavelengths 10cm-10m must be in direct sight of trasmitter → doesnt bend or travel far through buildings

• Bluetooth → Short wavelength radio devices used to send data over short distances without wires

Microwaves

• Satellites - certain wavelengths of microwave can easily pass atmosphere and reach satellites (without being reflect,refracted,diffracted or absorbed)

• transmitter - satellite - satellite dish

• however there is a slight time delay due to long distance being travelled

• Cooking - microwaves abosorbed by food molecules,transferring energy to water molecules in food causing it to heat up → which quickly cooks food

Infared Radiation

• Emitted from all object that have thermal energy

• Hotter it is more infared radiation

IR radiotin applications

• Cooking Food → causes objects to get hotter - means food can be cooked using IR radiation (as temp increases when it absorbs IR radiation)

• Infrared Cameras → heat loss can be detected through different parts of the camera/ night vision equipment (hotter object is brighter it appears)

• Electrical Heating → contain long piece of wire that heat up when current flows through it → lots of IR radiation is emitted and absorbed by objects in the room

• which increases thermal store of object causing temp to increase

Visible light applications

• em radiation that can be seen with naked eye

  • so used in CAMERAS

• fibre optic communications →

  • optical fibres are thin glass /plastic fibres that carry data

  • over long distance due to pulses of visible light

  • reflection (so they are bounced back and forth)

  • visible light is used as it is not easily absorbed or scattered as it travels along a fibre

UV radiation

• Ionise atoms inside our cells leading to sunburn — cancer sometimes

• Flourescent light,tanning beds,security ink,destroying microorganisms

• Flourescent - ultraviolet light absorbed and emitted as visible light (due to layers of phosphorus → makes them energy efficient)

UV radiation applications

• Energy efficent lamps → flourescent light generate uv radiation abosrbed then remitted as visible light

  • due to layer of phosphorus inside bulb

  • energy efficient so good to use over long periods of time

• Sun Tanning → UV lamps are used to give them artifical sunlight - overexposure to UV raadiation is dangerous (but floursecent emit low uv so its fine)

X ray process

• X rays are fired at patient

• Absorbed by material that are very dense like bones

• Trasmitted through materials that are not very dense like flesh

• X rays that pass through are dected by detector plate forming an xray image

Gamma Rays

• Changes in atoms and the nuclei of atoms can result in

• electromagnetic waves being generated or absorbed over a wide frequency range.

• Gamma rays originate from changes in the nucleus of an atom.

gamma xyra medical treatment

• radiographers use x rays to treat people with cancer (radiotherapy)

• high doses of these rays kill living cells so they are directed towards cancer cells

• precaution must be taken to avoid killing healthy cells

• Uses:

  • sterilise equipment/medical imaging (tracers) /treat cancer

Oscilloscope

Device used to display wavelength amplitude and frequency of voltage and current

Why cant microwaves in satellite communcation be abosrbed by water molecules?

• If they were absorbed by water molecules they would not be able to pass through the atmopshere

Dangers of EM radiation

• High frequnecy waves have lots of damage eg.

• Ultraviolet waves

  • can cause skin to age prematurely and increase the risk of skin cancer

• . X-rays and gamma rays

  • are ionising radiation that

  • can cause the mutation of genes and cancer.

• radiowaves

  • don’t have much energy so pass through soft tissue without being absorbed

Sieverts

• radiation does depends on exposure and type of radiation

• measured is sieverts

• 1000 milli sieverts = 1 sievert

Convex Lens

• lens that buldges outwards

• parallel rays of light are brough to a focus at the principal focus

• distance from lens to principal focus is called focal length

Convex/Converging Lense

Concave/Diverging Lense

Lens

• forms an image by refracting light and changing its direction

What is distance from the CENTRE lens to the principle focus called?

• focal length

Three rules for convex lenses

• incident ray travelling parallel to the axis refract through the lens and passes through the principal focus onto other side

• incident ray pass through centre carries on in the same direction

• incident ray passing through principal focus before meeting lens refracts through lens and travels parallel to axis

Concave lenses

• caves inwards - parallel rays of light diverge out

• image produced is always virtual

• principal focus of concave lens is point where rays hitting lens parallel to axis appear to all come from (you can trace them back until they appear to meet up at a point behind the lens)

Three important rules for concave lenses

• incident ray travelling parallel to the axis refracts through the lens and travels in line with near sided principal focus (so appear to have come from principal focus)

• incident ray passing through centre of lens carries on in the same direction

• incident ray passing through lens towards far sides principal focus refracts through the lens and travels parallel to axis

How to draw ray diagrams concave

1. Lens with arrows facing outwards

2. Draw line going from the top of object through middle and touching top of lense.

3. Draw a virtual line from the top of the lense meeting point to the focal point and continue that line on the other side with a solid line.

4. Point where they meet is where image is reflect.

How to draw ray diagrams convex

1. Lens with arrows facing inwards.

2. Draw line going from the top of object through middle and touching top of lense.

3. Draw a real line through the focal on the other side draw until two lines meet.

4. Point where they meet is where image is reflect.

(Sometimes when it is too small to meet go backwards and draw a virtual image)

How to describe image

- Virtual or Real

- Inverted or Upright

- Smaller or Larger

magnification produced by a lens can be calculated

• convex lenses virtual image - object magnified must be closed to the lens than focal length

• image height/object height

• magnification is a ratio and has no units

• Image height and object height should both be measured in either mm or cm.

Qualities of high power lenses

• more curved

• material that refracts light more strongly

What does each colour in the visible light spectrum have?

• has its own narrow band of wavelength

• and frequency

• eg. violent at 400nm and red at 700nm

Visible Light

• waves cover a very large spectrum

• however we can only see a tiny part of the spectrum

What happens when you mix colours?

• colour can be mixed together to form other colours

• only colours that you can make are primary colours red/green/blue

White light

• contains wavelengths of all colours

• some object appear white as they reflect all wavelengths EQUALLY

Black

• absence of light’

• some object appear black as all wavelengths are absorbed

How do colour filters work?

• absorbing certain wavelengths (and colour)

• and transmitting other wavelengths (and colour).

Colour of opaque objects

• determine by which wavelengths of light are more strongly reflected

• opaque objects do not transmit light

• they absorb wavelengths of light and reflect others

• eg. apple reflect red

For opaque object that are not a primary colour:

• they may be reflecting either the wavelengths of light corresponding to that colour

• or wavelengths of primary colours that can mix to make that colour

• eg. banana looks yellow as it reflects red and green light

Transparent objects

• transmits light in straight lines (all light through)

• so you can see image clearly

• eg. glass

Translucent objects

• transmit light but all scatter it

What does colour of trannsparent/translucent object depend on?

• wavelengths of light transmitted and reflect by it

• some wavelengths of light still abosrbed/reflected

Colour Filters

• only transmits certain wavelengths of light

• and absorbs all other wavelengths

• eg. look at blue object through blue colour filter object is blue (light reflect and transmitted by filter)

• eg. if object was red the object will appear black if viewed through blue colour filter (as all light reflect by object is absorbed by filter)

• eg. transmits that colour eg. white light is shone at blue colour filter → only blue light will be let through rest is abosrbed

Non - Primary colour filter

• let both wavelengths of light for that colour

• and the wavelengths of the primary colours that can be ADDED TO MAKE THAT COLOUR

Black body radiation

• perfect black body is an object that absorbs all of the radiation falling upon it

• it does not reflect or transmit any radiation

• it is the best possible emitter of radiation

Radiation emitted by objects?

• some object emit radiation due to their thermal energy stores

• dont just emit infared radiation → emit a range of wavelengths and frequencies across EM spectrum

• intensity and distribution of radiation emitted depends on objects temperature

Intensity of wavelengths emitted

• power per unit area

• intesity of radiation depends on temerpature

• As temperature of object increases the intensity of every emitted wavelength increases

What do all bodies do?

• that all bodies (objects) emit radiation

• that the intensity and wavelength distribution of any emission depends on the temperature of the body.

What happens to a body temp icnreases?

• body absorbs radiation faster than it emits radiation