Ch 8 · 2.3.3 Radiation · O Level Physics 5054
Topic 8 of 9 in Thermal Physics — O Level Physics 5054 — 40 lessons.
Introduction
The Sun is 150 million kilometres away across empty space, yet you feel its warmth on your face. There are no particles in space to conduct or convect the energy. It reaches us by infrared radiation. (Syllabus section 2.3.3.)
Key ideas — Thermal radiation is infrared
Every object gives out (emits) infrared radiation, a type of electromagnetic wave. The hotter the object, the more it emits.
🧑🏫 Teacher's explanation
Imagine you are standing near a campfire on a cold night. Even if the air is still (no convection) and you aren't touching the logs (no conduction), you feel a sudden wave of warmth on your face. That warmth is infrared radiation. It is a form of energy traveling as electromagnetic waves, meaning it doesn't need atoms or molecules to 'carry' it; it can zip through the empty vacuum of space.
📖 Definitions
🔍 Worked example — Easy
An object is placed in a vacuum. It emits infrared radiation. Does the energy transfer require a medium? Explain why.
🔍 Worked example — Standard
A black, dull surface and a white, shiny surface are both at 100^° C. Which surface will cool down faster? Justify your answer.
🔍 Worked example — Exam-style
An object with surface area A emits radiation at a rate P. If the surface area is doubled to 2A while keeping the temperature constant, what happens to the rate of emission P_new?
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
(a) Which container will lose heat to the surroundings more quickly? [1] (b) Explain your answer to part (a) in terms of infrared radiation. [2] [3 marks]
Key ideas — Surface colour and texture
A good absorber is also a good emitter. A good reflector is a poor absorber and a poor emitter.
🧑🏫 Teacher's explanation
Imagine you are standing near a campfire on a cold night. Even if you aren't touching the flames or sitting in the smoke, you feel a sudden warmth on your face. This is because the fire is sending out electromagnetic waves called infrared radiation. Unlike conduction (which needs contact) or convection (which needs moving air), radiation travels through the vacuum of space at the speed of light.
📖 Definitions
🔍 Worked example — Easy
A student uses a thermometer to compare two metal cans, one painted dull black and one shiny silver. If both are at the same temperature, which one will cause the thermometer to drop in temperature faster when placed near it?
🔍 Worked example — Standard
A solar water heater uses a dull black surface to maximize heat gain. If the surface area of the collector is A = 2.5 m^2 and the rate of energy absorption per unit area is P/A = 400 W/m^2, calculate the total power absorbed by the panel.
🔍 Worked example — Exam-style
An experiment is set up to test if a surface is a good absorber. A black metal cylinder and a silver metal cylinder are both heated to 100^circC. They are placed in a vacuum chamber at 20^circC. After 5 minutes, the black cylinder's temperature is 85^circC and the silver cylinder's temperature is 95^circC. Explain why the black cylinder cooled more.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
Key ideas — What affects the rate of emission
An object emits infrared radiation faster when:
🧑🏫 Teacher's explanation
Imagine you are standing near a campfire. You feel the warmth on your face even if there is no air moving toward you. This is because the fire is sending out waves of energy called infrared radiation. Unlike conduction (which needs contact) or convection (which needs a fluid like air or water), radiation travels through the vacuum of space. It is the only way the Sun's heat reaches Earth.
📖 Definitions
🔍 Worked example — Easy
An object has a surface area of 0.5 m^2 and emits radiation at a rate of 200 W/m^2. Calculate the total power radiated by the object.
🔍 Worked example — Standard
A metal block is heated to 80circC. If its surface area is doubled while keeping the temperature constant, what happens to the rate of infrared emission?
🔍 Worked example — Exam-style
An object with surface area A emits radiation at rate R. If the temperature is increased such that the emission rate per unit area increases by 50\%, find the new total power in terms of R and A.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
(a) Which sphere emits more infrared radiation? (b) Explain your answer in terms of surface properties. (c) If the black sphere's surface area is doubled, what happens to its rate of emission? [3] [3 marks]
Practical notes
(Another version: two identical cans of cold water, one dull black and one shiny, at the same distance from a heater; the water in the black can warms faster.)
Worked example
The silver surface is shiny and light, so it reflects most of the Sun's infrared radiation and absorbs little. The black surface is a good absorber, so it absorbs more infrared and its temperature rises more.
🔍 Worked example — Easy
A shiny silver spoon and a dull black spoon are placed in a room with a constant temperature. Which spoon will reach a higher temperature if exposed to infrared radiation, and why?
🔍 Worked example — Standard
A black metal plate has a surface area of 0.05 m^2 and a temperature of 350 K. A white metal plate has a surface area of 0.10 m^2 and a temperature of 350 K. Which plate emits more infrared radiation per second?
🔍 Worked example — Exam-style
In an experiment to test absorption, two identical glass tubes are used. Tube A is painted matte black and Tube B is painted shiny silver. Both are placed at the same distance from an infrared lamp. If the initial temperature of the liquid in both tubes is 20 ^°C, which tube will show the greatest temperature increase after 5 minutes? Explain.
⚠️ Common errors
Common mistakes
Practice
Answers: (1) Space is a vacuum; conduction and convection need particles, but infrared radiation does not need a medium. (2) Dull black is the best emitter; shiny white or silver is the best reflector. (3) A higher surface temperature and a larger surface area. (4) Place a dull black plate and a shiny silver plate, each with a pin fixed by wax on the back, at equal distances either side of a radiant heater; the pin on the better absorber (dull black) falls first.