INDEX of my physics notes: The visible spectrum and colour of objects

Key points for COLOUR - primary and secondary colours

Information sources for Doc Brown's key points: IGCSE-GCSE physics are based on textbooks & syllabus-specifications for students taking the UK AQA, Edexcel, OCR 21st Century Science, OCR Gateway science suite, WJEC, CCEA & CIE GCSE physics 9-1 level science examinations.

A syllabus-aligned revision guide on primary and secondary colours of light, tailored for students preparing for GCSE/IGCSE Physics across WJEC, CCEA, CIE, AQA, Edexcel, and OCR:

What Are Primary Colours of Light?

• The primary colours of light are:
  • Red
  • Green
  • Blue
• These are the basic colours that cannot be made by mixing other colours of light.
• When combined in different ways, they produce all other colours in the visible spectrum.

Mnemonic Tip: RGB - like your TV or computer screen!

What Are Secondary Colours of Light?

• Secondary colours are made by mixing two primary colours in equal amounts:

• These are the complementary colours to the primary colours.

Colour Mixing in Practice

• All three primary colours mixed together in equal amounts produce white light.
• This is called additive colour mixing - used in screens, projectors, and stage lighting.

Required Practical Applications

• Use colour filters and light boxes to:
  • Observe how different colours combine.
  • Investigate how filters absorb or transmit specific wavelengths.
• Understand how TVs and monitors use tiny red, green, and blue pixels to create full-colour images.

Exam Tips for Students

• Don’t confuse light mixing with paint mixing - they follow different rules!
• Be ready to label diagrams showing how colours combine.
• Know that filters subtract colours (e.g., a red filter only lets red light through).
• Practice past paper questions that ask about colour mixing and filter effects.
• Use precise terminology: additive mixing, primary/secondary colours, complementary colours.

EXTRA: Examples colour - addition and subtraction of colours

Colour mixing isn’t just a classroom concept - it’s everywhere in the real world, from the screens we stare at to the clothes we wear.

Here's a breakdown of real-world applications of both additive (light-based) and subtractive (pigment-based) colour mixing:

Additive Colour Mixing (Red, Green, Blue – RGB)

Used when mixing light - combining colours adds up to white.

Real-World Applications:

• Television & Computer Screens: Each pixel contains red, green, and blue sub-pixels. Varying their brightness creates millions of colours.
• Stage Lighting: Lighting designers mix coloured spotlights to create mood and atmosphere.
• Digital Projectors: Use RGB light to project full-colour images.
• Virtual Reality & Augmented Reality Displays: Rely on precise RGB mixing for immersive visuals.
• Photography & Film: Colour correction and grading use additive principles to adjust tones and hues.

Student Tip: Additive mixing is used when the source is light-emitting.

Subtractive Colour Mixing (Cyan, Magenta, Yellow – CMY)

Used when mixing pigments or dyes - combining colours subtracts light, leading to black.

Real-World Applications:

• Inkjet & Laser Printers: Use CMYK (Cyan, Magenta, Yellow, Black) to reproduce full-colour images on paper.
• Painting & Art: Artists mix paints to create custom shades and tones.
• Textile & Fashion Design: Dyes are mixed to achieve specific fabric colours.
• Packaging & Branding: Accurate colour reproduction is vital for logos and product design.
• Interior Design: Paints and finishes are mixed to match desired aesthetics.

Student Tip: Subtractive mixing is used when the source is light-reflecting (like paint or ink).

Exam-Ready Advice

• Know the difference between additive and subtractive mixing - it’s a common exam question!
• Be able to explain why RGB is used in screens and CMYK in printing.
• Use real-world examples to back up your answers in extended response questions.

Keywords, phrases and learning objectives for visible light and primary and secondary colours

Know that in physics the primary colours (colors) are red, green and blue.

Know that in physics, the secondary colours are produced by mixing two of the three primary colours.