Radiation

Radiant heat energy can be detected using a device called a thermopile. A thermopile is an instrument that converts radiant heat energy into electrical energy.

When the terminals of the thermopile are connected to a galvanometer using wires, and the thermopile is pointed at a hot object like an electric lamp, a current flows and a deflection is seen on the galvanometer. When the electric lamp becomes hotter (by increasing its current), the galvanometer shows a larger deflection. This shows that more radiant heat energy is being received.

The amount of heat energy radiated by a body depends on:

• The temperature of the body
• The nature of the surface
• The surface area of the body

To demonstrate how the surface type and area affect radiation, an instrument called Leslie's Cube is used.

Leslie's Cube is a metal box with four different surfaces:

• One side is painted black
• Another is painted grey
• One is painted green
• One side is highly polished (shiny)

The cube is placed on a turntable and filled with steam to keep it hot. A thermopile connected to a galvanometer is placed a short distance away.

As the cube is turned, the thermopile faces each surface one by one. The black surface causes the highest deflection on the galvanometer, meaning it radiates the most heat. The polished surface causes the lowest deflection, meaning it radiates the least heat.

This experiment shows that dark, dull surfaces are good emitters of radiant heat, while shiny, polished surfaces are poor emitters (but good reflectors).

To show how radiant heat is absorbed, two small metal plates (one blackened and one shiny) can be placed near a Bunsen burner. The black plate will get hotter faster, showing that black surfaces absorb more radiant heat than shiny ones.

A vacuum flask is a good example of how we reduce heat loss from radiation in daily life. It was invented by Sir James Dewar to store liquefied gases but is now commonly used to keep drinks hot or cold.

A vacuum flask works in several ways:

1. It has double glass walls with a vacuum between them to prevent heat loss by convection and conduction.
2. The glass itself is a poor conductor of heat, and the stopper (usually made of cork or rubber) also reduces heat loss by conduction.
3. The vacuum between the walls prevents air movement, stopping convection.
4. The silvered surfaces reflect radiant heat, reducing heat loss by radiation. Any heat coming from outside or inside the flask is reflected back.

This way, the vacuum flask keeps hot liquids hot and cold liquids cold for a long time.