ZIMSEC O-Level Combined Science (4003)

Waves, Light and Sound: notes

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Waves and their parts

Transverse wave: the vibrations are at right angles to the direction the wave travels. Examples: light, ripples on water.
Longitudinal wave: the vibrations are parallel to the direction of travel. Example: sound, which has compressions and rarefactions.
Wavelength (λ): the distance from one crest to the next, or from one compression to the next, in metres (m).
Frequency (f): the number of waves passing a point each second, in hertz (Hz).
Amplitude: the maximum distance a particle moves from its rest position. Larger amplitude means more energy.

Wave equation: v = f × λ, where v is speed (m/s), f is frequency (Hz) and λ is wavelength (m).
Worked example: a wave has f = 20 Hz and λ = 3 m. So v = 20 × 3 = 60 m/s.
Electromagnetic (EM) spectrum: all EM waves are transverse and travel at 3 × 10⁸ m/s in a vacuum. From longest to shortest wavelength: radio, microwave, infrared, visible light, ultraviolet, X-rays, gamma rays.
Uses: radio for broadcasting, microwaves for cooking and satellites, infrared for remote controls, X-rays for medical scans, gamma rays for treating cancer.
Dangers: ultraviolet causes sunburn and skin cancer, X-rays and gamma rays damage cells, and microwaves can heat body tissue.

Reflection: light bounces off a surface. The angle of incidence equals the angle of reflection.
Plane mirror image: virtual, upright, the same size, and as far behind the mirror as the object is in front. It is laterally inverted.
Refraction: light changes speed and bends when it passes between media at an angle. It bends towards the normal when entering a denser medium.
Total internal reflection: light travelling from a denser to a less dense medium is fully reflected when the angle of incidence exceeds the critical angle. It is used in optical fibres and prisms.

Convex (converging) lens: thicker in the middle. It brings parallel rays to a focus. It is used in magnifiers and corrects long sight.
Concave (diverging) lens: thinner in the middle. It spreads rays out. It corrects short sight.
Real image: formed where rays actually meet, so it can be projected onto a screen. Virtual image: rays only appear to come from it, so it cannot be projected.
The eye: the cornea and lens focus light onto the retina. Ciliary muscles change the lens shape to focus on near and far objects.

Sound production: sound is made by vibrating objects. It needs a medium and cannot travel through a vacuum.
Speed of sound: about 330 m/s in air. It travels faster in solids and liquids than in gases.
Pitch: how high or low a sound is. It depends on frequency.
Loudness: how loud a sound is. It depends on amplitude.
Echo: a sound reflected from a hard surface. Distance = (speed × time) ÷ 2.
Ultrasound: sound above 20 000 Hz. It is used in medical scans and for finding flaws in metal.

Mixing up terms: wavelength is a distance along the wave, while amplitude is the height from the rest position.
Units: convert kHz to Hz (1 kHz = 1000 Hz) before using v = f × λ.
Forgetting the echo halving: divide by 2 when the time covers the trip there and back.
Refraction direction: light bends towards the normal when entering a denser medium, not away from it.
Pitch and loudness: pitch depends on frequency, and loudness depends on amplitude.