Chapter 5 - Waves [Complete] Flashcards
What is a progressive wave?
A wave that transfers through a substance or space, transferring energy and information
What is the amplitude of a wave?
The maximum perpendicular displacement from the particle’s undisturbed position. The larger the amplitude, the larger the energy transferred
What is the wavelength of a wave?
The distance between wave peaks or two equivalent points on two successive cycles
What is the frequency of a wave and what is its unit?
The amount of cycles or vibrations of the wave per second, measured in Hertz (Hz)
What is the equation linking time period and frequency?
f = 1/T
What does it mean if two particles moving along a wave are in phase?
They are moving in the same direction at the same speed
What does it mean if two particles are moving in antiphase?
They are moving with the same speed, but in the exact opposite perpendicular directions. Particles moving in anti phase are separated by a distance of a whole number n of wavelengths + an extra half wavelength
How do you calculate an abstract phase difference? (aka one that is not exactly half a wavelength or a full wavelength etc.)
Find the difference in distance between the two points, and then times it by 2Pi radians and divide it by the wavelength
What is the equation to find the vertical displacement of a particle in a wave at any point of its cycle, given that the wave is sinusoidal?
V. displacement = Amplitude x sin([2 x Pi x t] / Time period)
What happens to a wave’s phase when it is reflected off a surface?
It undergoes a phase change of 180 degrees
What is a longitudinal wave?
A wave where the particles vibrate parallel to the direction the energy is travelling in
What is a transverse wave?
A wave where the vibrations are at right angles to the direction the energy is travelling in
What are electromagnetic waves?
A transverse waves that travel at the speed of light (3 x 10^8), they are made up of two component waves - one that is oscillating within the electric field, and the other in the magnetic field
What is a mechanical wave?
A wave that needs a medium to pass through, and thus cannot travel in a vacuum
Through which state does sound travel quickest?
Solids, 5100 m/s
What 3 wave groups have wavelengths shorter than that of visible light (10^-6)m?
Gamma rays, X-rays, UV
What effect does radio waves have on the body?
Minimal to negligible
What is the threat of X-rays, gamma rays, and UV light?
They can be ionising to cells/increase the risk of cancer
What are radio waves used for commercially?
To transmit radio and TV through carrier waves or through radio telescopes that can receive and interpret radio waves from space
Why is normal light unpolarised?
As the excited electrons that cause the emission of the light can vibrate in any direction, and thus the electric and magnetic fields of light can oscillate in any direction
Which type of wave can be polarised?
Transverse waves, and only transverse waves
What is it called when two polarising filters are held at right angles to one another and no light gets through?
Crossing the polarisers
Other than through polarising filters, how else can light sometimes be polarised?
Through reflection off some surfaces at certain angles of incidence
What are two applications of polarised electromagnetic waves?
For sunglasses and goggles which reduce eyestrain and allow people to see through water or off snow more effectively. For radio waves where interference needs to be reduced so plane polarised waves are picked up by receivers with the same alignment
What is refraction?
The change in direction at a boundary between one medium and another
What is the refractive index?
The ratio of a wave’s speed between two materials. It is normally quoted for light travelling in a vacuum into a material
What does it mean if a material has a higher refractive index than another material?
The light travelling through it bends a more significant amount in the material with the higher refractive index. Essentially, the implication is that the material with the higher refractive index is denser than the other one
What is the formula for refractive index and what are its units?
refractive index (n) = speed of light in a vacuum (c) / speed of light in the material (cs), since refractive index is a ration, it has no units
What is Snell’s Law?
Refractive index of M1 x sin (angle of incidence in M1) = Refractive index of M2 x sin (angle of refraction in M2)
When total internal reflection happens, is the boundary the light is attempting to enter one where the light is going from high to low refractive index or low to high?
High refractive index to low refractive index
How does a step index optical fibre work?
The core has a uniform refractive index and the cladding a lower refractive index, causing repeated total internal reflection and thus the transmission of light
Why does diversion happen?
As different colours of light travelling through the glass slow down by differing amounts since the refractive index is different for each of the wavelengths. This is why you can see each of the different colours separated from white light in a glass prism
What are the two types of dispersion that take place within step index optical fibres?
Modal and material dispersion
What is pulse broadening and what causes it?
When the duration of a pulse increases due to different wavelengths of light travelling at different speeds - thus a sharp pulse separates over distance into a broader signal
What happens in modal dispersion?
Rays within the fibre take different paths and thus pulse broadens
What happens in material dispersion?
Different wavelengths of light travel at varying speeds within the fibre thus the pulse broadens
What can absorption of light within an optical fibre cause in terms of signals?
A weakening of the signal, resulting in it often needing to be boosted or amplified over long travelling distances in the optical fibre
What has more energy, long wavelength radio waves or shorter wavelength X-rays and why?
The X-rays have more energy since the energy of a photon is inversely proportional to its wavelength
