Unit 3 Waves and Optics ! Flashcards
What is a progressive wave?
A wave that transfers energy from one place to another without transferring matter
What are waves caused by?
Waves are caused by something oscillating the particles in the medium or oscillating the field through which the wave travels
Define the displacement of a wave and state its unit
Distance and direction an oscillating particle moves from its rest position, in metres
Define the amplitude of a wave and state its unit
Maximum displacement an oscillating particle in the medium moves from its rest position in metres
Define the frequency of a wave and state its units
Number of complete waves passing a fixed point per second, in hertz
Define the wavelength of a wave and state its unit
The shortest distance between two adjacent points that are in phase with each other, in metres
What is the phase of a particle on a wave?
Fraction of a cycle it has completed since the start of the cycle
What is the phase difference between two particles on a wave?
Fraction of a cycle by which their oscillations are separated
Define a wavefront
Line or a surface on which the vibrations of the medium have the same phase at all points
What are mechanical waves?
Waves requiring a physical substance to move through
Why are electromagnetic waves not mechanical waves?
They can travel through a vacuum
What is a longitudinal wave?
Wave where the oscillations of the medium are parallel to the direction of energy transfer
What is a transverse wave?
Wave where the oscillations of the medium are perpendicular to the direction of energy transfer.
What are electromagnetic waves?
Oscillations in electric and magnetic fields that permeate space, which travel at the speed of light in a vacuum
Define polarisation
Restriction of the oscillations of a transverse wave into a single plane
What type of waves can be polarised?
Only transverse waves.
Describe a node on a stationary wave + phase ?
Point where there is no oscillation
The two waves will always meet with a phase difference of 180 degrees (π radians)
State the principle of superposition
When two waves meet, the total displacement at a point is equal to the sum of the individual displacements at that point
How is a stationary wave formed?
When two progressive waves with the same frequency and amplitude, moving in opposite directions, superpose. They are often the result of the reflection of a progressive wave superposing with the original wave.
Describe an antinode on a stationary wave + phase
Point that oscillates with maximum amplitude
The two waves always meet in phase at that point
If a stationary wave oscillates freely, does it transfer energy to its surroundings?
No.
Consider what happens to a stationary wave produced in a microwave.
There is no heating at the nodes and maximum heating at the antinodes
Consider what happens to a stationary wave formed by sound waves.
There is silence at the nodes and maximum volume at the antinodes
When stationary waves are formed on a string, what is the first harmonic?
Stationary wave that occurs at the lowest frequency
Define coherent waves
Waves with the same frequency and a constant phase difference
Define path difference.
Path difference between two waves is the difference in the distance travelled by the waves to the point at which they superpose.
What is an interference pattern?
When waves with a constant phase difference superpose to produce cancellation and reinforcement at fixed positions.
When does constructive interference occur?
Constructive interference occurs when two waves in phase superpose to give a wave with a larger amplitude.
When does destructive interference occur?
Destructive interference occurs when two waves out of phase superpose to cancel each other out.
What is the path difference between two waves if they constructively interfere at a point?
A whole number of wavelengths nλ
What is the path difference between two waves if they destructively interfere at a point?
A whole number of wavelengths (n+½)λ
What did Young’s double slit experiment provide evidence for?
For the wave nature of light because the resulting fringe pattern is due to diffraction and interference which cannot happen with particles
What happens in Young’s double slit experiment?
A single beam of light is split into two coherent beams as it passes through the closely spaced double slits. This then produces a pattern of dark and bright fringes on a screen.
When do bright fringes form in Young’s double slit experiment?
They form when the path difference between light from each slit is nλ, so the waves arrive in phase and superpose constructively
When do dark fringes form in Young’s double slit experiment?
They form when the path difference between light from each slit is (n+½)λ, so the waves arrive out of phase and superpose destructively.
What is the fringe separation in a fringe pattern?
Distance between the centres of two adjacent maxima or minima
List some of the precautions taken when using lasers
Turn it off when not in use
Never shine it at a person
Avoid shining it at reflective surfaces
Display a warning sign on lab door
Wear laser safety goggles
Why are the fringes produced by red light farther apart from those produced by blue light in the double slit experiment?
Red light has a longer wavelength
What is seen when monochromatic light is diffracted through a narrow single slit?
An interference pattern is seen where:
The central fringe is much brighter and is twice as wide as the outer fringes
The outer fringes are the same width and their brightness decreases with distance from the centre.
What is seen when white light is diffracted through a narrow single slit?
An interference pattern is seen where with white light the different wavelengths are diffracted by different amounts, resulting in:
- A bright white central maximum
- Less bright fringes that are spectra, with blue light on the inner edge and red light on the outer edge
What is seen when monochromatic light is passed through a diffraction grating?
It will produce an interference pattern where the maxima are much sharper than for a single or double slit. The central maximum is the brightest and is called the zero order maximum.
As light from the source is diffracted through each slit of a grating, the diffracted waves constructively superpose in certain directions only, and destructively superpose in all other directions.
What is seen when white light is passed through a diffraction grating?
The central maximum is white and every other order of interference is a spectrum.
What are diffraction grating used in?
They are used in spectrometers to study the spectrum of light from any source.
What is refraction?
Refraction is the change of direction that occurs when light passes at an angle across a boundary between two transparent substances.
What is the refractive index of a substance?
Refractive index = (speed of light in a vacuum)/(speed of light in substance)
What does a greater refractive index mean?
The greater the refractive index of a substance, the more slowly light travels through it and the higher its optical density.
What is an optically dense substance?
Substance with a high refractive index in which light travels slowly
State Snell’s law
n1sinθ1 = n2sinθ2
Which direction do rays of light bend when they travel from a less optically dense medium to a more optically dense one?
Towards the normal
Which direction do rays of light bend when they travel from a more optically dense medium to a less optically dense one?
Away from the normal
When does total internal reflection occur?
When light is travelling from an optically dense material to a less optically dense material, the angle of incidence is greater than the critical angle.
What is the critical angle θc in total internal reflection?
Angle at which the angle of refraction is 90 degrees
What does the cladding of an optical fibre do?
Allows total internal reflection to take place, protects the core from being damaged, prevents light from leaking out of it
Give two ways in which signal degradation in an optical fibre occurs
Absorption, pulse broadening
What is pulse broadening in an optical fibre and its consequence?
The signal pulse gets broader as it travels along the fibre which reduces the quality of signal and can lead to loss of information if pulses overlap.
Caused by modal/material dispersion
What is absorption in an optical fibre and its consequence?
Some of the light is absorbed by the optical fibre every time it reflects, resulting in a decrease in the amplitude of the signal.
What is modal dispersion and how can it be reduced?
If the core is wide enough, rays of light entering it at different angles will take different paths through it and travel different distances. It can be reduced by using an optical fibre with a narrower core.
What is material dispersion and how can it be reduced?
If white light is used for the signal, the different wavelengths in it travel at slightly different speeds through the fibre, causing them to separate as they travel along the fibre. It can be eliminated by using monochromatic light.
