Spectrum — Part 2: Infrared, ultraviolet and scattering: visual lesson, ICSE Class 10 Physics (Science Paper 1)

Lesson

Light beyond red and violet

Part 2 — just beyond the red end of the spectrum lies infrared, which we feel as heat; just beyond the violet end lies ultraviolet, which tans skin. And the way air scatters sunlight paints the sky blue.

In this lesson you will

Give two properties and two uses each of infrared and ultraviolet radiation
Explain why the sky is blue using wavelength-dependent scattering
Explain why the Sun looks red at sunrise and sunset
Explain other effects of scattering such as red danger signals and white clouds

Table of radiations and effects with everyday examples
Radiation or effect	Everyday example
Infrared	Warmth felt near a heater or a hot tawa
Ultraviolet	Sun tan after a day at the beach
Scattering	Blue sky, red sunrise
Visible light	Everything we see

Concept

Infrared radiation: production, detection and properties

Infrared (IR) lies beyond the red end of the visible spectrum ( $8 \times 10^{-7}$ m to $10^{-3}$ m). It was found by placing a thermometer beyond the red end of a solar spectrum: the temperature rose.

Produced by all hot bodies: the Sun, a heater coil, a hot iron, our bodies.
Detected by its heating effect: a thermometer with a blackened bulb, or a thermopile; also by special photographic films and sensors.
Properties: strong heating effect; less scattered by haze and fog than visible light (longer wavelength); absorbed by water vapour and carbon dioxide; mostly reflected by shiny surfaces; obeys reflection and refraction.

Table of the position, source, detector and key property of infrared radiation
Feature	Infrared
Position	Beyond red
Source	Hot bodies
Detector	Thermopile, blackened-bulb thermometer
Key property	Heating effect; little scattering by haze

Concept

Uses of infrared

Remote controls of TVs and air conditioners.
Night vision and thermal cameras, and photography through haze or fog (little scattering).
Physiotherapy: IR lamps warm muscles and relieve pain.
Drying and heating: drying paint on cars, cooking, warming rooms; greenhouses trap infrared.
Weather forecasting with infrared photographs from satellites, and intruder alarms.

Table of four uses of infrared radiation and the property each uses
Use	Property used
TV remote control	Invisible signal, easily detected by a sensor
Thermal camera	Warm bodies give out more infrared
Photography in haze	Little scattering
Physiotherapy lamp	Heating effect

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