Inverse Square Law with Light

A lamp and light meter are used to show that illumination decreases with the square of the distance from the source. Students observe that doubling the distance reduces the light intensity to one quarter, demonstrating the inverse-square law.

1. Set up a lamp on a stand in a darkened room.
2. Place a light meter at a distance of 30 cm from the lamp and record the reading.
3. Move the meter to 60 cm and then to 90 cm, recording readings at each distance.
4. Compare the values: intensity should fall in the ratio 1 : 1/4 : 1/9.

The Inverse Square Law - Saturday Morning Astrophysics at Purdue:

📄 The inverse square law with light - Institute of Physics: https://spark.iop.org/inverse-square-law-light

• Use different types of light sources (e.g., LED, incandescent) and compare results.
• Measure at additional distances to refine the curve of 1/r².
• Repeat the experiment with a sound source and microphone to show the same principle with sound waves.
• Extend with a classroom analogy like the “butter gun” example, where spray thins out over greater distances.

• Use only standard tungsten lamps, not unfiltered halogen lamps (to avoid UV exposure).
• Ensure power supplies and lamp holders are handled safely.
• Work in a controlled, darkened environment to avoid tripping hazards.

• What happens to light intensity when the distance is doubled? (It decreases to one quarter.)
• Why does light spread out this way? (It radiates in straight lines and spreads over an area that grows with the square of the distance.)
• How does this principle connect to Newton’s idea of gravity? (Gravity also follows an inverse-square law with distance.)
• Can you think of other physical quantities that follow the inverse-square law? (Yes: sound intensity, radiation, and gravitational/electric field strength.)