What is the colour of red rose under a blue light?

Step 1: Analyze the question We need to find what colour a red rose appears when it is lit only by blue light.

Step 2: The concept An object's colour depends on which wavelengths of light it reflects:

• A red object reflects red light and absorbs other colours (including blue).
• A blue light source contains only blue light, with no red light in it.

Step 3: Apply it

• The blue light falls on the red rose.
• The rose's petals absorb blue light, because they can only reflect red.
• No red light is available to reflect, and the blue is absorbed, so almost no light reaches your eyes from the petals.
• Where no light is reflected, we see black.

(The leaves are green, so they would also look black under pure blue light.)

Step 4: Check the options

• A. Green: the rose doesn't reflect green. ✗
• B. Blue: the rose absorbs blue rather than reflecting it. ✗
• C. Yellow: yellow is not present in the light. ✗
• D. Black: all of the incident light is absorbed. ✓

Tip: An object can only reflect colours that are present in the light shining on it. If its own colour is missing from the light, it looks black. Remember: "Coloured object + wrong colour light = black."