Waves Summary Flashcards
What is a mechanical wave?
They require a medium such as air or water to propagate
What is an electromagnetic wave?
Propagates in a vacum
What is a transverse wave?
The direction of energy transfer is perpendicular to the direction of vibrations e.g. water waves and electromagnetic waves e.g. light waves
What is a longitudinal wave?
The direction of energy transfer is parallel to the direction of vibrations e.g. sound waves and seismic waves
What is the amplitude of a wave?
The maximum displacement of a wave particle from its undisturbed position
What is the wavelength of a wave?
The distance between two peaks on a wave
What is the frequency of a wave?
The number of waves that pass a point in one second (Hz)
What is the period of a wave?
The time taken to complete on wave cycle (seconds)
What is a peak and trough?
- Peak: The point of maximum displacement
2. Trough: the point of minimum displacement
What are wave pulses and continuous waves?
- A wave pulse involves a short or single disturbance of the medium it is travelling in. For example dipping an object in water may produce a wave pulse
- Continuous waves involve repeated disturbances of the medium. For example, to produce continuous waves in a. ripple tank, the dipper would have to be dipped into the tank at regular intervals
What do all waves undergo?
Reflection, refraction, interference and diffraction
What are the two laws of reflection?
- They apply to both plane (flat) and curved mirrors (or other reflecting surface fo waves other than light)
1. The incident ray and reflected ray lie on a single plane which is perpendicular to the surface at the point of incidence
2. The angle of incidence is equal to angle of reflection - The angles re always measured to the normal, NOT the reflecting surface
What is refraction?
-Refraction is the change in direction, as a result int he change of speed of a wave as it crosses a boundary between two material
What laws are refraction governed by?
- The incident ray and reflected ray lie in a single plane which is perpendicular to the surface at the point of incidence
- “Snell’s Law”: at the boundary between any two given material, the ratio of sine of the angle of incidents tot he sine of the angle of refraction is constant for rays of any particular wavelength
What is Snells law?
- sintheta1 / sinthet2 = 1n2
- 2n1 = 1/ 1n2
How do you define the refractive index?
1n2 = v1 / v2
where v1 = velocity of wave in material 1 and v2 = velocity of wave in material 2 and this defines the refractive index
Also since FREQUENCY DOES NOT CHANGE DURING REFRACTION 1n2 = lamda1/ lamda2
What is the critical angle and total internal reflection?
- When light travels from a material with a higher refractive index to one with a lower refractive index it is possible for the angle of refraction to be 90 degrees
- The angle at which this occur is called the critical angle
What happens for angles larger than the critical angle?
- If the angle of incidence is greater than the critical angle, than the ray cannot be refracted and instead it is totally internally reflected
- In tract a certain amount of reflection will always occur at the interface but for angle greater than the critical angle, only reflections an occur and incident and reflected ray obey the laws of refection
What is the principle of superstition?
When two waves combine the resultant displacement at a point is equal to the vector sum of the individual displacements at that point
What is phase difference?
- Phase difference is a way of measuring how far ahead one wave is of another, half a wavelength, quarter of a wavelength etc
- It is given as an angle, with a whole wavelength corresponding to 360 degrees, so if one wave lead another by a quarter of a wavelength this is a phase difference of 0.25 x 360 = 90 degrees
What happens if the phase difference is zero?
The waves are in phase
What happens if the phase difference is 180 degrees (half a wavelength)?
They are completely out of phase
When are sources of waves coherent?
They maintain a constant phase difference and have the same frequency e.g. lasers
What is a wavefront?
It is a line or surface in the path of a wave motion on which all the disturbances are in phase and it is perpendicular to the direction travel of the wave
What stays the same in diffraction?
- When waves pass an edge of an obstacle, or through a gap, they spread out and change shape
- The wavelength, frequency and velocity remain constant
When does appreciable diffraction occur?
-Appreciable diffraction only occurs if the gap is no bigger than the wavelength of the wave
What happens to diffraction the narrower the slit?
The narrower the slit, the greater the diffraction for a particular wavelength
What happens to diffraction the longer the wavelength?
The longer the wavelength for a constant slit width the greater the diffraction
What happens during the diffraction of light and the single slit?
Diffraction of light through a slit onto a screen leads to the production of light and dark fringes
What are the properties of the fringe pattern of single slit diffraction?
- It is symmetrical
- The central bright fringe is much brighter than the other fringes
- It is twice as wide as the other fringes
- The brightness (intensity of light) decreases with distance for the central fringe - so the outer fringes are the faintest
How can the position of the dark fringes be calculated?
Sintheta = nlamda / d
- d is slit width
- lamda wavelength
- n fringe number
What does double slit diffraction produce?
- This produces a difference in intensity within each bright fringe seen in the single slit pattern
- This results from interference between light from one slit to the other
- This interference results from the fact that light from the different slits travels a different distance to reach a given point on the screen; this is referred to as the path difference
When does destructive interference occur?
When the path difference becomes half a wavelength, producing a dark fringe (n + 0.5) lamda
When does constructive interference occur?
When the path difference is a whole number of wavelengths producing bright fringes nlamda
What is the fringe spacing between two adjacent bright fringes given by?
W = Dlamda / s
- S is slit width
- D is distance to screen
- lamda is wavlegnth
- In between light and dark fringes the interference is not perfectly constructive or destructive, so the intensity of light changes gradually
What is a polarised wave?
- Transverse waves may undergo polarisation
2. Longitudinal waves cannot be polarised because the oscillations are already in one direction only
How is polarised light produced?
- Using. apiece of Polaroid (as used in sunglasses)
- Polaroid works by only allowing through light which oscillates in a particular direction
What happens to light when it enters a polaroid?
- If the light is passed trough a vertical piece of polaroid, then the emerging ray will be polarised vertically
- It will have half the intensity of the original beam and this is why sunglasses work
- If this ray meet a horizontal piece of Polaroid, no light will pass through
What is a progressive wave?
They move in a particular direction, transferring energy along the direction the wave is moving
What is a stationary wave?
The wave does not move in a particular direction and energy is stored by the wave
How are stationary wave formed?
Stationary waves are the result of two progressive waves of the same frequency travelling in opposite directions along the same line
What are nodes?
The nodes never move and these occur where the two original waves interfere destructively
What are antinodes?
The antinodes are points of maximum displacement and they occur where the two original waves interfere constructively
What are the differences between the energy of stationary and progressive waves?
- Stationary: stores vibrational energy
2. Progressive: transmits vibrational energy
What are the differences between the amplitudes of stationary and progressive waves?
- Stationary: amplitude varies
2. Progressive: amplitude is constant
What are the differences between the points of stationary and progressive waves?
- Stationary: all points between any two adjacent nodes are in phase
- Progressive: phase varies smoothly with the distance along the path of the wave
What are the differences between the nodes of stationary and progressive waves?
- Stationary: nodes are half a wavelength apart; antinodes are midway between nodes
- Progressive: no nodes or antinodes
Why would the appearance or structure potentially change?
- At viewing angles below the critical angle, there is no TIR, and the block looks transparent
- At viewing angle above the critical angle light reflects off the internal surface and is seen by the viewer
If the refractive index of the core is 1.6 and the cladding is 1.5 work out the critical angle?
- Combined refractive index= corenair x airncladding
- sinc =(1/1.6) x 1.5 = 0.93
- c = 70 degrees
What happens to the central maximum in single slit diffraction is the single slit is reduced?
- Width of central maximum increases
2. Intensity of central maximum decreases
What is an experiment to demonstrate wave nature of sound?
Diffraction through a door
What steps would be taken for value of a quantity to be accepted?
- Method and value published
2. Other scientists repeat experiment using same method
In single slit diffraction if red laser light is replaced with non laser source white light how would pattern change?
- Central white fringe
- All subsidiary maximum composed of a spectrum with violet/blue nearest central maximum
- Fringe spacing less/ maxima is wider
What is monochromatic light?
Single frequency/wavelength
What is coherent light?
Constant phase difference
How would pattern of single slit diffraction change with longer wavelength?
Maxima further apart/central maximum wider
Why use a single slit in double slit diffraction
Single slit acts as a point diffracting light to both slits and the path lengths between the single slit and the double slits are constant
How are bright fringes formed in double slight diffraction?
- Superposition of waves from two slits (define?)
- Constructive interference
- Waves from each slit meet in phase
What happens if you change the violet laser for green laser in double slit?
Fringes further apart
What is a progressive wave?
A wave that transfers energy from one point to another without transferring material
