Waves, sound and light

Waves

Waves transfer energy without transferring matter.

• Transverse: vibrations at right angles to the direction of travel (light, water waves).
• Longitudinal: vibrations along the direction of travel (sound).
• Wave speed = frequency × wavelength (v = fλ).

Sound

Needs a medium; travels fastest in solids. Speed in air about 340 m/s. An echo is a reflected sound; used in echo-sounding. Pitch depends on frequency; loudness on amplitude.

Light

• Reflection: angle of incidence = angle of reflection; plane-mirror images are virtual, upright and the same distance behind the mirror.
• Refraction: light bends towards the normal when entering a denser medium. Refractive index n = sin i ÷ sin r.
• Total internal reflection: happens when light in a denser medium hits the boundary at more than the critical angle. Used in optical fibres.
• Converging lenses form real images (cameras, the eye) and act as magnifying glasses when the object is close.

The electromagnetic spectrum

Radio, microwaves, infrared, visible, ultraviolet, X-rays, gamma rays (increasing frequency). All travel at 3 × 10⁸ m/s in a vacuum.

Check yourself

1. A radio wave has frequency 100 MHz. Find its wavelength (c = 3 × 10⁸ m/s).
2. Give one use of total internal reflection.
3. A girl hears an echo from a cliff 1 s after shouting. How far away is the cliff (speed of sound 340 m/s)?