Cambridge IGCSE Physics 3 Waves Exam Questions

About Cambridge IGCSE Physics 3 Waves

Waves in Cambridge IGCSE Physics 0625 covers the general properties of waves, light, the electromagnetic spectrum and sound. You need to describe transverse and longitudinal waves, use the wave equation, explain reflection, refraction and diffraction using a ripple tank, draw ray diagrams for plane mirrors, refraction and thin converging lenses, and describe total internal reflection and dispersion. You also learn the order, properties, uses and dangers of electromagnetic waves, and the properties of sound and ultrasound. Extended candidates use refractive index equations. Examiners commonly set wave equation calculations, ray diagrams that must be drawn accurately with a ruler, and echo-timing questions.

What you need to know

• 3.1 Wave properties Waves transfer energy without transferring matter. Learn crest, trough, amplitude, frequency, wavelength and wavefront. Transverse waves vibrate at right angles to the direction of travel (light, water waves); longitudinal waves vibrate parallel to it (sound). Wave speed v = fλ.
• 3.1 Ripple tank effects Reflection at a plane surface, refraction due to a change of speed (for example in shallower water), and diffraction through a gap or round an edge. Extended candidates know diffraction is greatest when the gap is similar in size to the wavelength.
• 3.2 Reflection and refraction of light Angle of incidence = angle of reflection; a plane mirror image is virtual, upright, the same size and the same distance behind the mirror. Light bends towards the normal entering a denser medium. Extended: refractive index n = sin i / sin r.
• 3.2 Total internal reflection When light travels from a denser medium and the angle of incidence exceeds the critical angle, it is totally internally reflected. This is used in optical fibres for communications and endoscopes. Extended: n = 1 / sin c.
• 3.2 Lenses and dispersion Converging lenses bring parallel rays to the principal focus; draw ray diagrams to find whether an image is real or virtual, enlarged or diminished, upright or inverted. A magnifying glass gives a virtual, enlarged image. A prism disperses white light into the seven colours of the spectrum.
• 3.3 Electromagnetic spectrum In order of increasing frequency: radio waves, microwaves, infrared, visible light, ultraviolet, X-rays, gamma rays. All travel at 3.0 x 10^8 m/s in a vacuum. Learn uses (e.g. microwaves for satellite communication, X-rays for medical imaging) and harmful effects (e.g. ultraviolet damage to skin and eyes).
• 3.4 Sound Sound is a longitudinal wave of compressions and rarefactions that needs a medium. Human hearing is about 20 Hz to 20 000 Hz, and sound in air travels at about 330 to 350 m/s, faster in liquids and solids. Louder sounds have larger amplitude; higher pitch has higher frequency. An echo is reflected sound.
• 3.4 Ultrasound (Supplement) Ultrasound is sound above 20 kHz. It is used in non-destructive testing of materials, medical scanning of soft tissue and sonar, by timing the echo from a boundary.

Common mistakes in 3 Waves

• Measuring angles from the surface instead of the normal. Angles of incidence, reflection and refraction are all measured from the normal.
• Forgetting to halve the distance or double it in echo calculations. The sound travels to the reflector and back.
• Saying the frequency changes during refraction. Speed and wavelength change; frequency stays the same.
• Listing the electromagnetic spectrum in the wrong order or saying the waves travel at different speeds in a vacuum. All travel at 3.0 x 10^8 m/s.
• Drawing rays freehand or without arrows. Use a ruler and show the direction of light on each ray.

Try three 3 Waves exam-style questions