Waves

The One Thing

Waves transfer energy and information from one place to another without transferring matter overall. Their behavior is described by measurable quantities and predictable processes, including reflection, refraction, diffraction and, for light, total internal reflection.

Reactions, Processes and Experiments

Key Terms

• Wave: A disturbance that transfers energy from one location to another without a net transfer of matter.
• Transverse wave: A wave in which the vibrations are perpendicular to the direction of energy transfer, such as light and waves on a rope.
• Longitudinal wave: A wave in which the vibrations are parallel to the direction of energy transfer, such as sound in air.
• Amplitude: The maximum displacement of a vibrating particle from its equilibrium position; it is related to wave energy.
• Wavelength: The distance between corresponding points on consecutive waves, such as crest to crest or compression to compression.
• Frequency: The number of complete waves or vibrations produced each second, measured in hertz (Hz).
• Period: The time taken for one complete wave or vibration.
• Wave speed: The distance travelled by a wave per unit time.
• Wavefront: A line or surface joining points that are at the same stage of vibration.
• Reflection: The change in direction when a wave bounces back from a surface.
• Normal: An imaginary line drawn at 90 degrees to a reflecting or refracting surface at the point where the wave meets it.
• Angle of incidence: The angle between the incoming wave or ray and the normal.
• Angle of reflection: The angle between the reflected wave or ray and the normal.
• Refraction: The change in direction of a wave as it enters a different medium because its speed changes.
• Diffraction: The spreading of waves as they pass through a gap or around an obstacle, especially when the gap or obstacle is similar in size to the wavelength.
• Sound: A mechanical longitudinal wave produced by vibrating objects and requiring a medium to travel.
• Pitch: The sensation related mainly to sound frequency; higher frequency means higher pitch.
• Loudness: The sensation related mainly to sound amplitude; greater amplitude usually produces a louder sound.
• Ultrasound: Sound with a frequency above the upper limit of typical human hearing, approximately 20,000 Hz.
• Electromagnetic wave: A transverse wave made of changing electric and magnetic fields that can travel through a vacuum.
• Electromagnetic spectrum: The complete range of electromagnetic radiation arranged by wavelength, frequency or energy.
• Total internal reflection: The complete reflection of light inside a denser medium when the angle of incidence is greater than the critical angle.
• Critical angle: The angle of incidence in the denser medium that produces a refracted ray travelling along the boundary.
• Refractive index: A measure of how much a material slows light, often expressed as the ratio of the speed of light in vacuum to its speed in the material.

Easily Confused

• Transverse and longitudinal waves: Transverse vibrations are perpendicular to the direction of energy transfer, whereas longitudinal vibrations are parallel to it.
• Amplitude and frequency: Amplitude is related mainly to energy transfer, whereas frequency is the number of waves or vibrations per second.
• Pitch and loudness: Pitch depends mainly on frequency, whereas loudness depends mainly on amplitude.
• Reflection and refraction: Reflection is a wave bouncing back from a boundary, whereas refraction is a change in direction caused by a change in speed when entering another medium.
• Angle of incidence and angle of reflection: The angle of incidence is measured for the incoming ray; the angle of reflection is measured for the reflected ray. Both are measured from the normal.
• Frequency and wavelength during refraction: Frequency remains constant at a boundary, whereas wave speed and wavelength may change.
• More optically dense and less optically dense media: Light slows and bends towards the normal in a more optically dense medium; it speeds up and bends away from the normal in a less optically dense medium.
• Total internal reflection and ordinary reflection: Total internal reflection occurs entirely inside a denser medium when the angle of incidence exceeds the critical angle; ordinary reflection can occur at a boundary without this condition.
• Real and virtual images: A real image can be formed on a screen, whereas a virtual image cannot.
• Convex and concave lenses: A convex lens converges parallel rays, whereas a concave lens diverges them.
• Sound and electromagnetic waves: Sound is mechanical and requires a medium; electromagnetic waves can travel through a vacuum.
• Radio waves and microwaves: Both are electromagnetic waves, but radio waves are used for broadcasting and communication, while microwaves are used in satellite communication, radar and cooking.
• Visible light and ultraviolet: Visible light is detected by the human eye; ultraviolet is not visible and can damage skin and eyes.
• X-rays and gamma rays: Both are penetrating and can damage living cells or tissue, but X-rays are used for medical imaging and security scanning, whereas gamma rays are used in medical treatment and sterilization.

What Gets Asked

• Calculating wave speed, frequency or wavelength: Use v = fλ with consistent units; the specific mark-losing error is failing to express wavelength in metres, frequency in hertz or speed in m/s.
• Calculating frequency or period: Use f = 1/T or T = 1/f; do not treat frequency and period as directly proportional.
• Drawing or interpreting reflection diagrams: Apply i = r and measure both angles from the normal, not from the surface.
• Explaining refraction: State that frequency remains constant while speed and wavelength change; identify bending towards the normal in a more optically dense medium and away from it in a less dense medium.
• Explaining sound: Identify sound as a mechanical longitudinal wave and state that it cannot travel through a vacuum because it requires vibrating particles in a medium.
• Ordering or applying the electromagnetic spectrum: Recall the order from radio waves through gamma rays and link each named region to its stated application and hazard.
• Explaining total internal reflection and optical fibres: State that the angle of incidence must exceed the critical angle and connect repeated total internal reflection with communication and medical instruments.