Cambridge IGCSE Physics · Syllabus 0625 · Waves
Frequency
What is Frequency?
Frequency is the number of wavelengths that pass a point per unit time; symbol f, unit hertz, where one hertz is one wave per second. Frequency is fixed by the source and does not change when a wave crosses a boundary.
This definition is part of the Waves chapter in Cambridge IGCSE Physics.
Frequency in context
A wave is a disturbance that travels, carrying energy away from a source while the material it passes through only oscillates about fixed positions. Describe the disturbance with wavelength, frequency and amplitude; link them with \(v = f\lambda\). Every wave can reflect, refract and diffract. Light is an electromagnetic wave, so mirrors, blocks, prisms, fibres and lenses are all the same three behaviours applied to rays. Sound is a longitudinal mechanical wave, so it needs a medium, its pitch follows frequency and its loudness follows amplitude, and its echoes measure distance.
A wave is a disturbance that travels through space or a material, transferring energy from a source to a receiver without transferring matter. The particles of the medium oscillate about fixed equilibrium positions and end up where they began. Describe the wave with its wavelength, frequency and amplitude, and link them to the speed with \(v = f\lambda\). Waves of every kind can be reflected at a plane surface, refracted when their speed changes, and diffracted at a gap or an edge.
White light is a mixture of all the visible frequencies. A glass prism separates them because the glass slows each frequency by a slightly different amount: violet is slowed most and so deviated most, red least. The colours therefore emerge along slightly different directions and spread out into a spectrum. The prism does not create the colours — it separates colours that were in the white light all along.
The electromagnetic spectrum is one continuous family of transverse waves. They all transfer energy, they all travel at the same high speed in a vacuum — far faster than sound — and they all need no material medium. In order of increasing frequency and therefore decreasing wavelength the main regions are: radio, microwave, infrared, visible, ultraviolet, X-ray, gamma. Each region is used for the jobs its particular frequency and wavelength suit, and the hazards change as you move along the spectrum: heating at the low-frequency end, and ionising damage to cells at the high-frequency end.
Sound is produced by a vibrating source and travels as a longitudinal wave: the particles of the medium oscillate back and forth parallel to the direction the wave travels. Because the disturbance is handed on from particle to particle, sound cannot travel through a vacuum. Its frequency sets the pitch and its amplitude sets the loudness. Humans hear roughly 20 Hz to 20 000 Hz; above 20 kHz the sound is ultrasound.
Common mistakes with Frequency
- 4. “Frequency changes when a wave crosses a boundary.” Core 3.1.7 Why wrongIf fewer crests left the boundary each second than arrived at it, crests would have to pile up there without limit. They do not. CorrectFrequency is fixed by the source. Speed changes at the boundary, and since \(v = f\lambda\) with \(f\) fixed, the wavelength changes in the same ratio. Say this“The frequency is unchanged because it is determined by the source; the speed and wavelength both change.” CheckA 500 Hz sound passes from air into water where it travels about four times faster. State what happens to \(f\) and to \(\lambda\).
- 15. “A prism creates the colours.” Core 3.2.4.1 Why wrongIsolate one colour and send it through a second prism: it stays that colour. Nothing new is manufactured. CorrectWhite light is already a mixture of frequencies; the prism separates them because glass refracts each frequency by a slightly different amount. Say this“The prism separates the colours already present in white light, because the refractive index of glass is slightly different for each frequency.” CheckWhat is seen if a second, inverted prism is placed after the first?
- 18. “All electromagnetic waves are equally dangerous.” Core 3.3.4 Why wrongThe hazard depends on frequency, intensity and exposure time. Radio waves at everyday intensities have no measurable effect; gamma rays ionise molecules in cells. CorrectExcessive exposure to any region heats tissue and can burn. Ultraviolet, X-rays and gamma rays are additionally ionising: ultraviolet damages surface cells and the eyes, leading to skin cancer and eye conditions, and X-rays and gamma rays cause mutation or damage to cells in the body. Say this“Excessive exposure to any electromagnetic radiation can heat tissue and cause burns, but only ultraviolet, X-rays and gamma rays are ionising.” CheckState one hazard of microwaves and one of gamma rays, and say which is ionising.
- 19. “Pitch depends on amplitude.” Core 3.4.7 Why wrongPlay the same guitar string harder: it sounds louder, not higher. The note is unchanged. CorrectPitch is set by frequency. Higher frequency gives higher pitch. Say this“A higher frequency gives a higher pitch.” CheckTwo traces have the same height but one has twice as many waves. Which sound is higher in pitch?
- 20. “Loudness depends on frequency.” Core 3.4.7 Why wrongThe mirror image of the previous error. A high whistle can be barely audible and a low drum deafening. CorrectLoudness is set mainly by amplitude. Greater amplitude gives a louder sound. Say this“A larger amplitude gives a louder sound.” CheckWhich measurement on a waveform display tells you the loudness?
- 22. “Ultrasound means very loud sound.” Core 3.4.9 Why wrongIt confuses the two axes of a trace. “Ultra” here refers to frequency, not amplitude. A dog whistle is ultrasonic and very quiet. CorrectUltrasound is sound of frequency above 20 kHz, beyond the upper limit of human hearing. Its loudness is a separate matter entirely. Say this“Ultrasound is sound with a frequency higher than 20 kHz.” CheckIs a 30 kHz sound necessarily loud? Explain.
Examiner tips on Frequency
- Answering “what does monochromatic mean?” Say light of a single frequency. “One colour” is the translation of the word, not the physics, and on its own it is unlikely to score. If the question then asks what happens when monochromatic light enters a prism, the answer is that it is refracted but not dispersed, because there is only one frequency to separate.