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). |
Worked Examples
Example 1 – Reflection
A light ray strikes a plane mirror at an angle of 35° to the mirror surface. Find the angle of reflection.
Step 1: Angle of incidence is measured from the normal, not the surface. Step 2: Angle with normal = 90° − 35° = 55°. Step 3: By law of reflection, angle of reflection = 55° (from the normal).
Example 2 – Speed of Sound
Thunder is heard 4 seconds after lightning is seen. Estimate the distance of the lightning (speed of sound = 340 m/s).
Step 1: Light travels almost instantaneously; use time for sound. Step 2: Distance = speed × time = 340 × 4 = 1360 m (or 1.36 km).
Example 3 – Wave Equation
A tuning fork vibrates at 256 Hz. If the speed of sound is 340 m/s, find the wavelength.
Step 1: Use v = f × λ → λ = v / f. Step 2: λ = 340 / 256 ≈ 1.33 m.
Common Mistakes
| Wrong Thinking | Correct Fix |
|---|---|
| Measuring angle of incidence from the mirror surface. | Always measure from the normal (perpendicular to the surface). |
| Believing sound travels faster in air than in solids. | Sound travels fastest in solids, slower in liquids, slowest in gases. |
| Confusing loudness with pitch. | Loudness relates to amplitude (energy); pitch relates to frequency. |
| Thinking light bends away from normal when entering glass from air. | Light bends towards the normal when entering a denser medium. |
| Assuming heat radiation requires a medium like conduction. | Radiation is electromagnetic and does not need any medium. |
Quick Reference
- Law of Reflection: i = r; all rays and normal in one plane.
- Refraction rule: Denser medium → bends towards normal; rarer → away.
- Heat transfer trio: Conduction (solids), Convection (fluids), Radiation (no medium).
- Sound in vacuum: Cannot travel—needs a material medium.
- Wave formula: v = f × λ.
- Echo distance: At least 17 m from the reflecting surface.
- Human hearing range: 20 Hz – 20,000 Hz.