Complementary colours are those which

Step 1: Analyze the question The question asks what "complementary colours" means in physics (light and colour).

Step 2: The concept Complementary colours are two colours of light that, when mixed (added) together in the right proportions, produce white light. They come from the idea of colour addition:

• Red + Cyan = White
• Green + Magenta = White
• Blue + Yellow = White

This works because the primary colours of light are red, green and blue. A secondary colour (such as yellow, made from red + green) combines with the remaining primary (blue) to give all three primaries, which gives white.

Step 3: Check each option

• A. Same refractive index: Incorrect. Different colours have different refractive indices in a medium, which is why a prism disperses light.
• B. Same wavelength: Incorrect. Different colours have different wavelengths. Two colours with the same wavelength would be the same colour.
• C. Add up to produce black light: Incorrect. "Black" is the absence of light, not something produced by adding lights.
• D. Add up to produce white light: Correct. This is the definition of complementary colours.

Step 4: Answer The correct option is D.

Exam tip: Remember "complementary = complete the set to white." Yellow + Blue, Cyan + Red, and Magenta + Green are the three pairs to remember. Don't confuse this with mixing paints (pigments), which is colour subtraction and tends toward dark or black.