Physics - waves

• Waves may be either transverse or longitudinal.
• In a transverse wave the oscillations are perpendicular to the direction of energy transfer. The ripples on a water surface are an example of a transverse wave.
• In a longitudinal wave the oscillations are parallel to the direction of energy transfer. Longitudinal waves show areas of compression and rarefaction.
• Sound waves travelling through air are longitudinal.
• Waves are described by their amplitude, wavelength, frequency and period.
• The amplitude of a wave is the maximum displacement of a point on a wave away from its undisturbed position.
• The wavelength of a wave is the distance from a point on one wave to the equivalent point on the adjacent wave
• The frequency of a wave is the number of waves passing a point each second.

  $𝑃𝑒𝑟𝑖𝑜𝑑 [𝑇] =$$1/𝑓$
• period, T, in seconds, (s)

  frequency, f, in hertz, (Hz)
• The period of a wave is how long it takes for one wave to pass a point.
• The wave speed is the speed at which the energy is transferred (or the wave moves) through the medium.
• All waves obey the wave equation:

  $𝑤𝑎𝑣𝑒𝑠𝑝𝑒𝑒𝑑=$$𝑓𝑟𝑒𝑞𝑢𝑒𝑛𝑐𝑦 *$$𝑤𝑎𝑣𝑒𝑙𝑒𝑛𝑔𝑡ℎ$
• wave speed, v, in metres per second, m/s

  frequency, f, in hertz, Hz

  wavelength, λ, in metres, m
• 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 and their frequency.
• . Going from long to short wavelength (or from low to high frequency) the groups are: radio, microwave, infra-red, visible light (red to violet),ultra-violet, X-rays and gamma-rays.
• Our eyes detect visible light and so only detect a limited range of electromagnetic waves.
• Radio waves can be produced by oscillations in electrical circuits.
• 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.
• When radio waves are absorbed they may create an alternating current with the same frequency as the radio wave itself, so radio waves can also produce oscillations in an electrical circuit.
• Ultra-violet waves, X-rays and gamma rays can have hazardous effects on human body tissue
• The effects depend on the type of radiation and the size of the dose. Radiation dose (in Sieverts) is a measure of the damage caused by the radiation in the body.
• Ultra-violet 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 mutation of genes and cancer.
• •radio waves –television and radio

  •microwaves –satellite communications, cooking food

  •infrared –electrical heaters, cooking food, infra-red cameras

  •visible light –fibre optic communications

  •ultraviolet –energy efficient lamps, sun tanning

  •X-rays –medical imaging and treatments.
• All objects, no matter what temperature, emit and absorb infra-red radiation. The hotter an object is the more infra-red radiation it radiates in a given time
• Raging Martians Invaded Venus Using X-ray Guns.
• Matt black surfaces are the best absorbers and the best emitters of infrared radiation.
• All objects, no matter what temperature, emit and absorb infrared radiation.
• A hotter object will emit more infrared radiation in a given time,

  compared with a cooler object.
• Both the wavelength and the intensity of radiation depend on the

  temperature of the object.
• Very hot objects emit shorter wavelength radiation than cooler objects.
• As the object gets hotter, it emits more short wavelength radiation. That is why very hot objects produce visible light eg a piece of very hot metal. The intensity of the radiation also increases at higher temperatures.
• A perfect black body absorbs all of the radiation incident on it. No radiation is reflected and no radiation is transmitted (passed through).
• an object that absorbs radiation well will also emit radiation well.
• A perfect black body is also the best possible emitter of radiation.
• If an object is warmer than its surroundings, it will emit more radiation than it absorbs. Its temperature will decrease.
• If an object is cooler than its surroundings, then it will absorb

  more radiation than it emits. Its temperature will increase
• The Sun emits short wavelength radiation such as visible light and ultraviolet which travels to the earth