General Properties Of Waves
Visão Geral
Drop a pebble into a still pond and something strange happens. Rings race outwards across the surface, faster than you could swim, and a leaf floating two metres away lifts, tips and settles back down. The leaf does not travel. It ends the afternoon exactly where it began. Yet something crossed those two metres of water, because the leaf moved and it was the pebble that moved it. Whatever made that journey carried no water with it at all.
That is the idea this lesson is built on, and it is one of the most useful in the whole of physics. The same description covers the ripple on the pond, the note from a guitar, the light reaching your eye from this page and the signal your phone pulled out of the air a moment ago. You will learn the two ways a wave can oscillate, the five words physicists use to measure one, the single equation that ties speed, frequency and wavelength together, and what happens when a wave runs into a boundary, changes speed or squeezes through a gap. By the end you will be able to say why a hill blocks your car radio in one band but not another, and why a hospital scanner uses sound far too high for you to hear.
Objetivos
- A wave is a disturbance caused by an oscillating source that transfers energy and information in the direction of wave travel, without transferring matter.
- In a transverse wave the oscillations are perpendicular to the direction of energy transfer.
- In a longitudinal wave the oscillations are parallel to the direction of energy transfer. Longitudinal waves have areas of compression and rarefaction.
- Electromagnetic waves and water waves are transverse, sound waves are longitudinal and mechanical waves may be either transverse or longitudinal.
- Waves can be reflected, transmitted or absorbed (or a combination of these) at the boundary between two different materials.
- Waves can undergo refraction due to a change in velocity and diffraction through a narrow gap or at an edge. Students should appreciate that for appreciable diffraction to take place the wavelength of the wave has to be comparable to the size of the obstacle or gap. Students may be required to apply these ideas to the reduction of diffraction in optical instruments, ultrasound waves in medicine and radio wave reception.
- Wave motion can be described in terms of their frequency, wavelength, period, amplitude and wavefront. Students should be able to explain the meaning of these terms.
- The relationship between wave speed, v, frequency, f, and wavelength, λ, is: v = f ×λ
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- What does a wave transfer from its source? A. Matter only B. Energy and information, but not matter C. Matter and energy together D. Mass but not energy Answer: B
- In a longitudinal wave, how are the oscillations arranged? A. At right angles to the direction of energy transfer B. Parallel to the direction of energy transfer C. At 45 degrees to the direction of energy transfer D. In circles around the direction of energy transfer Answer: B
- Which list contains only transverse waves? A. Sound waves and light waves B. Water waves and light waves C. Sound waves and radio waves D. Sound waves and water waves Answer: B
- A wave has a frequency of 50 Hz and a wavelength of 6.0 m. What is its speed? A. 0.12 m/s B. 8.3 m/s C. 56 m/s D. 300 m/s Answer: D
- Appreciable diffraction of a wave passing through a gap happens when: A. the gap is much wider than the wavelength B. the gap is comparable in size to the wavelength C. the gap is much narrower than one hundredth of the wavelength D. the wave is longitudinal rather than transverse Answer: B
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