Question

Maximum photosynthesis occurs in
A. Red light
B. Green light
C. Blue light
D. None of the above

Answer 1

Photosynthesis is the process by which green plants make their own food using sunlight, carbon dioxide from the atmosphere, water from the soil in presence of chlorophyll in the leaves. The food formed is a carbohydrate sugar called glucose. However, photosynthesis cannot take place at all wavelengths of light. The chlorophyll absorbs light of certain wavelengths and reflects light of other wavelengths.

Complete answer: Photosynthesis is the process in which plants manufacture their food in the form of carbohydrates using carbon dioxide and water, in presence of sunlight and chlorophyll.
The chlorophyll molecules absorb light. These pigments absorb light of certain wavelengths and reflect light of other wavelengths. The action spectrum of the process also depends on the presence of accessory pigments like carotenoids. This action spectrum is responsible for the colour of the plants.
Red light has long waves of around 700 nm. Red light exists at its maximum during morning and evening, that is when the sun is low in the sky. The Red light contains a lesser amount of energy being a longer spectrum of light. It is absorbed by the chlorophyll and reflected by carotenoids. Therefore, the use of red light is maximum and efficiently done by the plants. Hence photosynthesis is maximum in Red light.
Green light has shorter wavelengths of about 550nm. This light is reflected by the plants. Reflection of green light by the chlorophyll molecules is the reason why plants appear green to our eyes. Therefore, photosynthesis does not take place in green light.
Blue light has a short wavelength of about 500 nm. Blue light is high at midday when the intensity of heat is more. Blue light contains a very high amount of energy. This high heat intensity causes the chlorophyll molecules to move to the bottom in order to protect themselves. Moreover, blue light is also absorbed by the carotenoids that cannot transfer the energy to the chlorophylls efficiently.
Therefore, photosynthesis occurs in both Red light and in Blue light, but is maximum in Red light.

Hence, option (A) is correct.

Note: Photosynthetic pigment is chlorophyll. It is activated by absorption of light energy. However, chlorophylls cannot absorb light of all wavelengths – it reflects light of certain wavelengths. Since, the green light is reflected by chlorophyll, plants appear green to us. Photosynthesis can take place in both Red and Blue light, but its rate is maximum in Red light.