UPSC Prelims 2007 Q118: Science & Technology – Biology
Which of the following light types of light are strongly absorbed by plants:
- (a) Violet and Orange
- (b) Blue and Red
- (c) Indigo and Yellow
- (d) Yellow and Violet
Answer: (b) Blue and Red
Explanation
Light absorbed by plants for photosynthesis
| Light | Absorption by chlorophyll |
|---|---|
| Blue (~430-450 nm) | Strongly absorbed |
| Red (~640-680 nm) | Strongly absorbed |
| Green (~500-560 nm) | Least absorbed — reflected, so leaves look green |
Chlorophyll a and chlorophyll b absorb most in the blue-violet and red parts of the spectrum.
This is shown by the absorption spectrum and the action spectrum (Engelmann's experiment).
Greenhouse and indoor-farm LED lights often use red + blue for this reason.
Answer: (b) Blue and Red.
Why each option is right or wrong
- (a) Incorrect. Orange is only partly absorbed; the peak pair is blue and red.
- (b) CORRECT. Chlorophyll absorbs blue and red light most strongly.
- (c) Incorrect. Yellow is poorly absorbed.
- (d) Incorrect. Yellow is poorly absorbed, even though violet is absorbed.
Key point
Chlorophyll absorbs blue and red most; reflects green. That is why leaves look green.
Remember
Plants absorb blue and red light most strongly.
Common mistakes
- Thinking plants absorb green light — they reflect it.
- Mixing up the absorption spectrum (pigment) with the action spectrum (photosynthesis rate) — both peak at blue and red.
- Picking violet + yellow because violet is absorbed — yellow is not.
Exam tip
Leaves look green because green is thrown away. The 'useful' colours are at the two ends: blue and red.
Sources
- NCERT Class XI — Biology, Ch 13 (Photosynthesis in Higher Plants)
- NCERT Class X — Science, Ch 6 (Life Processes: Nutrition in plants)