demonstrations:colour_subtraction_with_filters
Colour Subtraction with Filters
Materials: ★★☆ Available in most school laboratories or specialist stores
Difficulty: ★☆☆ Can be easily done by most teenagers
Safety: ★☆☆ Minimal safety procedures required
Categories: Light
Alternative titles: Light Absorption and Transmission
Summary
Light filters and surfaces selectively transmit, reflect, or absorb different colors of light. This subtraction of colors explains why objects and filtered images appear in certain colors or may even appear black if no light is reflected or transmitted.
Procedure
- Shine white light through a colored filter (such as red, green, or blue cellophane). Observe which colors pass through and which are blocked.
- Try secondary filters (cyan, magenta, yellow) and notice that they transmit two primary colors while absorbing the third.
- Place objects of different colors behind the filters to see how their appearance changes.
- Direct white light onto colored surfaces and observe which colors are reflected and which are absorbed.
- Compare results with white and black surfaces: white reflects all colors, while black absorbs all colors.
Links
Colour Filter Demonstration - Physics with Simon Poliakoff:
📄 Colour subtraction, absorption and reflection - BBC Bitesize: https://www.bbc.co.uk/bitesize/articles/z6mpywx#zwwmh4jv
Variations
- Use 3D glasses or sweet wrappers as inexpensive filters.
- Shine different colored lights (red, green, blue, cyan, magenta, yellow) onto objects and compare their appearance.
- Mix filters together (for example, red + blue cellophane) to explore subtractive color mixing.
Safety Precautions
- Do not look directly into strong light sources.
- Use LED lamps or stage lights rather than lasers to avoid eye damage.
- Supervise children when working with electrical light sources.
Questions to Consider
- Why does a red filter only transmit red light? (Because it absorbs green and blue light, subtracting them from the white light.)
- Why does a cyan filter appear cyan under white light? (Because it transmits green and blue light, absorbing red.)
- Why does a surface sometimes look black under colored light? (Because the surface absorbs the available wavelengths and reflects none back to the eye.)
- How is color subtraction different from color addition? (Addition mixes light sources to make new colors, while subtraction removes wavelengths through filtering or absorption.)