Waves, light and sound

Learning objectives

• Apply the wave equation.
• Construct simple ray diagrams.
• Compare sound with electromagnetic waves.

The big idea

Waves reflect at boundaries and can refract when their speed changes. Frequency remains unchanged across a stationary boundary, so a speed change also changes wavelength.

Ray diagrams approximate the direction energy travels when diffraction is small. Sound is a mechanical longitudinal wave and needs a medium. Electromagnetic waves are transverse and share the same speed in a vacuum.

Topics to connect

• Wave properties
• Reflection and refraction
• Lenses
• Electromagnetic spectrum
• Sound

1. Wave properties and boundaries

Describe waves using amplitude, wavelength, frequency and speed. Transverse oscillations are perpendicular to travel; longitudinal oscillations are parallel. Reflection changes propagation direction at a boundary. Refraction occurs when speed changes between media, with frequency unchanged at a stationary boundary. Diffraction is spreading through a gap or around an edge and is especially noticeable when wavelength is comparable to the gap size. It is not the same as refraction.

2. Reflection and refraction of light

Measure ray angles from the normal. A plane mirror gives equal incidence and reflection angles and a virtual image apparently behind the mirror. Entering glass from air generally slows light and bends an oblique ray towards the normal. Total internal reflection requires travel from higher to lower refractive index and incidence beyond the critical angle. It does not occur simply whenever a ray meets any boundary steeply.

3. Lenses and image construction

A converging lens bends parallel rays towards a principal focus. Use standard rays from one point on an object: a ray parallel to the axis refracts through the far focus, while a ray through the optical centre is approximately undeviated for a thin lens. Their intersection locates a real image when one forms. An object inside the focal length instead gives a magnified virtual image. A diverging lens spreads rays and gives an upright virtual image for a real object.

4. The electromagnetic spectrum

Radio, microwave, infrared, visible, ultraviolet, X-ray and gamma radiation form a continuous spectrum. In a vacuum all have the same speed; frequency and wavelength vary inversely. Uses depend on interactions: microwaves can transfer energy to food, infrared supports thermal sensing and X-rays penetrate soft tissue more readily than bone. Risk depends on intensity, exposure and photon energy; ultraviolet, X-rays and gamma rays can cause ionisation and biological damage.

5. Sound and echoes

Sound consists of mechanical compressions and rarefactions and needs a medium. Frequency influences pitch, while amplitude relates to loudness under otherwise similar conditions. An echo time includes travel to a reflector and back: at 340 m/s, a 0.10 s echo indicates about 17 m distance. Ultrasound has frequency above typical human hearing and can support echo measurements; it is not electromagnetic radiation.

A method you can reuse

1. Identify the wave type and its medium.
2. Read angles from the normal and label boundaries.
3. Keep frequency fixed when considering a stationary boundary.
4. Use a ray diagram or wave diagram suited to the scale.
5. For echoes, distinguish one-way distance from round-trip path length.

Equations and relationships

Investigate and explain

Trace a low-power ray through a transparent block with teacher-approved equipment. Keep the light away from eyes.

Use wavefronts for interference and diffraction, rays for lens images and refraction. Ray models leave out diffraction, and simple sound models do not predict hearing damage or clinical performance.

Open the interactive investigation

Check your understanding