The Standard
Assignments
General Information
Longitudinal waves oscillate in the same direction as they are travelling.
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Transverse waves oscillate at right-angles to their direction of motion.
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Interference - Assessed (PAGE 52)
Wave interference is all to do with phase relationship and how phase changes with path difference from different sources.
If two (or more) waves met at a point and they are in phase you get constructive interference. If the waves arriving at the point are ALWAYS in phase, regardless of where they are in their cycle, that point is an anti-node. If two (or more) waves met at a point and they are 180 degrees out of phase you get destructive interference. If the waves arriving at the point are ALWAYS 180 degrees out of phase, regardless where they are in their cycle, that point is a node. If you have multiple sources of waves that are coherent (the waves leaving the sources are in phase) you will see an interference pattern. The more sources you have (i.e. using a diffraction grating instead of double slits) the brighter, narrower and further apart the anti-nodes will be. |
Young's double slit experiment - Assessed (PAGE 54 - 58)
In 1801, Thomas Young carried out a simple experiment to investigate and confirm the wave nature of light (Physicists were divided at the time as to whether light was a particle or a wave - surely it couldn't be both!)
The 'Doc' explains this experiment very nicely in the video below
The 'Doc' explains this experiment very nicely in the video below
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Below are some printable notes showing the derivation of Young's formulas that you will use in the exam)
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The Diffraction grating - Assessed (PAGE 59 - 61)
Diffraction gratings use the phenomenon of diffraction and interference to split white light up into its constituent colours i.e. they create spectral dispersion (similar to a glass prism) as shown below. Some gratings will have up to 1800 slits per mm!
The greater the wavelength of the light, the greater the angle of diffraction. That's why red light (longer wavelength) diffracts further from the central maximum.
Let's check in with Doc Physics to see how this works.
Let's check in with Doc Physics to see how this works.
Absorption and Emission Spectra - It turns out that atoms of a particular element will emit light of certain wavelengths (and, therefore, frequencies) when 'excited'. However, no two elements emit light of the same wavelength i.e. all elements have a unique emission signature. The same is true for the frequencies of light they will absorb.
This information, literally, provides us with a list of ingredients (elements) for all stars and nebulae that we can receive light from. The video below explains a bit more about what's going on inside the atoms.
Wave/Particle duality - Not assessed
Although not assessed at this level, it's these theories that make Physics the most mind blowing science of all (not that we should differentiate between the sciences, there cannot be one without them all - but Physics is way cooler!)
Standing waves and music - assessed (PAGE 63 - 76)
Standing waves are formed when two identical waves, travelling in opposite directions interfere with one another. Furthermore, in musical instruments, the waves must be of the correct frequency (and wavelength) in order to produce standing waves and resonance. These frequencies are known as harmonics and represent the natural frequencies of a system.
Follow this link for a detailed explanation of the formation of standing waves
http://www.acs.psu.edu/drussell/Demos/SWR/SWR.html
Musical instruments are designed to resonate at specific frequencies (harmonics) to produce amplified sound waves. Even though we often look at these harmonics individually, the reality is that many can be played at once. This gives the instrument it's specific timbre or tone.
Check out these series of videos to learn more about the mathematical patterns we observe in harmonics and music.
Follow this link for a detailed explanation of the formation of standing waves
http://www.acs.psu.edu/drussell/Demos/SWR/SWR.html
Musical instruments are designed to resonate at specific frequencies (harmonics) to produce amplified sound waves. Even though we often look at these harmonics individually, the reality is that many can be played at once. This gives the instrument it's specific timbre or tone.
Check out these series of videos to learn more about the mathematical patterns we observe in harmonics and music.
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The diagram below shows the series of harmonics for a stringed instrument, such as a guitar. Notice the pattern in the harmonics - they are all multiples of the fundamental frequency. The pattern is that a node is added for each additional harmonic, adding a half wave to the string. Since speed is constant, this decrease in wavelength (L also constant) corresponds to an increase in frequency.
Here is the pattern for open and closed pipes. Notice the similarity in the harmonic pattern for the open pipe and waves on a string above. Since the fundamental of a closed pipe is 1/4 wavelength, only odd numbered harmonics can exist in this type of musical instrument (the open end is an anti-node so always a region of max displacement).
In reality, when a musical note is played, multiple harmonics will be played at once. The harmonics interfere to give the instrument its unique tone (or timbre). The note that is heard is determined by the frequency of the 1st harmonic (fundamental). Notice that the resultant wave below (green) has an odd shape but it's wavelength (and frequency) are unchanged from the fundamental.
Musical beats - assessed
Musical beats are produced when sound waves of slightly different frequencies interfere. These waves will move in and out of phase at regular intervals producing alternating loud and soft sound (beats). The beat frequency is calculated as the difference between the two frequencies.
The doppler effect - assessed (PAGE 77 - 82)
When a wave source is moving in a straight line path, emitted wave fronts in front of the source tend to 'bunch up' (wavelength shortens) and those behind tend to 'stretch out' (wavelength lengthens). This change in wavelength is most often observed as a corresponding change in frequency.
Things to note when applying the Doppler equation:
- Is the observer ahead of or behind the source?
- Are you measuring the observed frequency or the frequency of the source?
- Vw is the speed of the wave.
- Vs is the speed of the source.
The shape of sound! - not assessed
I keep telling you, Physics is awesome!
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