IIT JEE Physics (1978-2018: 41 Years) Topic-wise Complete Solutions

# Is glass opaque or transparent for heat radiation?

## Introduction

Glass is transparent for visible light. Heat radiation is also an electromagnetic wave with frequency lower than that of visible light. So the question is relevant. This simple demo probes this question and it can also be used to talk about other modes of heat transfer.

## Apparatus

A glass plate, candle, plate to put the candle, water mug to put burnt stick and matchbox.

## Procedure

1. Keep the glass plate vertical. Lit a candle and put it close to the glass plate.
2. Just after bringing the candle close to the plate, put your palm on the other side of the plate for 1 or 2 seconds (careful, it may be hot). Do you feel the heat? Is heat radiation coming out of the plate? In other words: Is glass transparent or opaque for heat radiation?
3. Let the candle be close to the plate for about 30 seconds or so. Now put your palm on the other side of the plate (careful, it may be hot). Do you feel the heat? Is heat radiation coming out of the plate? In other words: Is glass transparent or opaque for heat radiation?
4. What is the temperature of the candle flame? The temperature of the sunâ€™s surface is 6000K and the mean wavelength of the radiation, from the sun is around 600 nm. What would be the mean wavelength of the radiation from the flame?

## Discussion

Heat radiation is essentially electromagnetic wave. The mean wavelength of the radiation emitted by a hot object depends on the temperature of the object.

When radiation falls on a material, it can travel through the medium or it can get absorbed, it will increase the temperature locally. This will cause conduction in solids and connection in liquids. Generally, lower the frequency, more is the absorption.