Flashcards · 5054 · Topic 3

Waves flashcards

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

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Transverse vs longitudinal waves, with examples: Transverse: vibration at right angles to the energy transfer (electromagnetic waves, water waves, seismic S-waves). Longitudinal: vibration parallel to it, with compressions and rarefactions (sound, seismic P-waves).
Wave speed and period. Waves of 12 Hz have λ = 0.25 m: speed?: v = fλ and T = 1/f. v = 12 × 0.25 = 3.0 m/s.
What happens to speed, wavelength and frequency when a wave refracts?: Speed and wavelength change; frequency stays the same. Entering a slower medium the ray bends towards the normal.
When is diffraction through a gap greatest?: When the gap is about one wavelength wide. A much wider gap gives little spreading; a longer wavelength spreads more.

Plane mirror: law of reflection and the image: Angle of incidence = angle of reflection (from the normal). The image is the same size, as far behind the mirror as the object is in front, and virtual.
Refractive index and critical angle. Find c for glass with n = 1.5.: n = sin i / sin r (i in air). sin c = 1/n = 0.667, so c = 41.8°.
Two conditions for total internal reflection, and two advantages of optical fibres: Light travels from the more dense to the less dense medium, and the angle of incidence is greater than the critical angle. Fibres carry a large amount of information and avoid electrical interference.
Converging lens images: object beyond 2F, between F and 2F, inside F: Beyond 2F: inverted, real, diminished. Between F and 2F: inverted, real, magnified. Inside F: upright, virtual, magnified (magnifying glass).

Electromagnetic spectrum order and speed: Increasing frequency: radio, microwaves, infrared, visible, ultraviolet, X-rays, gamma rays. All travel at 3.0 × 10⁸ m/s in a vacuum.
Which electromagnetic waves are ionising, and what harm do they do?: Ultraviolet (skin cancer, cataracts), X-rays and gamma rays (cell mutation, cancer). Microwaves and infrared cause heating of tissue and burns.
Uses of radio waves, microwaves and infrared: Radio: radio and television communications, astronomy. Microwaves: satellite television, mobile phones, Bluetooth, microwave ovens. Infrared: remote controllers, intruder alarms, thermal imaging, optical fibres, household appliances such as toasters.
Uses of visible light, ultraviolet, X-rays and gamma rays: Visible: vision and photography. Ultraviolet: security marking, detecting forged bank notes, sterilising water. X-rays: medical imaging, security scanners, detecting cracks in metal. Gamma: killing cancer cells, sterilising food and medical equipment.

Sound: nature, audible range, speed in air, ultrasound: Longitudinal; cannot travel in a vacuum. Audible range 20 Hz to 20 000 Hz. Speed in air about 330–350 m/s (faster in liquids, faster still in solids). Ultrasound: above 20 kHz.
A sonar pulse returns after 0.40 s; sound travels at 1500 m/s in water. Depth?: distance = v × t ÷ 2 = 1500 × 0.40 ÷ 2 = 300 m. Halve because the pulse goes there and back.
How do amplitude and frequency affect a sound?: A larger amplitude gives a louder sound. A higher frequency gives a higher pitch.
Why can sound not travel through a vacuum, and how is this shown?: Sound is passed on by vibrating particles, and a vacuum has none. A ringing electric bell in a jar goes quiet as the air is pumped out, although the bell can still be seen vibrating.
A student stands 85 m from a wall and hears the echo of a clap 0.50 s later. Speed of sound?: The sound travels to the wall and back: 2 × 85 = 170 m. v = 170 ÷ 0.50 = 340 m/s.