Waves

Longitudinal and transverse waves

• Waves transfer energy from one place to another but don’t transfer any matter

  • They vibrate/ oscillate to do this

• Amplitude is the max displacement from equilibrium

• A time period is the time it takes for one complete oscillation

• F=1/t

  • Frequency(Hz) equals the number of complete oscillations per second

• V=f\lambda

  • Wave speed (m/s) = frequency (Hz) x wavelength (m)

• Transverse waves

  • Oscillations are perpendicular to the direction of energy transfer

    • Energy moves right by the wave moving up and down

  • For example, all electromagnetic waves, ripples and guitar strings

• Longitudinal waves

  • Oscillations are parallel to the direction of energy transfer

    • Energy moves right though the wave compressing left and right

  • For example, sound waves

Reflection

• When a wave arrives at a boundary, it is either:

  • Absorbed (energy transfers to the surface)

  • Transmitted (energy passes through - can be refracted)

  • Reflected

    • This depends on the wave length and properties of materials

• Ray diagrams show how waves act at boundaries

• When boundaries are flat, all the normals are in the same direction

  • This is called specular reflection, where a clear image is formed

• When boundaries are rough, like paper, the normals are all in different directions

  • They are perpendicular to where they hit the surface

  • This is called diffuse/scattered reflection, where light is reflected in different directions with no image being formed

Refraction

• Refraction is when waves change direction as they pass from one medium to another

• Waves travel at different speeds in different materials

  • This is due to different densities

  • For electromagnetic waves, the higher the density, the slower the wave travels

  • If it travels from air into glass, it will slow down

• If the wave travels perpendicularly to the surface, it continues straight

  • It can still change speed depending on density

• If the wave travels at an angle, it refracts (changes direction)

• If it is travelling into a denser material, it refracts towards the normal

• If it travels from a denser material to a less dense one, the emergent ray is faster and at a larger angle from the normal than from when it enters

• Throughout, the wave speed changes but frequency stays the same

  • This keeps the wavelength constant

• Different wavelengths refract different amounts

  • If we put white light (all the colours combined) through a triangular prism, they would all spread out at different colours

Electromagnetic waves

• The electromagnetic spectrum contains only transverse waves, which can travel in a vacuum

  • All of these waves travel at the speed of light - 300000000m/s (x10⁸)

  • They refract through different mediums at different speeds

• V=f\lambda

  • Speed is constant, so as wavelength decreases, frequency increases

• Humans can only detect a small part of the spectrum

• Ultraviolet, x-rays and gamma rays are all ionising, so can damage cells and cause cancers

• Radio and microwaves and infrared are all used for communication as they can travel long distances

• All of these waves can travel through empty space

• When in contact with a surface, they reflect, refract/transmit or are absorbed

Microwaves and infrared

• Microwaves and infrared have long wavelengths, and low frequencies

• Microwaves are either absorbed by water molecules, or not

  • If they are not, they are used for communications using satellites

    • They have to pass through the earth’s atmosphere, so can’t be transported by water

    • They are received and transmitted by satellite dishes

  • If they are absorbed by water molecules, they are used for heating food in microwaves

    • Energy is absorbed from the waves into the water molecules, which causes vibrations that spread through the food and heat it

• Infrared (IR) is emitted by all objects that have thermal energy, and the amount depends on the object’s temperature

  • Infrared is used for:

    • Infrared cameras - they allow objects to be seen in the dark and can spot living organisms due to a change in temperature (animals have higher temperature, so appear bright)

    • Cooking - we can heat metal, which emits IR and heats food by transferring energy

    • Electrical heaters - they use electrical energy to heat heat metal and emit IR to the surroundings

  • It doesn’t penetrate surfaces

    • Why toast is crispy and not just warm throughout 🥖🥖

• Infrared and microwaves are only harmful in large quantities (if IN a microwave)

Radiowaves

• Radiowaves have the longest wavelengths and lowest frequency in the electromagnetic spectrum

• Oscilloscopes allow us to see the frequency of an alternating current, which determines the wave that is produced

  • These can be connected to a transmitter, which generates the wave

  • This wave can be detected by a receiver, which absorbs energy and generates another alternating current, displayed on another oscilloscope

• This can be used to transfer information through frequencies

• The main uses of radiowaves are for communications

  • Long wavelengths - can be transmitted huge distances

    • Diffract/bend around the curved surface of earth between receivers

  • Short wavelengths can travel long distances

    • They can’t curve, but are instead reflected from the ionosphere (electrically charged layer of the atmosphere)

    • Can also send data short distances

  • Very short wavelengths - travel directly from transmitter to receiver

    • TV remotes, FM radio

    • They are affected by obstacles

Visible light and ultraviolet

• Visible light is visible to humans

  • Different colours appear depending on wave lengths

  • Are viewed from red to violet

  • Can be used for communications

    • Optic fibres - thin glass/plastic that transmits pulses of light through reflections

      • With coded information, messages can be sent with fat transmission and without absorption

      • They can also transmit more information and are less likely to be disrupted than wires

• Ultraviolet is emitted from the sun, and can be generated on earth 🙂

  • Fluorescence is a property of some chemicals where UV is absorbed and re-emitted as visible

  • Fluorescent lights generate UV radiation, which is absorbed by a layer of phosphorous in a bulb and then is re-emitted as visible light

    • They are really energy efficient, cheaper and have lower CO₂ emissions

  • They can be used for security

    • Passports, bank notes, security pens

    • Invisible images can be printed, which are only visible when UV is shone on them, which makes forgeries more difficult

  • They can sterilize water as UV destroys microorganisms, so sanitizes

X-rays and gamma rays

• Shortest wave length and highest frequency waves

• X-rays are mainly used to view internal structure of objects

  • Some waves are absorbed by denser materials, like bone, so a detector plate registers only which pass through to create an image

    • Most pass through air, partially pass through middle density objects (flesh)

  • In X-rays, the white image shows where wave radiation didn’t reach/affect

  • They are used to detect broken bones

    • Small amounts of radiation, but usually worth the risk

    • Quick and cheap

• Gamma rays

  • Used for medical imaging, to treat cancer (radiotherapy) and sterilisation (medical and food)

  • Sterilisation - can kill microorganisms without causing other damage

    • No risk to equipment/food as it only kills harmful organisms within and makes no alterations

    • Makes food fresher for longer, and makes it safer

• X-rays and gamma rays are ionising radiation

  • They can be dangerous medically, so needs to be worth the risks

Lenses

• Conex lenses refract parallel rays of light inwards to a single point - converging

• Concave lenses refract parallel rays of light outwards - disperse light

• Lenses are usually symmetrical, so light can go both ways

• The principle focus is always on an axis

  • The distance between the focal point and centre is the focal length

  • Shorter focal length = more powerful lens (stronger refraction)

• The strength of lenses can be changed by changing the material or changing the amount the lens is curved

• Images are formed at points where all light rays from a particular point on an object appear to come together

• Real images are formed where light rays come together to form an image, that will be inverted

  • It happens in our eyes, and the brain then flips the image

  • From convex lenses only

• Virtual images are formed when light rays don’t actually come together where the image appears, but where the rays can be traced back to the focal point

  • This happens in mirrors - can’t actually be behind the mirror as rays don’t reach it but appears to be

  • Forms upright images

  • From concave lenses and sometimes convex lenses

Visible light and colour

• Colours in visible light

  • The range of electromagnetic light waves our eyes can see - red → violet

    • Red has longer wavelengths, and violet has shorter wavelengths

  • Black is an absence of light - a perfect black body is theoretical and would be a material that absorbs all radiation and reflects/transmits none of it

    • Are good emitters

  • White is a mix of all visible light rays

• The colour of an object depends on the wavelength of light hitting it and the properties of the object

  • This can determine absorption, reflection or transmission

• Opaque objects don’t transmit any light

  • Wave lengths are either reflected or absorbed

  • Wave lengths that are reflected are the colour that the object appears

  • For example, an apple only reflects red wavelengths so appears red

• Transparent objects transmit most light, but might absorb/reflect some light

  • Appears the colour that is transmitted

  • A bottle may appear green as wave lengths pass through or are reflected

• Translucent objects transmit some light but scatter it (refract), so it appears fuzzy

  • Colour appears as the wavelengths that are transmitted and reflected

• Colour filters are materials that only transmit certain wavelength of light and absorb/reflect the rest

  • Primary colour filters only transmit one of the primary colours - red, green or blue

  • Other colours transmit a range of wavelengths, that include primary colours that mix to certain colours

Absorbing radiation (temperature)

• Absorbing radiation increases temperature

• Emitting radiation decreases temperature

  • Radiation is pure energy

  • If more radiation is emitted than is being absorbed, the temperature will drop

• Intensity is the power of the radiation per unit area

  • Energy transferred in a given area in a time period

• As temperature increases, the intensity of every emitted wave length increases

  • Hotter = shorter wavelengths

  • Intensity of shorter wave lengths increase more (sun)

• Balance of absorbing and emitting

  • The atmosphere absorbs radiation, reflects and emits it

  • Infrared is reemitted

  • In day, more energy is absorbed by the earth and atmosphere than is emitted, which increases local temperature

  • At night, more energy is emitted than absorbed, which decreases local temperature

  • It is always day somewhere, so it roughly balances out

Sound waves

• Sound waves are vibrations that pass through the molecules of a medium

  • Through compressions and rarefactions

  • Longitudinal waves - need a medium to travel

• Denser objects cause the wave to move faster as particles are closer together so transmit wave faster

  • Sound waves travel fastest through solids than gases

  • Sound can’t travel in a vacuum as there are no particles to vibrate

• As mediums change, frequency doesn’t change but speed does due to density

  • v = f\lambda - frequency remains the same, but speed does so therefore wave length changes

• Solids = faster → longer wavelengths

• Gases = slower → shorter wavelengths

  • This means sound can be refracted (due to change in speed), absorbed and reflected

• Human hearing

• Sound waves cause the ear drum to vibrate, which is transmitted through the ossicles and ear canal and to the cochlea

  • This then converts vibrations into electrical signals, passed along auditory nerve to the brain

    • Higher frequencies = higher pitch

    • More intensive signal = louder

• Humans can hear frequencies from around 20 Hz to 20000 Hz

Ultrasound

• Ultrasounds are sounds that vibrate at frequencies above 20000Hz (human hearing)

  • Some animals use these for communications

• They are produced by electrical oscillations converted into ultrasound waves

• When passing through a boundary (between materials), only some waves are reflected and some are refracted

  • Partial reflection

• If ultrasound waves are transmitted through an object, some are reflected back to us

  • If we know their speed and the time taken, we can calculate how far away the boundary is

  • If we repeat this with all reflections, we can find different boundaries within an object, which tells us about its internal structure

    • Can use to form images - s=d/t

    • Is completely safe 🙂

• Ultrasound machines - scanning foetuses

  • Waves pass from one medium to another

  • Some waves are reflected back, and some are refracted through

  • Timings and distributions of echoes are processed by a computer

  • This produces a live image - used to check health

• Ultrasound is also used to check product quality

  • Wave should pass straight through if the product is solid with only two reflections

  • If there are more than expected there is a crack or fault which is detected

• Echo sounding/sonar

  • Boats and submarines fire ultrasounds at the sea floor/objects in the ocean to find out how far away they are

Seismic waves

• Large scale events, like volcanoes and earthquakes, cause or produce waves that spread in all directions into the earth - seismic waves

• P-waves are longitudinal, travel through both solids and liquids and are faster

• S-waves are transverse, only travel through solids and are slower

• By studying how the waves pass through the earth, scientists can find the internal structure and their materials

• They can detect waves with seismometers and compare results with others around the world to see how long it took for the waves to travel through the earth, after earthquakes for example

• Boundaries

  • Seismic waves can be reflected, absorbed and transmitted

  • This usually causes refraction as waves change speed between different densities

• P-waves can pass through both solids and liquids so refract between the mantle and outer core

  • As they pass through the outer core, the density varies so there are constant slight refractions (curves)

  • This happens again as the wave passes back through the mantle

• S-waves can’t travel through liquids, so can’t pass through the outer core

  • This is how scientists discovered the outer core is liquid

DONE!!!