WAVES AS Flashcards
What are the identifying features of longitudinal waves?
Distinct identifying features of longitudinal waves:
The oscillations are parallel to the direction of energy transfer, and there are compressions and rarefractions.
What are the identifying features of transverse waves?
- the oscillations are perpendicular to the direction of energy transfer
- there are peaks and troughs.
Give 2 examples of transverse and longitudinal waves.
Transverse waves include:
-EM waves
-surface water waves
-seismic-S waves
-waves on a rope
Longitudinal waves
-sound
-seismic-P waves
Define a progressive wave.
A moving wave that carries energy from one point to another without transferring any matter.
Give 2 uses of polarising filters and explain them.
- polaroid sunglasses/polaroid photography
- use lenses with vertical filters
- which almost completely absorb surface reflected light, which is partially horizontally polarised
- light from the underwater object is not polarised and is transmitted through the filter, albeit at a lower intensity
-reduced glare, and underwater images possible - Microscopes - to identify minerals, and determine their structure
For a transverse progressive wave, define the following:
Displacement of a particle
Amplitude
Wavelength
Period
Frequency
-Displacement of a particle is the distance a point on a wave has moved from it’s equilibrium position.
-Amplitude is the maximum magnitude of displacement of a point on a wave from it’s equilibrium position
-Wavelength is the minimum distance between two adjacent points on a wave that are in phase.
-Period is the time taken for one full wave cycle to pass a point
-Frequency is the number of waves passing a point per second or being produced per second
Define what an unpolarised wave is.
A wave which has oscillations in more than one plane that is perpendicular to the direction of wave travel.
Define what a polarised wave is.
A wave which has oscillations in only one plane that is perpendicular to the direction of wave travel.
This question is about stationary waves.
Describe the properties that two waves must have, so that when they undergo superposition, a stationary wave is formed.
CONDITIONS:
-Two progressive waves
-same frequency/wavelength
-same speed
-same amplitude
-travelling in opposite directions.
Give 3 differences between a progressive wave and a stationary wave.
- a progressive wave transfers energy from one point to another, while a stationary wave only stores energy
- a progressive wave moves through space while a stationary wave does not
- amplitude of oscillations of all points in a progressive wave are equal, while in a stationary wave, the amplitude varies from min to max
Give the equation for the frequency of the first harmonic of a string
(1/2L) * sqrt(T/u), u is mass per unit length
Draw a diagram of how amplitude changes across a stationary wave, and also what points are anti-phase/ in-phase with each other.
reminder:
-amplitude varies from 0 at nodes, to maximum at anti-nodes
-the set of all points S between two adjacent nodes are in-phase with each other
-adjacent sets of points are in antiphase with each other
-sets of points with one set inbetween are in phase with each other
[insert diagram of microwave oven: a wall at one end, and a transmitter at the other.]
A student wants to use stationary waves formed in the oven to measure the frequency of the microwaves emitted by the transmitter.
Suggest how stationary waves are formed (in the microwave oven).
- transmitted produces waves that travel in one direction and are reflected off the far wall. -The original and reflected waves interfere and superimpose.
-both progressive waves
-both same frequency, wavelength, speed and amplitude
-both travelling in opposite directions,
so a stationary wave is formed.
In an arrangement to investigate stationary waves, a student sets up a stretched string attached to a vibration generator.
On the string, there are two marked points P and Q, which are equal distances from either side of the string.
The distance PQ is 0.55m.
The string is made to vibrate at the second harmonic.
Compare the motion of points P and Q on the string.
PHASE
- P and Q must be in anti-phase, so are displaced in opposite directions at any given time
AMPLITUDE
- P and Q oscillate with similar amplitude
FREQUENCY
- P and Q have the same frequency of oscillation
In Quincke’s tube, sound fed into point X is divided between two tubes which meet back up again at a point Y. One tube is extendable, one is fixed. As the movable tube is slowly extended outwards, the sound heard at Y varies.
Explain the variation in the loudness of the sound at Y as the movable tube is slowly pulled out.
VARIATION
- sound loudness varies through a series of minima and maxima
EXPLANATION
-Initially, path difference for the two waves is 0
- phase difference is 0
- sound waves constructively interfere
- as the tube is extended, the path difference and phase difference increases
- when phase difference is (n+1/2)lambda, sound waves are in anti-phase and destructive interference occurs
- when phase difference is nlambda, sound waves in phase and constructive interference occurs
In the Young’s double slit experiment, coherent light waves are passed through two slits to produce an interference pattern, with a fixed fringe spacing.
Define what is meant by “coherent” light waves.
Define fringe spacing.
Coherent waves are waves with:
- same frequency and wavelength
- same fixed phase difference
Fringe spacing is defined as:
- The distance between the centres of 2 adjacent bright/dark fringes
A student carries out Young’s Double Slit experiment, passing coherent, monochromatic light through two slits.
When he places a screen behind the double slits, he observes an interference pattern on the screen.
Describe and explain the interference pattern.
DESCRIPTION
- Bright and dark fringes evenly spaced out
- intensity of the bright fringes max at centre, decreases further away from the center
EXPLANATION
- light diffracts through both slits and interferes
- when path difference is n lambda, waves in phase and constructive interference occurs, leading to a maxima
- when path difference is (n+1/2) lambda, waves in antiphase and destructive interference occurs, leading to a minima
Describe and explain the difference between the fringe seperation for red light compared with blue light.
- red light has a greater wavelength than blue light
- since w is d.p to wavelength, red light has a larger fringe spacing than blue light
Describe and explain the interference pattern produced when white light is used in Young’s double slit experiment.
DESCRIPTION:
- central bright, white fringe
- outer bright fringes show a spectrum of colours, with violet closest to centre and red furthest
- outer bright fringes are wider, so dark fringes may no longer be visible
State and explain 3 safety precautions in using lasers.
- wear safety goggles : to prevent permanent eye damage
- place a “laser in use” warning sign outside the room : to warn others of the danger
- stand behind laser
In Young’s Double Slit experiment, for an interference pattern to be observed on the screen, the incident light must be emitted by two coherent sources.
Explain what is meant by coherent sources.
Explain how the use of a single slit before the double slits makes the light sufficiently coherent for fringes to be observed.
COHERENCY
- sources which produce waves with
-same wavelength and frequency
-a constant fixed phase difference.
SINGLE SLIT
- single slit acts as a point source
- light waves travel the same distance to both slits, so arrive in phase
Polarising filters can be used in many applications, such as in quantum optic experiments.
Describe how a polarising filter polarises waves.
- Pass waves through a polarising filter
- which allows only one plane of oscillations to be transmitted
- all other planes are absorbed
How can waves be partially polarised?
- by reflection from a reflective surface like the surface of a pond
Describe how stationary waves are formed on a string.
- source produces progressive waves
- which reflect off fixed points at ends
- the original and reflected waves superpose
- two waves have same frequency,wavelength,speed,amplitude and travel in opposite directions
- nodes and antinodes, two nodes at ends of string
6 MARKS
A student uses a semi-circular glass block to demonstrate refraction of light. A ray of light is aimed through it’s semi-circular arc, exiting the block at it’s centre. The angle of incidence with the normal is increased from 0 degrees up to the critical angle and beyond.
Describe and explain the observations seen by the student as this is done.
You should:
-Describe what happens to the light ray in each stage of the experiment
-Explain why the light ray follows the path it does.
Initially, for theta < critical angle:
- The light is refracted away from the normal as it passes from glass to air
- This is because it passes into a medium with a lower optical density, so the speed of the wave increases, which means the wavelength increases, as frequency is constant.
- Refraction occurs according to Snell’s Law. Since n2 < n1, sintheta2 > sintheta1, theta2 > theta1: away from normal!
- since the angle of incidence < critical angle, no total internal reflection occurs.
Then, when theta = critical angle:
- The angle of refraction becomes it’s maximum of 90 degrees, and the light just escapes the block along the glass-air boundary.
- The critical angle is given by arcsin(n2/n1).
Then, when theta > critical angle:
- The light ray is totally internally reflected, and the angle of incidence = angle of reflection.
difference between Young’s double slit interference pattern and a diffraction grating pattern?
Define path difference
- the difference in the distance travelled by two waves from their sources to the point where they meet
