Waves, Light and Sound for Singapore O-Level Physics (6091): general wave properties, reflection and refraction of light, thin converging lenses, and the electromagnetic spectrum and sound

What this module covers

Waves, Light and Sound in O-Level Physics (SEAB 6091) shows that one set of ideas, the properties of waves, describes phenomena as different as ripples, light, the colours of the rainbow and the sounds we hear. The module builds general wave properties first, then applies them to light through reflection, refraction and lenses, and finally to the electromagnetic spectrum and to sound as a longitudinal wave.

Light and sound are everyday examples of energy transfer without transfer of matter, the defining feature of a wave. Each dot point below has full worked answers and practice questions.

General wave properties

General wave properties introduces amplitude, wavelength, frequency, period and speed, and the distinction between transverse waves (vibration across the direction of travel, such as light) and longitudinal waves (vibration along it, such as sound). The central relationship is the wave equation

A wave transfers energy from place to place without moving matter along with it, an idea worth stating in full when asked.

Reflection and refraction of light

Reflection and refraction of light treats light as a wave that obeys the law of reflection (angle of incidence equals angle of reflection from the normal) and bends on changing medium. Entering a denser medium light slows and bends towards the normal; leaving it light speeds up and bends away. Beyond the critical angle total internal reflection occurs, the basis of optical fibres.

Thin converging lenses

Thin converging lenses shows how a convex lens brings parallel rays to a focus and forms images. Ray diagrams predict the image: distant objects give small, real, inverted images (as in a camera), while objects inside the focal length give magnified, virtual, upright images (as in a magnifying glass).

The electromagnetic spectrum and sound

The electromagnetic spectrum and sound places visible light within the full electromagnetic spectrum, from radio waves through to gamma rays, all transverse waves travelling at the speed of light in a vacuum. It contrasts these with sound, a longitudinal wave that needs a medium and travels far more slowly, and links pitch to frequency and loudness to amplitude.

How this module is examined

• Apply $v = f\lambda$ cleanly. Convert to SI units, then substitute and quote units.
• Measure angles from the normal. Both reflection and refraction angles are taken from the normal, not the surface.
• Order the spectrum and link properties. Know the order of the regions and that shorter wavelength means higher energy.

Check your knowledge

Recall and calculation questions across the module. Try them, then check the worked solutions.

1. State the difference between a transverse and a longitudinal wave, giving one example of each. (2 marks)
2. A wave has a frequency of $20\ \text{Hz}$ and a wavelength of $1.5\ \text{m}$. Calculate its speed. (2 marks)
3. State the law of reflection. (2 marks)
4. Name the regions of the electromagnetic spectrum in order of increasing wavelength, starting from gamma rays. (2 marks)
5. State what happens to the pitch of a sound when its frequency increases. (1 mark)