Infra-red thermometers work by detecting the

1. Analyze the question

The question asks for the physical principle behind the operation of an infra-red thermometer. We need to identify how these devices measure temperature without making physical contact with an object.

2. Explain the concept

All objects with a temperature above absolute zero emit electromagnetic radiation. This is known as thermal radiation or black-body radiation.

• Infra-red (IR) thermometers are designed to detect the intensity of the infra-red radiation emitted by an object.
• The device uses a lens to focus the infra-red energy onto a detector called a thermopile.
• The thermopile absorbs the IR radiation and converts it into an electrical signal.
• The device then uses an internal algorithm to convert that electrical signal into a temperature reading.

Because this process relies on radiation, it does not require physical contact (unlike conduction) or the movement of air (unlike convection).

3. Show the solution

• Option A: Infra-red thermometers measure the thermal radiation emitted by an object. This is the correct physical principle.
• Option B: Conduction requires physical contact between the thermometer and the object. IR thermometers are "non-contact" devices.
• Option C: Convection involves the transfer of heat through the movement of fluids (like air). IR thermometers do not measure air pulses.
• Option D: Absorption of heat by the body describes how an object gains heat, not how a thermometer measures the temperature of an object.

4. Identify the answer

The correct option is A.

5. Exam Tip

When you see "Infra-red" in a Physics question, immediately associate it with radiation. Remember that infra-red radiation is a form of electromagnetic wave that travels through a vacuum and does not require a medium to transfer heat. This is why you can measure the temperature of a hot object from a distance!