IP Physics Notes (Upper Secondary, Year 3-4): 8) Waves

Quick recap -- Waves transport energy without carrying matter. Understand the language (wavelength, frequency, period), track phase with wavefronts, and apply the same maths from microwaves to ultrasound echoes.

Electromagnetic Spectrum

• EM waves are transverse oscillations of electric and magnetic fields and travel at \( c = \pu{3.00 \times 10^8 m.s-1} \) in vacuum.
• They obey \( c = f \lambda \) and do not require a medium.
• Order (long \( \lambda \) -> short): radio, microwave, infrared, visible, ultraviolet, X-ray, gamma.
• Typical uses and hazards:
  • Radio: broadcasting, navigation.
  • Microwave: radar, satellite links, cooking (heating via water dipoles).
  • Infrared: remote controls, thermal imaging.
  • Visible: fibre optics, cameras.
  • UV: sterilisation, fluorescence; overexposure damages skin/eyes.
  • X-ray: medical/industrial imaging; ionising radiation.
  • Gamma: cancer therapy, sterilising equipment; highly penetrating.
• Entering a medium changes wave speed and wavelength but leaves frequency constant.

Wave Terminology & Equation

• Wavelength \( \lambda \): distance between successive points in phase.
• Frequency \( f \): oscillations per second (\( \pu{Hz} \)).
• Period \( T = \dfrac{1}{f} \).
• Amplitude: maximum displacement.
• Wavefront: locus of points in phase.
• Wave speed: \[ v = f \lambda \]
• Displacement-time graphs show period; displacement-distance graphs show wavelength.

Transverse vs Longitudinal Waves

• Transverse (displacement travel): EM waves, waves on strings.
• Longitudinal (displacement travel): sound, compression waves.
• Demonstrate via slinkies or ripple tanks; circular and plane waves obey the same rules.

Reflection & Refraction of Wavefronts

• Plane water waves reflect with ( i = r ) just like light.
• Crossing boundaries alters speed, so wavelength changes while frequency remains fixed.
• Waves bend toward the normal when slowing down, away when speeding up.

Sound Waves

• Longitudinal pressure variations; typical speed in air \( \approx \pu{330 m.s-1} \) at room temperature.
• Hearing range: \( 20 \pu{Hz} \le f \le 20 \pu{kHz} \).
• Echo rangefinding: \( s = \tfrac{1}{2} v t \) (half because of the out-and-back path).

Ultrasound & Applications

• Ultrasound: \( f > \pu{20 kHz} \).
• Pulse-echo method calculates distance by \( s = \tfrac{1}{2} v t \).
• Uses: prenatal scans, industrial flaw detection, sonar depth sounding.
• Reflections arise at boundaries with different acoustic impedances.

Cathode Ray Oscilloscope (CRO)

• Microphone converts sound to voltage; CRO displays voltage vs time.
• Y-gain sets volts per division; time-base sets seconds per division.
• Peak voltage = (vertical divisions) x (Y-gain).
• Period = (horizontal divisions) x (time-base); frequency \( f = \dfrac{1}{T} \).

Worked Example: Determining Sound Speed

A hall echo returns \( \pu{0.25 s} \) after a clap. With sound speed \( \pu{343 m.s-1} \), the wall is at \[ s = \tfrac{1}{2} v t = \tfrac{1}{2} \times \pu{343 m.s-1} \times \pu{0.25 s} \approx \pu{42.9 m}. \]

Key Takeaways

• Waves move energy, not matter; use ( v = f \lambda ) to connect frequency and wavelength.
• Boundary changes keep frequency fixed but adjust speed and direction.
• Ultrasound timing always divides by two for the round trip.
• CRO measurements translate sound questions into voltage and time scales.