Waves

How do you measure the wavelength? (3)

1. Using a metre ruler, measure across as many loops as you can

2. Each loop is a half wavelength

3. Divide the distance by the number of loops and multiply by two to get the wavelength

Electromagnetic Waves

They are transverse waves that transfer energy from a source to an absorber.

They form a continuous spectrum of a range of frequencies and all EM waves travel at the same speed in air/vacuum.

Focal Length

The distance between the centre of a lens and its principal focus.

Define wave.

• a transfer of energy - from a source to a destination

Define electromagnetic wave.

a wave that does not need matter to travel through

Define transverse wave.

a wave that has a perpendicular oscillation to the direction of travel (energy)

Define a longitudinal wave.

a wave that has a parallel oscillation to the direction of travel (energy)

The ripples on a water surface are an example of a _________ wave.

transverse

Describe the oscillations in longitudinal waves.

parallel oscillations with areas of compression and rarefaction

When a wave arrives at a boundary between two different materials, what three things can happen?

• the wave is absorbed - the wave is transmitted (leading to refraction) - the wave is reflected

What happens when a wave is absorbed by a material the wave is trying to pass into?

Energy is transferred to the material's energy stores (this is how a microwave works)

What determines whether a wave is absorbed, transmitted or reflected?

the wavelength of the wave and the properties of the wave involved

Draw a ray diagram to show the reflection of a wave at a surface.

(Draw: incident ray hits surface, normal line perpendicular, reflected ray bounces off at equal angle)

What type of image is seen in a mirror?

virtual image

What is the angle of incidence always equal to, in reflection?

the angle of reflection

How do all objects interact with radiation and under what condition?

• All bodies (objects), - no matter what temperature, - emit and absorb infrared radiation

As the temperature of a body increases, what happens to the infrared radiation that it radiates?

• The hotter the body, - the more infrared radiation it radiates - in a given time.

A perfect black body is an object that ________ all of the radiation incident on it.

absorbs

A black body does not _______ or ________ any radiation.

reflect, transmit

A black body radiator is a perfect absorber. Since a good absorber is also a good _______, a perfect black body would be the ______ _________ _____________.

emitter, best possible emitter

Describe a black body radiator. (4)

• perfect absorber - best possible emitter - zero reflector - zero transmitter

Define body

object

What does the temperature of a body determine in terms of its emission?

• the intensity and wavelength distribution of any emission (depends on the temperature of the body)

What determines the intensity and wavelength distribution of any emission from a body?

the temperature of the body

If a body is at a constant temperature, what does that mean about the rate of absorption and emission of radiation?

• A body at constant temperature is - absorbing radiation - at the same rate - as it is emitting radiation.

If a body is absorbing and emitting radiation at the same rate, what does that mean about the temperature of the body?

the temperature is constant

In terms of radiation absorption and emission, how does an object's temperature increase?

• The temperature of a body increases - when the body absorbs radiation - faster - than it emits radiation.

When a body emits more radiation than it absorbs, what happens to the temperature?

the temperature decreases

When a body absorbs more radiation than it emits, what happens to the temperature?

The temperature increases

What does the temperature of the Earth depend on?

Many factors e.g: - the rates of absorption and emission of radiation - reflection of radiation into space

How does a lens form an image?

by refracting light

In a convex lens, what happens to parallel rays of light?

• converge (are brought together) - to a focus - at the principal focus

Define focal length.

the distance from the lens to the principal focus

Define principal focus.

the point where parallel rays of light focus from a convex lens.

What are ray diagrams used to show?

the formation of images by convex and concave lenses

What type of image is formed by a convex (converging) lens?

real or virtual

What type of image is formed by a concave (diverging) lens?

always virtual

What is the equation for magnification?

magnification = image height ÷ object height

What is the unit for magnification?

Magnification is a ratio and so has no units

In the equation for magnification, what should image and object height be measured in?

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

In ray diagrams, how is a convex (converging lens) represented?

(Draw: line with outward-curving arrowheads at both ends)

In a ray diagram, how is a concave lens represented?

(Draw: line with inward-curving arrowheads at both ends)

What 3 pairs of adjectives are used to describe images?

Real / Virtual; Magnified / Diminished; Upright / Inverted

What determines the colour of a light?

• Each colour within the visible light spectrum - has its own narrow band - of wavelength and frequency.

What does the wavelength and frequency of visible light determine?

the colour

Define specular reflection.

Reflection from a smooth surface (e.g. a mirror) in a single direction

Define diffuse reflection.

when a wave is reflected by a rough surface (e.g. a piece of paper) and the reflected waves are scattered in lots of different directions

How do colour filters work?

by absorbing certain wavelengths (and colour) and transmitting other wavelengths (and colour)

How is the colour of an opaque object determined?

The colour of an opaque object is determined by which wavelengths of light are more strongly reflected. Wavelengths that are not reflected are absorbed.

For an opaque object what happens to the wavelengths that are not reflected (to show its colour)?

Wavelengths that are not reflected are absorbed.

Objects that transmit light are either __________ or __________.

transparent, translucent

How does a primary colour filter work?

It only transmits that primary colour

How would a blue object appear through a blue filter and why?

• blue - because the light reflected from the object's surface - is transmitted by the filter (as the filter only transmits blue light).

How would a red object appear through a blue filter and why?

• black - all of the light reflected by the red object will be absorbed by the filter (because the filter only transmits blue light).

Can sound waves travel through solids?

yes - they travel fastest in solids

Order the states of matter from sound travelling fastest in it to sound generally travelling slowest in it.

Solids - fastest; Liquids - medium; Gases - slowest

How does a sound wave travel through a solid?

• by causing it to vibrate - as a series of compressions and rarefactions (longitudinal)

What is the normal human hearing range?

20Hz to 20kHz

How does an ear allow us to hear?

• sound waves cause the ear drum to vibrate - these vibrations are passed on to the ossicles (tiny bones) - then through the semicircular canals - and to the cochlea - The cochlea turn these vibrations into electrical signals which are sent to the brain - and allow us to hear the sound.

Define amplitude.

the maximum displacement of a point on a wave away from its undisturbed position

Define wavelength.

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

Define frequency.

the number of waves passing a point each second

Define period.

The length of time between two adjacent waves passing a point

What is the equation and units for the period of a wave?

period, T, in seconds, s = 1 ÷ frequency, f, in hertz, Hz

Define wave speed.

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

What is the equation and units for wave speed?

wave speed, v, in metres per second, m/s = frequency, f, in hertz, Hz × wavelength, λ, in metres, m

What happens to a sound wave when it meets a hard flat surface?

• It is reflected - Echoes are reflected sound waves

What is an echo?

a reflected sound wave

What happens to sound waves as they enter a different medium?

• They refract - due to a change in speed - because the wavelength changes - but the frequency stays the same (so speed must change)

How does frequency change when a sound wave enters a different medium?

it remains the same

How does speed change when a sound wave enters a different medium?

• speed changes - because wavelength changes - but frequency stays the same - so the speed must change (causing refraction) wave speed = frequency × wavelength

Define ultrasound.

Sound waves which have a frequency higher than the upper limit of hearing for humans (20kHz).

How is ultrasound used for imaging?

• Ultrasound waves are partially reflected when they meet a boundary between two different media. - The time taken for the reflections to reach a detector can be used to determine how far away such a boundary is. - This allows ultrasound waves to be used for both medical and industrial imaging.

Give an example of a medical ultrasound imaging and the boundary?

• pre-natal scanning of a foetus - boundary e.g. fluid in the womb and the skin of the foetus

Give an example of industrial imaging.

finding flaws in materials e.g. pipes

What are seismic waves?

waves produced by earthquakes

What are P-waves? How does their speed relate to material?

• longitudinal seismic waves - P-waves travel at different speeds through solids and liquids.

What is an S-wave?

• transverse seismic wave - they cannot travel through liquids

Can S-waves travel through liquids?

no

What do P-waves and S-waves provide evidence for?

the structure and size of the Earth's core

What is echo sounding?

• detection of objects in deep water - and measuring water depth - through the use of high frequency sound waves (including ultrasound)

What are radio waves used for?

television and radio

What are microwaves used for?

• satellite communications - cooking food

What is infrared used for?

• electrical heaters - cooking food - infrared cameras

What is visible light used for?

fibre optic communications

What is ultraviolet light used for?

• energy efficient lamps - sun tanning

What are x-ray and gamma rays used for?

medical imaging and treatments

Why are radio waves used for TV and radio communications?

• They can travel long distances within the Earth's atmosphere - because their long wavelengths (1 - 10km) - mean they can diffract around the curved surface of the earth

Why are microwaves used for satellite communications?

because they can easily pass through the Earth's watery atmosphere

Why are microwaves used for cooking food (in a microwave)?

• the microwaves are absorbed by the water molecules - this transfers energy to the water molecules - the energy is then transferred to the food - causing it to heat up

How do infrared cameras work?

• the camera detects IR radiation - turns it into an electrical signal - and displays it on a screen

How is ultraviolet used for energy efficient lamps?

• fluorescent lights generate UV - which is then absorbed by a layer of phosphorus - and re-emitted as visible light

Why are X-rays used for medical imaging?

dense materials, e.g. bones, absorb X-rays and so are not detected

Why are X-rays and gamma rays used for medical treatments?

a high dose will kill all living cells so can be directed at cancer cells

Why is gamma used as a medical tracer?

it can be detected from outside of the body

Which EM wave has the longest wavelength?

radio wave

Which EM wave has the lowest frequency?

Radio wave

Which EM wave has the shortest wavelength?

Gamma ray

Which EM has the highest frequency?

Gamma ray

Which EM wave is the highest energy?

Gamma ray