Waves B2.01

Measuring waves Displacement Amplitude Wavelength Period Frequency Phase difference

A Wave is… …the transfer of energy through a medium without the net transfer of the medium

Aims Describe the oscillation of the particles of the medium for a progressive wave Be able to label amplitude, wavelength, period. Be able to use c = f λ Know the definitions and examples of longitudinal and transverse waves. Describe polarisation, using a diagram

Label an arrow to show how point A oscillates Wave Direction  Amplitude  Label an arrow to show how point A oscillates Label amplitude of point B Label wavelength from point B, label it  Label one wavelength from point C, also label it  Describe how the oscillation of particle A compares with the direction of motion of the wave State whether particle B is about to move down or up as the wave passes through it Describe the motion of particle C over the next full cycle of movement Perpendicular Down Up, then down, before returning to original position

Examples of transverse waves Electromagnetic waves Ripples on water Waves on ropes Secondary earthquake waves (S-waves)

ag Label an arrow to show how point A oscillates Wave Direction ag   Label an arrow to show how point A oscillates Label one wavelength from point B, label it  Label one wavelength from point C, also label it  Now write these in the box below the diagram you have just labelled Describe how the oscillation of particle A compares with the direction of motion of the wave What is the Displacement of particle B has travelled after one complete wave has passed through it State how far the energy carried by the wave has travelled after one complete wave has passed through point B. Parallel 0m One Wavelength

Examples of longitudinal waves Sound Primary earthquake waves (P-waves)

Using the two wave equations Fill in the table: Now complete these in the box below the table If a wave travels along a string with a wavelength of 0.75m and a frequency of 20Hz, calculate its speed. If an object oscillates once every 0.02 seconds calculate its frequency and the wavelength of sound leaving this object at a speed of 330 ms-1 Calculate wavelength of wave with speed 3.0x108 ms-1 and frequency 88.3MHz Try to explain in words why frequency x wavelength = speed symbol Stands for…. Measured in.. c f λ T Wave speed m s-1 Frequency Hz ( = s-1) Wavelength m Time Period s 20 x 0.75 = 15 ms-1 f= 1/T = 50Hz ;  = 330 / 50 = 6.6 m  = 3.0x108 / 88.3x106 = 3.4 m f = 1/T; because Speed = Distance / Time, Speed = Distance x Frequency

Now write these in the box below the diagram you have just labelled C Wave Direction D In-phase with B In-antiphase with B Ideas of PHASE Label next point along the wave that is in phase with B. (We say that this has a phase difference of 0° or say that they are in phase) Label next point along the wave that is ½ cycle out of phase (this is called antiphase) with B Now write these in the box below the diagram you have just labelled Write the phase difference between points A and B as an angle, (remember 1 full cycle = 360° ) State whether this phase difference will stay the same or change over the next cycle of motion What is the simplest way of expressing the phase difference between points B and D? 90 Same 180

Point to another region of rarefaction Wave Direction A B C E D The regions where particles are close together in a longitudinal wave are called regions of compression. The regions where particles are spread out in a longitudinal wave are called regions of rarefaction. What is the phase difference (in degrees) between points B (rarefaction) and C (compression)? Point to another region of rarefaction What is the simplest way of expressing the phase difference between points B and D? 180 180

Extended response time: a chance to practise your written skills… Answer the following taking care to use correct English Using a long spring (stretched horizontally)it is possible to send longitudinal or transverse waves by oscillating one end. Describe: How to move the end to create transverse waves How to move the end to create longitudinal waves The difference this will make to how each particle of spring moves What is happening in terms of energy What values could be measured about the movement of one end How these values could be measured from the wave motion Extra if finished: - If you timed how long it took a wave to travel 10metres along the spring, explain how you could work out two extra values for this wave

Read each others… how well have they included these ideas Read each others… how well have they included these ideas? (add ticks or comments in a different colour) Using a long spring (stretched horizontally)it is possible to send longitudinal or transverse waves by oscillating one end. Describe: How to move the end to create transverse waves… oscillate up and down, vertically How to move the end to create longitudinal waves …oscillate forwards and back, horizontally The difference this will make to how each particle of spring moves… in transverse wave each particle oscillates perpendicular to direction of wave (or just up and down) in longitudinal wave each particle oscillates parallel to direction of wave (or just forwards and back) Overall, the particles do not change their mean positions What is happening in terms of energy…..the energy of oscillating the end is transferred along the wave (you may notice some of it reflects back from the far end) What values could be measured about the movement of one end amplitude (how far up/down/forwards/back) from starting position it is moved frequency (how many complete oscillations in each second) How these values could be measured from the wave motion amplitude (how far up/down/forwards/back from rest position any part of the spring moves) frequency (how many complete oscillations any part of the spring makes each second) Extra if finished: - If you timed how long it took a wave to travel 10metres along the spring, explain how you could work out two extra values for this wave

Extra if finished: - If you timed how long it took a wave to travel 10metres along the spring, explain how you could work out two extra values for this wave Calculate wave speed using distance divided by time Calculate wavelength using wave speed divided by frequency (showing this in symbols is OK)

Polarisation

Polarisation

Polarisation can only happen for transverse waves Now check pages 73-75 in new CGP textbooks…then answer questions on p75 and 76

Most light you see is unpolarised Reflected light is partially polarised (horizontally) LCDs on calculators use polarised light. Bolt systems 4 By horizontally, I mean that the plane of polarisation is horizontal, like this. Imagine a bright sunny day. You’re on the beach and the sun is reflecting off the water. Aagh bright! I reach for my...polarising filters. What direction do I want the filter/slits? Liquid crystal display

Aims Describe the oscillation of the particles of the medium for a progressive wave Be able to label amplitude and wavelength. Be able to use c = f λ Know the definitions and examples of longitudinal and transverse waves. Describe polarisation, using a diagram

Homework You will be given a sheet to complete Note that this is part of your numbered homework tasks on this week’s e-mail. Please have it ready for collection in next week’s practical session (Monday or Tuesday lesson) Any questions? Well done for today… more on waves this time next week