Waves, Sound and Light

All EM waves travel at $c$ in a vacuum, so frequency and wavelength are always inversely related — as one goes up, the other goes down proportionally. From lowest to highest frequency the order is radio, microwave, infrared, visible, ultraviolet, X-ray, gamma, and photon energy rises with frequency according to $E=hf$. Gamma rays carry the most energy per photon of any EM wave; radio waves carry the least. UV, X-ray and gamma are ionizing — energetic enough to strip electrons from atoms — which is exactly why X-rays image bone (dense tissue absorbs them strongly) but need shielding and dose limits.

Choosing the right EM wave for a technology always comes down to matching a wave property to the job. Radio waves have long wavelengths that diffract around buildings and hills, which is perfect for broadcast and emergency radios — but thick, extended barriers like tunnels and mountains still block them because too little of the wave can diffract through such dense obstacles. Optical fibres send infrared light pulses along a glass core using total internal reflection, forming the backbone of broadband and phone networks, while microwaves' short wavelength gives a narrow, directional beam ideal for satellite and mobile links.

Every wave, mechanical or electromagnetic, is described by amplitude, wavelength, frequency, period and speed, linked by $v=f\lambda$ and $T=1/f$. Transverse waves oscillate perpendicular to the direction of travel (light, water, waves on a string); longitudinal waves oscillate parallel to travel, creating compressions and rarefactions (sound is the standard example). Amplitude relates to the energy a wave carries, while wavelength and frequency together fix its speed — keep these two roles separate.

Sound is the classic longitudinal wave, travelling as compressions and rarefactions through a medium — unlike light, sound cannot travel through a vacuum because it needs particles to pass the disturbance along. This medium-dependence is central to exam questions: sound's speed changes depending on whether it travels through a solid, liquid or gas. Echo-timing questions build directly on this idea, using the time for a reflected sound pulse to return to calculate distance or speed.

Ray diagrams and refraction questions are marked strictly on measuring angles from the normal — the imaginary line perpendicular to the boundary — never from the surface itself. Total internal reflection happens when light travels from a denser medium toward a less dense one and hits the boundary above a critical angle, calculated with $\sin {\theta }_c=\frac{1}{n}$, where $n$ is the refractive index of the denser medium. This is exactly the principle that keeps light trapped inside an optical fibre core.