MCQ Detailed View

The correct answer is Blue light. Photosynthesis, the process by which green plants, algae, and some bacteria produce food in the form of glucose using sunlight, requires light energy absorbed by pigments, primarily chlorophyll a and chlorophyll b.

The effectiveness of different wavelengths of light depends on how well they are absorbed by photosynthetic pigments. Blue light (around 450–495 nm) and red light (around 620–700 nm) are the most effective in driving photosynthesis.

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  Blue light (short wavelength, high energy): Strongly absorbed by chlorophyll a, chlorophyll b, and carotenoids. It plays a vital role in vegetative growth, stomatal opening, and overall photosynthetic activity.

  
  


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  Red light (longer wavelength): Also highly effective and is strongly absorbed by chlorophyll a. It contributes greatly to photosynthesis, particularly in flowering and fruiting phases.

  
  


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  Green light: Least effective because chlorophyll reflects green wavelengths instead of absorbing them, which is why leaves appear green.

  
  

The classic Engelmann’s experiment (1882) demonstrated this concept. Using a prism to split light into different wavelengths, Engelmann showed that oxygen production (indicating photosynthesis) was highest under blue and red light.

Thus, while both blue and red light are crucial, blue light is considered the most effective wavelength for photosynthesis because it is absorbed more efficiently and provides higher energy photons.

🔑 Key Facts:

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  Blue light (450–495 nm) → most effective.

  
  


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  Red light (620–700 nm) → also very effective.

  
  


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  Green light → least effective (reflected, not absorbed).

  
  


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  Chlorophyll a & b are the main pigments absorbing these wavelengths.

  
  


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  Engelmann’s experiment proved photosynthesis is strongest under blue/red regions.

  
  

👉 Final Answer: The most effective wavelength of light in photosynthesis is Blue light.