Visible light
Visible light is the narrow band of EM waves our eyes can detect: ~400 nm (violet) to 700 nm (red). The colour we see depends on the wavelength.
Spectrum colours
In rainbow order (longest to shortest λ): red, orange, yellow, green, blue, indigo, violet (ROYGBIV).
White light is a mixture of all visible wavelengths. A prism separates them by refraction (different λ refracts by different amounts) — that's how rainbows form.
Why objects have colour
Materials absorb some wavelengths and reflect others. The reflected wavelengths are what we see.
- A red apple: absorbs all visible wavelengths except red, which it reflects.
- A blue shirt: reflects blue, absorbs others.
- White paper: reflects all wavelengths approximately equally.
- Black ink: absorbs all wavelengths.
Transparent / translucent / opaque
- Transparent — light passes through without much scattering (clean glass, water).
- Translucent — light passes but is scattered (frosted glass, tracing paper). You can see light but not detail.
- Opaque — no light passes; absorbed or reflected (wood, metal).
Colour filters
A filter transmits some wavelengths and absorbs others.
- Red filter: lets red through; absorbs other colours.
- Green filter on a red object: object appears black (no green to reflect; red absorbed by filter).
✦Worked example— Worked example — what colour appears?
A blue object viewed through a red filter:
- Blue object reflects only blue.
- Red filter blocks blue (only transmits red).
- Result: black (no light reaches the eye).
Adding light vs mixing pigment
- Adding light: red + green + blue light = white (additive colour, used in screens).
- Mixing pigment: cyan + magenta + yellow = black (subtractive, used in print).
⚠Common mistakes
- Saying objects have intrinsic colour — colour depends on which wavelengths the object reflects.
- Mixing additive and subtractive colour theories (different rules).
- Forgetting that a filter does not "add" colour — it removes other colours.
- Saying black objects "have no colour" — they absorb all visible.
AI-generated · claude-opus-4-7 · v3-deep-physics