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Flashcards · 8463 · Topic 6
Waves flashcards
28 cards on Topic 6, subtopics 4.6.1 to 4.6.3. Pick a subtopic or study them all.
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4.6.1 Waves in air, fluids and solids
- Transverse vs longitudinal waves
- Transverse: oscillations perpendicular to the direction of energy transfer (ripples, EM waves). Longitudinal: oscillations parallel to it, with compressions and rarefactions (sound in air).
- Amplitude vs wave height
- Amplitude is the maximum displacement from the undisturbed position: half the crest-to-trough height.
- Wavelength and frequency
- Wavelength: distance from a point on one wave to the equivalent point on the next wave. Frequency: number of waves passing a point each second, in Hz.
- Wave speed equation, and period equation
- v = fλ (m/s, Hz, m) T = 1/f (T in s)
- Evidence that a wave moves but the medium does not travel with it
- A floating object bobs up and down on ripples but stays in place; air particles vibrate about fixed positions as sound passes.
- A wave has speed 12 m/s and frequency 4.0 Hz. Find its wavelength.
- λ = v / f = 12 / 4.0 = 3.0 m
- Specular vs diffuse reflection
- Specular: from a smooth surface, in a single direction. Diffuse: from a rough surface, scattered in many directions.
- Range of normal human hearingHigher
- 20 Hz to 20 kHz.
- UltrasoundHigher
- Sound above 20 kHz. It is partly reflected at boundaries between media, and the echo time gives the distance. Used for medical and industrial imaging.
- P-waves vs S-wavesHigher
- P-waves: longitudinal, travel through solids and liquids. S-waves: transverse, cannot pass through liquids, so the S-wave shadow shows part of the Earth’s core is liquid.
- A sonar pulse (1500 m/s) returns after 0.20 s. How deep is the water?Higher
- 1500 × 0.20 = 300 m there and back, so the depth is 150 m.
4.6.2 Electromagnetic waves
- Order of the electromagnetic spectrum, long wavelength to short
- Radio, microwave, infrared, visible, ultraviolet, X-ray, gamma.
- Properties shared by all electromagnetic waves
- Transverse; all travel at the same speed in a vacuum (or air); transfer energy from source to absorber; form a continuous spectrum.
- One use each: radio, microwaves, infrared, visible light
- Radio: television and radio. Microwaves: satellite communications, cooking. Infrared: electric heaters, infrared cameras. Visible: fibre optic communications.
- Hazards of ultraviolet, X-rays and gamma rays
- Ultraviolet: premature skin ageing and increased risk of skin cancer. X-rays and gamma: ionising radiation that can cause gene mutation and cancer.
- A radio wave has frequency 200 MHz. What is its wavelength? (c = 3.0 × 10⁸ m/s)
- λ = 3.0 × 10⁸ ÷ 2.0 × 10⁸ = 1.5 m
- What causes refraction, and how does a wave front diagram show it?Higher
- A change in wave speed. The part of the wave front that enters the slower medium first slows first, so the wave front turns and the fronts get closer together.
- How are radio waves produced and detected?Higher
- Oscillations in electrical circuits produce them. When absorbed, they can induce an alternating current with the same frequency as the radio wave.
- Convex vs concave lens
- Convex: parallel rays converge at the principal focus; image can be real or virtual. Concave: parallel rays spread out as if from the principal focus; image always virtual.
- Magnification equation
- magnification = image height ÷ object height. It is a ratio, so it has no units (use the same units for both heights).
- What colour does a blue object appear in red light?
- Black. It reflects only blue light, and there is no blue in red light, so all the light is absorbed.
- Required practical 10: how do you compare infrared emission from different surfaces?
- Fill a Leslie cube with hot water and hold an infrared detector at the same distance from each face. Matt black emits most; shiny silver emits least.
4.6.3 Black body radiation
- Which objects emit and absorb infrared radiation?
- All bodies, at any temperature. The hotter the body, the more infrared radiation it emits in a given time.
- Perfect black body
- An object that absorbs all the radiation incident on it, reflecting and transmitting none. It is also the best possible emitter.
- What does the temperature of a body change about the radiation it emits?
- Both the intensity and the wavelength distribution of the emitted radiation depend on the temperature.
- A body stays at a constant temperature. What is true about its radiation?Higher
- It absorbs radiation at the same rate as it emits radiation.
- A body absorbs radiation faster than it emits it. What happens?Higher
- Its temperature increases.
- What does the temperature of the Earth depend on?Higher
- The rates at which light and infrared radiation are absorbed and emitted by the Earth and its atmosphere, and the radiation reflected back into space.