Heat and Light
Overview
Heat and Light form the foundation of thermal physics and optics in the upper primary science curriculum. For TS TET Paper II, this topic carries significant weight as it connects everyday phenomena—cooking, seeing objects, wearing glasses—to scientific principles. Questions typically test conceptual clarity on modes of heat transfer, laws of reflection and refraction, and ray diagrams for mirrors and lenses.
Mastery requires understanding the distinction between heat and temperature, recognizing how energy moves through different media, and applying mirror/lens formulas to solve numerical problems. This topic frequently appears in both content-based and pedagogy-linked questions, making it essential for candidates targeting the Mathematics and Science section.
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Key Concepts
- **Heat vs Temperature**: Heat is the total kinetic energy of molecules (measured in Joules or calories); temperature is the average kinetic energy (measured in °C, °F, or K). Heat flows from higher to lower temperature.
- **Three Modes of Heat Transfer**: Conduction (through solids via molecular collision), convection (through fluids via bulk movement), and radiation (through electromagnetic waves, no medium needed).
- **Thermal Expansion**: Solids, liquids, and gases expand on heating. Linear expansion occurs in solids; volume expansion is significant in liquids and gases.
- **Laws of Reflection**: (i) Incident ray, reflected ray, and normal lie in the same plane. (ii) Angle of incidence equals angle of reflection (∠i = ∠r).
- **Refraction**: Bending of light when it passes from one medium to another due to change in speed. Light bends toward normal when entering a denser medium.
- **Refractive Index**: n = speed of light in vacuum / speed of light in medium. Also, n = sin i / sin r (Snell's Law).
- **Lenses**: Convex (converging) lenses bring parallel rays to a focus; concave (diverging) lenses spread rays apart. Used in spectacles, magnifying glasses, and cameras.
- **Real vs Virtual Images**: Real images are formed by actual convergence of rays (can be projected); virtual images are formed by apparent divergence (cannot be projected).
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Formulas / Key Facts
| Concept | Formula / Fact | |---------|----------------| | Heat absorbed/released | Q = mcΔT (m = mass, c = specific heat, ΔT = temperature change) | | Conversion | 1 calorie = 4.18 Joules | | Kelvin scale | K = °C + 273 | | Mirror formula | 1/f = 1/v + 1/u (f = focal length, v = image distance, u = object distance) | | Lens formula | 1/f = 1/v − 1/u | | Magnification (mirror/lens) | m = −v/u = height of image / height of object | | Power of lens | P = 1/f (f in metres), unit = Dioptre (D) | | Snell's Law | n₁ sin i = n₂ sin r | | Critical angle | sin C = 1/n (for total internal reflection) | | Convex lens image for object at 2F | Image at 2F, real, inverted, same size |
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Worked Examples
**Example 1: Heat Calculation**
*Problem*: How much heat is required to raise the temperature of 2 kg of water from 25°C to 75°C? (Specific heat of water = 4200 J/kg°C)
*Solution*:
- Given: m = 2 kg, c = 4200 J/kg°C, ΔT = 75 − 25 = 50°C
- Q = mcΔT = 2 × 4200 × 50 = 4,20,000 J = 420 kJ
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**Example 2: Mirror Formula**
*Problem*: An object is placed 30 cm in front of a concave mirror of focal length 15 cm. Find the image position and nature.
*Solution*:
- Sign convention: u = −30 cm, f = −15 cm (concave mirror)
- Using 1/f = 1/v + 1/u:
- 1/(−15) = 1/v + 1/(−30)
- 1/v = −1/15 + 1/30 = (−2 + 1)/30 = −1/30
- v = −30 cm
- Image is at 30 cm in front of the mirror, real and inverted.
- Magnification m = −v/u = −(−30)/(−30) = −1 (same size, inverted)
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**Example 3: Refraction and Snell's Law**
*Problem*: A light ray passes from air (n = 1) into glass (n = 1.5) at an angle of incidence of 30°. Find the angle of refraction.
*Solution*:
- n₁ sin i = n₂ sin r
- 1 × sin 30° = 1.5 × sin r
- 0.5 = 1.5 × sin r
- sin r = 0.5/1.5 = 1/3 ≈ 0.333
- r ≈ 19.5°
Light bends toward the normal when entering the denser medium (glass).
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Common Mistakes
| Wrong Thinking | Correct Fix | |----------------|-------------| | Confusing heat with temperature—"hot objects have more heat" | Heat depends on mass, specific heat, AND temperature. A large cold object can have more heat than a small hot object. | | Forgetting sign convention in mirror/lens problems | Always apply: distances measured against incident light are negative; real images have negative v for mirrors, positive v for lenses. | | Assuming light always bends toward normal | Light bends toward normal only when entering a denser medium; it bends away when entering a rarer medium. | | Mixing up convex mirror and convex lens properties | Convex mirrors always form virtual, erect, diminished images; convex lenses can form real or virtual images depending on object position. | | Ignoring units in power of lens | Power P = 1/f requires f in metres. If f = 20 cm = 0.2 m, then P = 5 D, not 0.05 D. |
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Quick Reference
- **Heat flows**: High temperature → Low temperature (never reverse spontaneously)
- **Best conductor**: Metals (silver > copper > aluminium); worst: air, wood, plastic
- **Radiation**: Only mode that works in vacuum (how Sun's heat reaches Earth)
- **Plane mirror**: Image is virtual, erect, laterally inverted, same size, at same distance behind mirror
- **Concave mirror uses**: Shaving mirrors, headlights, solar cookers
- **Convex lens uses**: Magnifying glass, camera, human eye correction (hypermetropia)
- **Concave lens use**: Correction of myopia (short-sightedness)
- **Total internal reflection condition**: Light must travel from denser to rarer medium; angle of incidence > critical angle