10.3 Waves, Heat, and the Electromagnetic Spectrum

10.3 Waves, Heat, and the Electromagnetic Spectrum

Waves, heat, and light recur on every Physical Science form because they tie directly to aircraft sensors, radios, and propulsion. These facts are pure recall — perfect for the 30-second pace.

Wave basics

A wave carries energy without carrying matter. Key relationship: v = fλ (speed = frequency × wavelength). For a fixed speed, higher frequency means shorter wavelength. Photon energy rises with frequency: E = hf, where h is Planck's constant. Two wave families to keep distinct:

• Mechanical waves (sound, water, seismic) need a medium. Sound cannot travel through the vacuum of space.
• Electromagnetic waves (light, radio, X-rays) need no medium and travel at the speed of light, c ≈ 3×10^8 m/s (about 186,000 miles/sec).

Sound moves far slower — roughly 343 m/s in air at sea level (about 767 mph). When an aircraft exceeds this speed (Mach 1), it generates a shockwave heard as a sonic boom.

The electromagnetic spectrum

Memorize the order — the AFOQT loves to ask which type has the longest or shortest wavelength:

Mnemonic for increasing energy: Radio Microwave Infrared Visible Ultraviolet X-ray Gamma. Visible light itself runs red (longest) to violet (shortest).

Heat transfer

Heat always flows from hot to cold by three mechanisms:

1. Conduction — direct contact; energy passes through a solid (a metal spoon heating in soup). Metals are good conductors.
2. Convection — bulk movement of a heated fluid (warm air rising, ocean currents).
3. Radiation — transfer by EM waves, requiring no medium; this is how the Sun warms Earth across empty space.

Temperature and phase changes

At standard sea-level pressure (101.3 kPa, 1 atm), water boils at 100°C (212°F) and freezes at 0°C (32°F). Lower atmospheric pressure lowers the boiling point — water boils below 100°C on a mountaintop, which is why cooking takes longer at altitude. The conversion to remember: °F = (°C × 9/5) + 32. Absolute zero is 0 K = −273°C, the temperature at which molecular motion theoretically stops.

Common traps

• Thinking radio waves carry the most energy — they carry the least; gamma rays carry the most.
• Assuming sound and light travel at the same speed; light is roughly a million times faster.
• Confusing the boiling point (100°C) with the freezing point (0°C).
• Believing radiation needs a medium — it does not, which is how solar energy crosses space.

Wave properties worth knowing

Two wave behaviors come up in distractor sets. Amplitude is the wave's height and corresponds to intensity — a louder sound or brighter light has greater amplitude, but amplitude does not change the wave's speed or frequency. Frequency (measured in hertz, cycles per second) sets pitch for sound and color for light; it is independent of amplitude. Waves also reflect (bounce off a surface, the basis of radar and sonar), refract (bend when passing between media, why a straw looks broken in water), and diffract (spread around obstacles).

The Doppler effect explains why an approaching siren sounds higher-pitched and a receding one lower — the same effect lets radar measure closing speed.

Temperature scales and thermal expansion

Three temperature scales appear: Celsius (water freezes 0°, boils 100°), Fahrenheit (freezes 32°, boils 212°), and Kelvin (the absolute scale starting at 0 K = −273°C). To convert Celsius to Kelvin, add 273. Most materials expand when heated and contract when cooled because added thermal energy increases molecular motion; this is why bridges have expansion joints and why a sealed container can rupture if heated. Specific heat measures how much energy a substance needs to change temperature — water has an unusually high specific heat, which moderates coastal climates and makes it an effective coolant.

Light behavior and the eye

White light is a mix of all visible colors; a prism separates it by refracting each wavelength differently, producing a spectrum (red bends least, violet most). Objects appear a given color because they reflect that wavelength and absorb the rest — a red flag reflects red light. These facts tie heat and light together: infrared just below visible red is felt as warmth, and ultraviolet just beyond violet carries enough energy to damage skin and equipment. Knowing where a wavelength sits on the spectrum lets you reason about its energy and effect even on an unfamiliar question.

Sound and the Mach scale

Sound is a longitudinal pressure wave that needs a medium, and its speed depends on that medium and its temperature — about 343 m/s in air at sea level, faster in water (~1,480 m/s) and faster still in steel. Because density and temperature drop with altitude, the speed of sound is lower in the thin cold air at cruising altitude. Mach number is an object's speed divided by the local speed of sound: Mach 1 is exactly the speed of sound, supersonic is above Mach 1, and subsonic is below. When an aircraft passes Mach 1 it overtakes its own pressure waves, which pile into a shockwave — the sonic boom — that trails behind as a cone.

These facts connect Physical Science directly to the kinds of flight concepts the AFOQT favors, so they are worth locking in cold.