waves Flashcards
transverse waves
A transverse wave is one where the direction of vibration is perpendicular to the direction of energy transfer.
e.g. water waves, Electromagnetic waves, S waves
longitudinal waves
A longitudinal wave is one where the direction of vibration is parallel to the direction of energy transfer.
e.g. sound waves (in air), p waves
Wave properties
For both ripples on a water surface and sound waves in air, it is the wave and not the water or air itself that travels.
amplitude
The amplitude of a wave is the maximum displacement of a point on a wave away from its undisturbed position.
wavelength
The wavelength of a wave is the distance from a point on one wave to the equivalent point on the adjacent wave.
frequency
The frequency of a wave is the number of waves passing a point each second.
The Wave Equation
π€ππ£ππ ππππ=πππππ’ππππ¦Γπ€ππ£ππππππ‘β
π£=ππ
v: m/s
f: Hz
Ξ»: m
Time Period
The time period of a wave is the time it takes for one complete wave (length) to pass a point.
πππππ’ππππ¦ (Hz) =1/(π‘πππ ππππππ - s)
π=1/π
specular reflection
diffuse reflection
how is an image formed by a mirror
Describe a method that can be used to measure the speed of sound in air:
two people (person A and person B) are placed a distance apart, eg 100 metres
person A fires a starterβs pistol (or hitting two blocks of wood)
person B times the difference between seeing the flash of the gun and hearing the sound - this is measured in seconds
The speed of sound can be calculated using this equation:
π ππππ=πisπ‘ππππ/π‘πππ
Sound waves
Sound waves can travel through solids causing vibrations in the solid.
Within the ear, sound waves cause the ear drum and other parts to vibrate which causes the sensation of sound. The conversion of sound waves to vibrations of solids works over a limited frequency range of 20 to 20 000 Hz. This restricts the limits of human hearing.
Ultrasound
Ultrasound waves have a frequency higher than the upper limit of hearing for humans. 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.
Seismic Waves
Seismic waves are produced by earthquakes. There are P - waves and S - waves. P-waves and S-waves provide evidence for the structure and size of the Earthβs core.
P - waves
P-waves are longitudinal seismic waves. P-waves travel at different speeds through solids and liquids.
S - waves
S-waves are transverse seismic waves. S-waves cannot travel through a liquid.
Echo Sounding
Echo sounding, using high frequency sound waves is used to
detect objects in deep water and measure water depth.
The Electromagnetic Spectrum
Electromagnetic waves are transverse waves that transfer energy from the source of the waves to an absorber.
Electromagnetic waves form a continuous spectrum and all types of electromagnetic wave travel at the same velocity through a vacuum (space) or air.
The waves that form the electromagnetic spectrum are grouped in terms of their wavelength (long to short) and their frequency (low to high frequency):
Radio waves, Microwaves, Infrared, Visible Light, Ultra Violet, X-rays, Gamma Rays
Our eyes only detect visible light and so detect a limited range of electromagnetic waves.
Radio
Radio waves can be produced by oscillations in electrical
circuits. When radio waves are absorbed they may create an alternating current with the same frequency as the radio wave itself, so radio waves can themselves induce oscillations in an electrical circuit. Radio waves are reflected by ionosphere (and ground) so can travel around the Earth.
uses: television and radio
Microwave
When microwaves are absorbed by water they cause the water molecule to vibrate faster. Because they are not easily absorbed by water they are able to penetrate several cm into food. They can penetrate the ionosphere
uses: satellite communications, cooking food
Infrared
All object emit infrared radiation, the hotter they are the more radiation is emitted
When the surface of an object absorbs infrared radiation it causes the molecules on the surface to vibrate faster.
uses: electrical heaters, cooking food, infrared cameras
visible light
Light reflects off the internal edges of
fibre optic cables.
Ultra Violet
UV is produced inside energy efficient bulbs. This is absorbed by the white coating on the surface of the bulb, which then emits the energy as visible light. (Fluorescence).
uses: energy efficient lamps, sun tanning,
dangers: ionising - can cause skin cancer
X-ray
X-rays can penetrate soft tissue (such as muscle) but not bone (absorbed by bone).
X-rays are ionizing radiation that can cause the mutation of genes and cancer, but can also kill cancer cells
uses: medical imaging and treatments
dangers: ionising - can cause cancer
Gamma Ray
Gamma are ionizing radiation that can cause the mutation of genes and cancer, but can also kill cancer cells and can kill bacteria (e.g. they can sterilize surgical equipment)
uses: medical imaging and treatments,
dangers: ionising - can cause cancer
Refraction
refraction, is due to the difference in velocity of the waves in different substances.
If a wave slows down it bends towards the normal
if a wave speeds up it bends away from the normal
When a wave changes speed is wavelength changes, but its frequency always remains constant.
refraction wavefronts
The wave travels faster in the first medium.
Side A of the wave (front) enters the new medium first so slows down.
But side B continues at a higher speed, so travels a greater distance in the same timw.
This causes a change in direction of the wave fronts.
The wavelength is shorter after it has crossed the boundary meaning that the wave must be slowing down
Visible Light
Each colour within the visible light spectrum has its own narrow band of wavelengths and frequencies.
Colour filters work by absorbing certain wavelengths (and colours) and transmitting other wavelengths (and colours).
The colour of an opaque object is determined by which wavelengths of light are more strongly reflected. Wavelengths that are not reflected are absorbed. If all wavelengths are reflected equally the object appears white. If all wavelengths are absorbed the objects appears black.
Global Warming
A body at constant temperature is absorbing radiation at the same rate as it is emitting radiation. The temperature of a body increases when the body absorbs radiation faster than it emits radiation.
Carbon dioxide (and other greenhouse gases) reduces the rate at which radiation is emitted into space, which means that the Earth absorbs more radiation than it emits causing it to increase in temperature.
As the Earth becomes hotter it emits more radiation until, eventually, the amount emitted into space becomes equal to the amount being absorbed.
Black Body Radiation
All bodies (objects), no matter what temperature, emit and absorb infrared radiation. The hotter the body, the more infrared radiation it radiates in a given time.
A perfect black body is an object that absorbs all of the radiation incident on it. A black body does not reflect or transmit any radiation. Since a good absorber is also a good emitter a perfect black body would be the best possible emitter.
graph to show the distribution of wavelengths emitted by a black body for two objects at different temperatures:
convex lenses
concave lens
