A sheet of white paper viewed through blue glass appears blue because

Analyze the question: White paper is viewed through blue glass and looks blue. We need the reason, which comes from how coloured filters work.

Concept: A coloured transparent material (a filter) works by selective absorption and transmission.

• White paper reflects all the colours of white light (red, orange, yellow, green, blue, indigo, violet).
• Blue glass transmits blue light and absorbs the other colours.
• Only blue light passes through to your eye, so the paper appears blue.

Step-by-step solution:

1. White light falls on the paper, and the paper reflects all colours towards you.
2. This reflected light must pass through the blue glass to reach your eye.
3. The glass absorbs red, green, yellow and the other colours.
4. Only blue is transmitted, so you see the paper as blue.

Checking the options:

• A. Wrong. Being a primary colour has nothing to do with the effect.
• B. Correct. The glass absorbs all colours except blue, and blue is transmitted.
• C. Wrong. The glass does not appear blue because of reflection. It transmits blue light.
• D. Wrong. White paper reflects all colours, not only blue.
• E. Wrong. The glass absorbs the other colours, not white light as a whole, and this doesn't explain why blue appears.

**Answer: B.

Exam tip: Remember "a filter subtracts": a coloured filter lets through its own colour and absorbs the rest. A red filter passes only red, and a blue filter passes only blue. If you look at a white object through a filter, you see the filter's colour. If the object is a different colour, for example red paper through a blue filter, it appears black, because no blue light is reflected for the glass to transmit.