Light and Sound

Light and Sound

🟢 Lite — Quick Review (1h–1d)

Rapid summary for last-minute revision before your exam.

Light is electromagnetic radiation visible to the human eye, travelling at roughly 3 × 10⁸ m/s in straight lines even through a vacuum. Sound is a mechanical, longitudinal wave produced by vibration, requiring a material medium and travelling at about 330–340 m/s in air at room temperature.

Two laws you must recall cold:

• Law of reflection: ∠i = ∠r, where both angles are measured from the normal (a line perpendicular to the reflecting surface), never from the mirror itself.
• Wave equation: v = fλ, where v is wave speed, f is frequency in hertz (Hz), and λ is wavelength in metres.

High-yield pointers for NCEE: (1) Pitch depends on frequency; loudness depends on amplitude — examiners swap these to trap candidates. (2) An echo requires a minimum gap of 0.1 s between the original sound and its reflection. (3) Luminous objects emit light (Sun, lit bulb); non-luminous objects (book, moon) are only seen because they reflect light from a luminous source.

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🟡 Standard — Regular Study (2d–2mo)

Standard content for students with a few days to months.

Nature of Light and Sound

Light is a transverse electromagnetic wave that needs no medium — it crosses the vacuum of space from the Sun to Earth. The visible spectrum spans roughly 380 nm (violet) to 750 nm (red). Sound is a longitudinal mechanical wave in which particles oscillate parallel to the direction of wave travel; it cannot exist in a vacuum because there are no particles to vibrate.

Reflection of Light

When light strikes a smooth surface such as a plane mirror, it obeys two rules:

1. The incident ray, the reflected ray, and the normal all lie in the same plane.
2. The angle of incidence (i) equals the angle of reflection (r): ∠i = ∠r.

Angles are measured between the ray and the normal, not the mirror surface. A common NCEE trap reverses this — always draw the normal first.

Refraction of Light

When light crosses a boundary between two transparent media of different optical density (e.g. air to glass), it bends. The refractive index n = sin i / sin r. Light bends towards the normal when entering a denser medium (air → glass) and away from the normal when entering a less dense one (glass → air).

Properties of Sound

Pitch is controlled by frequency (f, in Hz); period T = 1/f. Loudness is controlled by amplitude. Sound travels fastest in solids (≈ 5000 m/s in steel), slower in liquids (≈ 1500 m/s in water), and slowest in gases (≈ 330–340 m/s in air) because particle spacing governs how quickly vibrations transfer.

Worked Relationship

Using v = fλ: a sound of frequency 680 Hz and wavelength 0.5 m travels at v = 680 × 0.5 = 340 m/s, matching air at room temperature.

Property	Light	Sound
Medium needed	No (vacuum OK)	Yes (solid/liquid/gas)
Wave type	Transverse EM	Longitudinal mechanical
Speed in air	3 × 10⁸ m/s	≈ 340 m/s
Detected by	Eyes	Ears

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🔴 Extended — Deep Study (3mo+)

Comprehensive coverage for students on a longer study timeline.

Absorption, Transmission, and Colour

Materials interact with light in three principal ways. Transparent materials (clear glass, air) transmit light almost completely so objects behind are seen sharply. Translucent materials (frosted glass, tracing paper) scatter transmitted light, blurring the view. Opaque materials (wood, metal) absorb or reflect light, blocking transmission. A red apple looks red because it absorbs all visible wavelengths except red, which it reflects into your eye — the colour of an object is the colour of the light it reflects, not the colour it “is”.

Echoes and Reflection of Sound

An echo is produced when a sound wave reflects off a hard, large surface (cliff, wall) and returns to the listener. The human ear cannot distinguish a sound from its reflection unless the gap exceeds 0.1 s, so the reflecting surface must be at least d = v × t / 2 = 340 × 0.1 / 2 = 17 m away. Soft or irregular surfaces absorb sound instead of reflecting it, which is why furnished rooms echo less than empty halls.

Common Mistakes

• Measuring angles from the mirror instead of the normal.
• Claiming sound can travel through space (it cannot — explosions in space films are silent in reality).
• Stating the speed of sound is constant; it varies with medium and temperature.
• Confusing the refractive index formula’s sin i (in the rarer medium) with sin r.

Adjacent Topics to Link

This topic connects directly to Waves and Wave Motion (transverse vs longitudinal), The Human Eye (light reception), and Work, Energy and Power (light and sound as energy carriers). Mastering v = fλ here pays off across all wave questions.

Practice Prompts

1. A student standing 25 m from a wall claps once. Using v = 340 m/s, calculate the time gap between the clap and the echo heard, and state whether the echo will be distinguishable.
2. A ray strikes a plane mirror at 35° to the mirror surface. What is the angle of reflection measured (a) from the normal and (b) the angle between the incident and reflected rays?

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