Flashcards · 9203 · Topic 3

Waves flashcards

52 cards on Topic 3, subtopics 3.3.1 to 3.3.6. Pick a subtopic or study them all.

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3.3.1 General properties of waves

Wave: A disturbance from a vibrating source that carries energy and information without carrying the material itself along. A floating cork bobs up and down but does not travel with the ripples.
Transverse vs longitudinal waves, with examples: Transverse: oscillations at right angles to the direction energy moves (electromagnetic waves, water waves). Longitudinal: vibrations along the direction of energy travel (sound, a pushed and pulled spring).
Compression and rarefaction: Compression: region where the particles (or coils) are squashed together. Rarefaction: region where they are spread apart.
Amplitude and wavelength: Amplitude: the largest displacement of a point from its undisturbed position. Wavelength (λ): the length of one full wave, from a point to the matching point on the next wave.
Frequency, period and wavefront: Frequency: waves passing a point each second (Hz). Period: time for one complete wave (s); T = 1/f. Wavefront: a line joining points at the same stage of oscillation.
A wave of 1200 Hz has a wavelength of 0.28 m. Speed?: v = fλ = 1200 × 0.28 = 336 m/s.
What happens to a wave entering a different medium?: Its speed changes, so its wavelength changes; the frequency stays the same. If it arrives at an angle its direction changes (refraction).
When is diffraction noticeable?: When the wavelength is comparable to the size of the gap or obstacle. A gap much wider than the wavelength gives very little spreading.
Three things that can happen to a wave at a boundary: It can be reflected, transmitted or absorbed, or any combination of these.

3.3.2 The electromagnetic spectrum

Electromagnetic waves: type, speed and order: Transverse; all travel at the same speed in a vacuum. Increasing frequency (decreasing wavelength): radio, microwaves, infrared, visible, ultraviolet, X-rays, gamma rays.
Visible light: The part of the spectrum the eye detects; different wavelengths look like different colours. Red has the longest wavelength, violet the shortest.
Which objects emit and absorb infrared, and which surfaces are best?: All objects; a hotter object radiates more in a given time. Dark, matt surfaces are good absorbers and emitters; light, shiny surfaces are poor absorbers and emitters, and good reflectors.
Black-body radiation: The range of electromagnetic radiation emitted by an object at a particular temperature. As it heats up it radiates more infrared and at higher frequencies.
Uses of radio waves, microwaves, infrared and visible light: Radio: TV and radio, Bluetooth. Microwaves: mobile phones, satellite TV. Infrared: remote controls, night vision, heating. Visible: photography, fibre optic communications.
Uses of ultraviolet, X-rays and gamma rays: Ultraviolet: security marking. X-rays: medical imaging. Gamma: sterilising surgical instruments, killing harmful bacteria in food.
Hazards of microwaves, infrared, ultraviolet, X-rays and gamma rays: Microwaves: heating of body tissue. Infrared: skin burns. Ultraviolet: skin cancer and blindness. X-rays: high doses kill cells. Gamma: genetic mutations.
Low-energy vs high-energy electromagnetic waves: main effect on the body: Low energy: heating. High energy: ionisation, the removal of an electron from an atom or molecule.
Why do X-rays show bones on an image?: Soft tissue transmits most X-rays, exposing the film or detector; bone absorbs them strongly, so less reaches the film behind it. Used for fractures, dental problems and CT scans.
A radio station transmits at 100 MHz (c = 3.00 × 10⁸ m/s). Wavelength?: λ = c/f = 3.00 × 10⁸ ÷ 1.00 × 10⁸ = 3.0 m.
Precautions for people who work with ionising radiation: Less time near the source, more distance, shielding (lead aprons and screens), and a monitoring badge to record exposure.

3.3.3 Sound and ultrasound

Sound: type of wave, medium and range of human hearing: Longitudinal, with compressions and rarefactions. Needs a medium (cannot cross a vacuum). Human hearing: about 20 Hz to 20 000 Hz.
Pitch and loudness on an oscilloscope trace: Pitch depends on frequency: higher pitch, more cycles across the screen. Loudness is related to amplitude: louder, taller wave.
One cycle on a trace is 4 divisions at 0.5 ms per division. Frequency?: T = 4 × 0.5 = 2.0 ms = 0.0020 s. f = 1/T = 500 Hz.
An echo from a wall returns after 0.50 s (sound at 340 m/s). How far is the wall?: Halve the time (there and back): s = vt = 340 × 0.25 = 85 m.
Ultrasound: what it is and how it is produced: Sound above 20 000 Hz. An electrical signal makes a transducer vibrate at high frequency; the same device detects returning pulses. At a boundary it is partially reflected.
An ultrasound pulse in tissue (1500 m/s) returns after 50 µs. Depth of the boundary?: One-way time 25 µs: s = 1500 × 25 × 10⁻⁶ = 0.0375 m, about 3.8 cm.
Two medical uses of ultrasound: Prenatal scanning: reflections from tissue boundaries build an image (non-ionising). Kidney stones: focused high-intensity ultrasound breaks them into small pieces.

3.3.4 Reflection

The normal, and how angles are measured: A construction line at 90° to the surface at the point of incidence. Angles of incidence, reflection and refraction are measured from it.
Law of reflection: Angle of incidence = angle of reflection.
Image in a plane mirror: Virtual, upright, laterally inverted, the same size, and the same distance behind the mirror as the object is in front.
How do you locate a plane-mirror image on a ray diagram?: Draw two rays from one point of the object to the mirror, reflect each with equal angles to the normal, then extend the reflected rays backwards with dashed lines; the image is where they cross.
Real vs virtual image: Real: rays actually pass through it, so it can be shown on a screen. Virtual: rays only appear to come from it; it cannot be shown on a screen.

3.3.5 Refraction and total internal reflection

Which way does light bend entering a denser medium? Along the normal?: Entering a denser (slower) medium: towards the normal. Entering a less dense (faster) one: away from the normal. Along the normal: speed changes, direction does not.
Refractive index in terms of speed. Light travels at 1.80 × 10⁸ m/s in a plastic:: n = c/v = 3.00 × 10⁸ ÷ 1.80 × 10⁸ = 1.67. n has no unit and is greater than 1.
Light enters glass (n = 1.60) from air at 30°. Angle of refraction?: n = sin i / sin r, so sin r = sin 30° ÷ 1.60 = 0.3125, r = 18.2°.
Critical angle. Find c for glass with n = 1.60.: The angle of incidence in the denser medium at which the refracted ray skims along the boundary. n = 1/sin c, so c = sin⁻¹(1 ÷ 1.60) = 38.7°.
Conditions for total internal reflection: Light travels in a denser medium towards a less dense one, and the angle of incidence is greater than the critical angle. All the light is then reflected back.
Dispersion by a prism: White light splits into a spectrum because each colour is bent by a different amount: red least, violet most.
Optical fibres: how they work and two uses: Light or infrared hits the wall above the critical angle each time, so it is totally internally reflected along the fibre. Uses: endoscopes in medicine; communications (large capacity, low loss, no electrical interference).
Refraction practical: how do you find n from a graph?: Measure i and r from the normal for at least five angles. Plot sin i against sin r; the gradient is n.

3.3.6 Lenses and the eye

Convex vs concave lens: Convex (converging): thicker in the middle; parallel rays are brought to a focus at the principal focus. Concave (diverging): thinner in the middle; parallel rays diverge as if from the principal focus.
Focal length: The distance from the lens to the principal focus.
Convex lens: images for an object beyond 2f, at 2f, and between f and 2f: Beyond 2f: real, inverted, smaller. At 2f: real, inverted, same size. Between f and 2f: real, inverted, larger.
Convex lens: object closer than the principal focus: Virtual, upright, larger: a magnifying glass.
Image formed by a concave lens: Always virtual, upright and smaller, on the same side of the lens as the object.
Magnification. A cell 0.040 mm across appears 6.0 mm across:: Magnification = image height ÷ object height = 6.0 ÷ 0.040 = 150 (no unit).
Functions of the cornea, iris, pupil and retina: Cornea: transparent front; refracts light. Iris: muscle that changes pupil size to control the light entering. Pupil: opening that lets light in. Retina: light-sensitive cells; the image must focus on it.
Accommodation: focusing on a near object: The ciliary muscle contracts, the suspensory ligaments go slack, and the lens becomes thicker and more strongly curved, so it bends light more. (Distant object: the reverse.)
Near point, far point and range of vision of a typical eye: Near point about 25 cm; far point infinity. Range of vision: from about 25 cm to infinity.
Long sight vs short sight: cause and correction: Long sight: eyeball too short (or lens cannot focus); near objects blurred; image behind the retina; convex lens. Short sight: eyeball too long; distant objects blurred; image in front of the retina; concave lens.
Eye vs camera: The retina corresponds to the film or CCD sensor. The eye focuses by changing the shape of its lens; a camera focuses by changing the distance between lens and sensor.
Uses of lasers: A laser is a concentrated source of light, used for cutting, cauterising and burning, including eye surgery to correct visual defects.