Crookes Radiometer – Thermal Radiation Demonstration [0461]

The Crookes Radiometer is a classic and visually engaging physics demonstration device designed to illustrate the effects of thermal radiation and gas molecule motion. Suitable for KS3 and above, this apparatus provides a clear and memorable way for students to explore several important principles of energy transfer and particle behaviour.

The instrument consists of a partly evacuated glass bulb, approximately 70mm in diameter, mounted securely on a round plastic base. Inside the bulb, a fine central pivot supports four lightweight metal vanes. Each vane has one side blackened and the reverse side white, allowing clear comparison of heat absorption.

When exposed to light or infrared radiation, the blackened surfaces absorb more energy than the white surfaces. This causes a temperature difference that produces a pressure imbalance in the surrounding low‑pressure gas, resulting in rotation of the vanes. The smooth, continuous motion provides a striking visual demonstration that naturally leads to discussion and enquiry.

While often associated with radiation pressure, the behaviour of the Crookes Radiometer opens up discussion across several areas of physics, making it a versatile teaching tool rather than a single‑concept demonstration.

Key Features

• Classic Crookes radiometer for physics education
• Partly evacuated glass bulb (~70mm diameter)
• Four lightweight metal vanes on a fine pivot
• Vanes with blackened and white faces
• Mounted on a stable round plastic base
• No electricity required
• Robust and suitable for repeated classroom use
• Designed for KS3, GCSE, and A‑Level physics

Physics Concepts Demonstrated

• Thermal radiation and absorption
• Heat transfer mechanisms
• Gas pressure in low‑density environments
• Kinetic theory and momentum of gas molecules
• Energy interaction with matter

Ideal for School Science Lessons

The Crookes Radiometer is ideal for KS3, GCSE, and A‑Level physics lessons, offering a highly visual starting point for discussing abstract concepts related to heat and particle motion. Its simplicity encourages curiosity and questioning, making it especially effective for demonstrations and enrichment activities.

• Demonstrating effects of thermal radiation
• Linking theory to observable physical motion
• Supporting lessons on energy transfer
• Introducing aspects of the kinetic theory of gases
• Stimulating discussion through hands‑on observation

Simple, elegant, and scientifically rich, the Crookes Radiometer is an excellent addition to any school physics department, providing a timeless demonstration that continues to fascinate students while supporting deep conceptual understanding.