The invisible part of the spectrum of white light consists of

Analyze the question: The question asks which parts of the spectrum of white light are invisible to the human eye.

Explain the concept: When white light passes through a prism, it disperses into a spectrum. The visible part is the seven colours, remembered as ROYGBIV: Red, Orange, Yellow, Green, Blue, Indigo, Violet.

Beyond the ends of this visible band are regions the eye cannot detect:

• Just beyond red is infra-red, which has longer wavelengths and is detected as heat.
• Just beyond violet is ultra-violet, which has shorter wavelengths.

Solution, checking each option:

• A. Infra-red and ultra-violet only. Both lie outside the visible range, so both are invisible. ✔
• B. Blue is visible, so this is wrong.
• C. Red, orange and green are all visible, so this is wrong.
• D. Red and violet are visible (they are the edges of the visible band), so this is wrong.
• E. Yellow and indigo are visible, so this is wrong.

Answer: The invisible parts of the spectrum of white light are infra-red and ultra-violet only.

Tip: Remember that the visible range is about 400 nm (violet) to 700 nm (red). Anything below 400 nm is ultra-violet and anything above 700 nm is infra-red. If an option lists a colour name from ROYGBIV, it is visible, so you can eliminate it right away.