Light, Heat and Sound

Overview

Light, Heat and Sound form a core segment of the upper-primary science syllabus and appear consistently in WB TET Paper II. These topics test a candidate's understanding of basic physical phenomena that children encounter daily—mirrors, lenses, cooking, musical instruments and echoes. The exam expects you to know definitions, laws, simple numerical relationships and real-life applications rather than advanced derivations.

Mastering this topic requires clarity on three fronts: (1) how light travels and bends, (2) how heat moves from one body to another, and (3) how sound is produced, transmitted and perceived. Questions often link concepts to classroom demonstrations or EVS-style scenarios, so always think of practical examples while revising.

Key Concepts

• **Rectilinear propagation of light**: Light travels in straight lines in a uniform medium; this explains shadows, eclipses and pinhole cameras.

• **Reflection**: When light bounces off a surface. The angle of incidence equals the angle of reflection (measured from the normal). Plane mirrors produce virtual, erect and laterally inverted images of the same size.

• **Refraction**: Bending of light when it passes from one medium to another of different optical density. Light bends towards the normal when entering a denser medium and away when entering a rarer medium.

• **Conduction, convection and radiation**: The three modes of heat transfer. Conduction occurs in solids (molecule-to-molecule), convection in fluids (bulk movement), and radiation needs no medium (electromagnetic waves).

• **Sound as a mechanical wave**: Sound requires a material medium; it cannot travel through a vacuum. It is a longitudinal wave—particles vibrate parallel to the direction of wave travel.

• **Characteristics of sound**: Loudness depends on amplitude, pitch depends on frequency, and quality (timbre) depends on the waveform produced by different sources.

• **Echo and reverberation**: An echo is a distinct reflected sound heard after a minimum distance of about 17 metres from the reflecting surface (at 20 °C). Reverberation is the persistence of sound due to multiple reflections.

Formulas / Key Facts

| Concept | Formula / Fact | |---------|----------------| | Law of reflection | Angle of incidence (i) = Angle of reflection (r); incident ray, reflected ray and normal lie in the same plane. | | Speed of light in vacuum | Approximately 3 × 10⁸ m/s. | | Refractive index (n) | n = Speed of light in vacuum / Speed of light in medium. | | Snell's law (basic form) | n₁ sin i = n₂ sin r (rarely asked at upper-primary level, but good to know). | | Speed of sound in air (20 °C) | Approximately 343 m/s (often rounded to 340 m/s). | | Relation: speed, frequency, wavelength | v = f × λ (speed = frequency × wavelength). | | Minimum distance for echo | d ≥ 17 m (round trip 34 m; time ≥ 0.1 s for human ear to distinguish). | | Audible frequency range | 20 Hz to 20,000 Hz for humans. | | Infrasound / Ultrasound | Below 20 Hz is infrasound; above 20,000 Hz is ultrasound. | | Heat transfer order in metals | Silver > Copper > Aluminium > Iron (conductivity decreases). |