Waves flashcards

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What is a wave, and what does it transfer?
A wave is a regular disturbance that transfers energy from place to place without transferring matter. The particles only oscillate about their rest positions.
What is the difference between transverse and longitudinal waves? Give an example of each.
Transverse: oscillations are at right angles to the direction of energy travel (light, water ripples). Longitudinal: oscillations are parallel to it, forming compressions and rarefactions (sound).
Define amplitude, wavelength, frequency and period of a wave.
Amplitude: maximum displacement from rest. Wavelength λ: distance between two matching points. Frequency f: waves per second (Hz). Period T: time for one wave, T = 1/f.
State the wave equation and what each symbol means.
v = fλ. v is wave speed (m/s), f is frequency (Hz), λ is wavelength (m).
A wave has frequency 500 Hz and wavelength 0.66 m. What is its speed?
v = fλ = 500 × 0.66 = 330 m/s.
What is a wavefront, and how are wavefronts drawn in a ripple tank?
A wavefront is a line joining points in phase (e.g. all crests). It is drawn as straight or circular lines, always at right angles to the direction of travel.
What happens to speed, frequency and wavelength when a wave reflects?
All three stay the same. The angle of incidence equals the angle of reflection, both measured from the normal.
What happens to a wave when it refracts, and which quantity never changes?
It changes speed and direction at a boundary between media (unless entering along the normal). Frequency never changes, so wavelength changes with speed (v = fλ).
Describe what happens to water waves moving from deep to shallow water.
They slow down, the wavelength decreases and the frequency stays the same. The wavefronts bend towards the normal if they hit the boundary at an angle.
What is diffraction and when is it most noticeable?
Diffraction is the spreading of waves as they pass through a gap or past an edge. It is most noticeable when the gap is about the same size as the wavelength, or narrower.
State the law of reflection and describe the image in a plane mirror.
Angle of incidence = angle of reflection. The image is virtual, upright, the same size as the object, and as far behind the mirror as the object is in front, and laterally inverted.
Define refractive index and state the equation.
n = sin i ÷ sin r, where i is the angle in the air and r is the angle in the material (both from the normal). It also equals the speed of light in air ÷ speed in the material.
Light enters glass of refractive index 1.5 at 45° to the normal. What is the angle of refraction?
sin r = sin 45° ÷ 1.5 = 0.471, so r = 28°.
What is total internal reflection and when does it occur?
All light is reflected back inside a denser medium. It occurs when the angle of incidence exceeds the critical angle and light is travelling towards a less dense medium.
(Extended) Relate the critical angle to refractive index, and find it for glass with n = 1.5.
sin c = 1/n. For n = 1.5: sin c = 0.667, so c = 42°.
Describe two uses of total internal reflection.
Optical fibres carry light signals for telecommunications and endoscopes. Prisms in periscopes and binoculars turn light through 90° or 180° without losing brightness.
What is the difference between a converging and a diverging lens, and what is the focal length?
A converging lens brings parallel rays to a focus; a diverging lens spreads them out. The focal length is the distance from the lens centre to the principal focus.
When does a converging lens form a real image and when a virtual one?
A real image forms when the object is further than the focal length (upside down, can be on a screen). A virtual image forms when the object is within the focal length (upright, magnified, seen through the lens).
Describe how white light is split by a prism, and why.
White light disperses into the spectrum from red (least bent) to violet (most bent). Different colours have different speeds in glass, so they refract by different amounts.
(Extended) What is monochromatic light, and how do its frequency and speed compare with white light in glass?
Monochromatic light has a single frequency (one colour). Its frequency does not change in glass, but its speed and wavelength decrease. Violet slows more than red.
List the electromagnetic spectrum in order from longest to shortest wavelength.
Radio, microwaves, infrared, visible light (red to violet), ultraviolet, X-rays, gamma rays. Frequency and energy increase in that order.
What do all electromagnetic waves have in common?
They are transverse, travel at 3.0 × 10⁸ m/s in a vacuum (the same speed), need no medium and transfer energy.
Give one use for each of: radio waves, microwaves, infrared, ultraviolet, X-rays, gamma rays.
Radio: broadcasting. Microwaves: satellite communication and cooking. Infrared: remote controls and thermal cameras. UV: detecting forged notes. X-rays: medical imaging. Gamma: sterilising equipment and treating cancer.
State the hazards of microwaves, infrared, ultraviolet, X-rays and gamma rays.
Microwaves: internal heating of cells. Infrared: skin burns. Ultraviolet: skin cancer and eye damage. X-rays and gamma rays: ionise cells, causing mutations and cancer.
How is sound produced and why can it not travel through a vacuum?
Sound is produced by vibrating objects, creating compressions and rarefactions in a medium. A vacuum has no particles to pass the vibrations on.
What range of frequencies can a healthy human ear hear, and what is ultrasound?
About 20 Hz to 20 000 Hz. Ultrasound is sound with a frequency above 20 000 Hz, used in medical scanning and cleaning.
How do the pitch and loudness of a sound relate to frequency and amplitude?
Higher frequency gives higher pitch. Larger amplitude gives louder sound. The waveform on an oscilloscope shows both.
Compare the speed of sound in solids, liquids and gases.
Fastest in solids, slower in liquids, slowest in gases, because particles are closest together in solids. In air it is about 330 m/s.
A sound echo is heard 1.5 s after a shout. How far away is the wall? (speed of sound 330 m/s)
Total distance there and back = 330 × 1.5 = 495 m. The wall is half of that: 247.5 m, about 250 m.

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