Flashcards · 0625 · Topic 3

Waves flashcards

43 cards on Topic 3, subtopics 3.1 to 3.4. Pick a subtopic or study them all.

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3.1 General properties of waves

What do waves transfer?: Energy, without transferring matter.
Wavelength: The distance between two identical points on consecutive waves, e.g. crest to crest. Never crest to trough.
Amplitude: The distance from the middle (undisturbed) position to a crest or a trough. Not crest to trough.
Frequency: The number of waves passing a point per second, in hertz (Hz).
The wave equation. Find v for f = 2.5 Hz and λ = 0.80 m.: v = fλ (also f = v/λ, λ = v/f). v = 2.5 × 0.80 = 2.0 m/s.
Transverse vs longitudinal waves, with examples: Transverse: vibration at right angles to the direction of travel (electromagnetic waves, water waves, seismic S-waves). Longitudinal: vibration parallel to the direction of travel (sound, seismic P-waves).
What can a ripple tank show?: Reflection (at a barrier), refraction (waves slow down in shallower water) and diffraction (through a gap or at an edge).
A wave is described by compressions and rarefactions. What type is it?Extended: Longitudinal.
How do wavelength and gap size affect diffraction?Extended: Diffraction through a gap increases as the wavelength increases relative to the gap size. Diffraction at an edge is more noticeable for longer wavelengths.

3.2 Light

Law of reflection: Angle of incidence = angle of reflection, both measured from the normal.
Image in a plane mirror: Same size as the object, the same distance behind the mirror as the object is in front, and virtual.
Light passes from air into glass at an angle. What happens?: It slows down and bends towards the normal. Leaving the glass, it speeds up and bends away from the normal.
What happens when the angle of incidence inside glass exceeds the critical angle?: There is no refracted ray at all: all the light is reflected back inside (total internal reflection).
Refractive index equations. Find n if the critical angle is 42°.Extended: n = sin i / sin r and n = 1/sin c. n = 1/sin 42° = 1.5.
How do optical fibres carry signals?Extended: Light (or infrared) is totally internally reflected along the fibre. Used in telecommunications.
Principal focus of a converging lens: The point where rays travelling parallel to the principal axis converge after refraction by the lens. Both parts are needed.
Focal length: The distance from the centre of the lens to the principal focus.
Converging lens: image of an object beyond 2F: Real, inverted, diminished.
Converging lens: image of an object between F and 2F: Real, inverted, enlarged.
Converging lens: image of an object closer than FExtended: Virtual, upright, enlarged: the lens acts as a magnifying glass.
How is a virtual image formed?: By extrapolating diverging rays backwards: the rays appear to come from the image but do not actually pass through it.
Correcting short sight and long sightExtended: Short sight (light focused in front of the retina): diverging lens. Long sight (focus behind the retina): converging lens.
Why does a glass prism split white light into a spectrum?: Different colours refract by different amounts: blue/violet refracts more than red. Order: red, orange, yellow, green, blue, indigo, violet.
Monochromatic lightExtended: Light of a single frequency (a single colour).

3.3 Electromagnetic spectrum

Electromagnetic spectrum in order of increasing frequency: Radio, microwave, infrared, visible, ultraviolet, X-ray, gamma. (Wavelength decreases along this order.)
What do all electromagnetic waves have in common in a vacuum?: They all travel at the same high speed.
Speed of electromagnetic waves in a vacuumExtended: 3.0 × 10⁸ m/s.
Uses of radio waves and of microwaves; harm from microwaves: Radio: radio and TV transmissions, astronomy, RFID. Microwaves: satellite TV, mobile phones, microwave ovens. Harm: internal heating of body cells.
Uses and harm of infrared: Electric grills, remote controls, intruder alarms, thermal imaging, optical fibres. Harm: skin burns.
Uses and harm of ultraviolet: Security marking, detecting fake bank notes, sterilising water. Harm: damage to surface cells and eyes, leading to skin cancer and eye conditions.
Uses and harm of X-rays and gamma rays: X-rays: medical scanning, security scanners. Gamma: sterilising food and medical equipment, detecting and treating cancer. Harm (both): mutation or damage to cells.
How do we communicate with satellites?: Mainly by microwaves. Some satellite phones use low-orbit satellites; other satellite phones and direct-broadcast satellite TV use geostationary satellites.
Which waves do mobile phones, Wi-Fi, Bluetooth and optical fibres use?Extended: Mobile phones and Wi-Fi: microwaves (pass through some walls, short aerial). Bluetooth: radio waves. Optical fibres: visible light or infrared (high data rates).
Analogue vs digital signals, and two benefits of digitalExtended: Analogue varies continuously; digital takes only set values (1 and 0). Digital: higher data rate, and the signal can be regenerated accurately, so it can travel further.

3.4 Sound

How is sound produced and transmitted?: By a vibrating source. Sound is a longitudinal wave and needs a medium: it cannot travel through a vacuum.
Range of human hearing, and ultrasound: About 20 Hz to 20 000 Hz. Ultrasound is sound above 20 kHz.
Approximate speed of sound in air: 330–350 m/s.
What affects the loudness and the pitch of a sound?: Loudness: amplitude (larger amplitude, louder). Pitch: frequency (higher frequency, higher pitch).
Echo: A reflected sound wave.
A method to measure the speed of sound in air: Measure a long distance between a sound source that can be seen (e.g. two blocks banged together) and an observer. Time from seeing the bang to hearing it. Speed = distance ÷ time; repeat and average.
Speed of sound in solids, liquids and gasesExtended: Fastest in solids, then liquids, slowest in gases.
A sonar echo from the sea bed returns after 0.40 s. Sound travels at 1500 m/s in sea water. How deep is it?Extended: depth = (speed × time) ÷ 2 = (1500 × 0.40) ÷ 2 = 300 m. Halve because the pulse goes down and back. (Do not use v = fλ.)
Two uses of ultrasound other than sonarExtended: Non-destructive testing (echoes from cracks in metal) and medical scanning of soft tissue (echoes from tissue boundaries).