Heat and Light

Overview

Heat and Light forms a critical portion of the Physics section in GTET Paper-2 (Mathematics and Science). This topic tests your understanding of fundamental thermal concepts and optical phenomena that students encounter in classes 6-8. Questions typically assess conceptual clarity rather than complex calculations.

For the exam, you must understand how heat transfers between objects, the relationship between temperature and heat, and how light behaves when it encounters different surfaces and media. Optical concepts like reflection, refraction, and lens properties are frequently tested through diagram-based and application questions. Mastering this topic also strengthens your ability to teach these concepts through everyday examples—a key pedagogical skill tested indirectly.

Expect 3-5 questions from this combined topic. Focus on definitions, laws, ray diagrams, and real-life applications rather than numerical derivations.

Key Concepts

• **Heat vs Temperature**: Heat is the total kinetic energy of molecules (measured in Joules or calories), while temperature measures the average kinetic energy (measured in °C, °F, or K). Heat flows from higher to lower temperature, not from "more heat" to "less heat."

• **Three modes of heat transfer**: Conduction (through direct contact in solids), convection (through fluid movement in liquids/gases), and radiation (through electromagnetic waves, no medium needed).

• **Thermal expansion**: Most substances expand when heated. Solids expand least, liquids more, gases most. This principle explains gaps in railway tracks and the working of thermometers.

• **Laws of reflection**: (1) Incident ray, reflected ray, and normal lie in the same plane. (2) 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 the normal when entering a denser medium, away when entering a rarer medium.

• **Refractive index**: n = speed of light in vacuum / speed of light in medium. Higher refractive index means denser optical medium.

• **Lenses**: Convex (converging) lenses are thicker in the middle; concave (diverging) lenses are thinner in the middle. Each forms different types of images depending on object position.

• **Total internal reflection**: Occurs when light travels from denser to rarer medium at an angle greater than the critical angle. Basis of optical fibers and diamond sparkle.

Formulas / Key Facts

| Concept | Formula/Fact | |---------|--------------| | Heat absorbed/released | Q = m × c × ΔT (mass × specific heat × temperature change) | | Specific heat of water | 4.18 J/g°C or 1 cal/g°C (highest among common substances) | | Celsius to Kelvin | K = °C + 273 | | Celsius to Fahrenheit | °F = (9/5)°C + 32 | | Law of reflection | ∠i = ∠r | | Snell's Law (refraction) | n₁ sin θ₁ = n₂ sin θ₂ | | Refractive index | n = c/v = real depth / apparent depth | | Lens formula | 1/f = 1/v - 1/u | | Magnification | m = v/u = height of image / height of object | | Power of lens | P = 1/f (in dioptres when f is in metres) | | Sign convention | Distances measured from optical centre; same direction as incident light is positive |