Question

The wavelength of light least effective in photosynthesis is
A. Violet
B. Blue
C. Green
D. Red

Answer 1

Hint: Light is an essential component for photosynthesis. A linear relationship exists between the incident light and carbon dioxide​ fixation at low light intensities.

Complete answer:
Plants use light energy to convert carbon dioxide and water into glucose. Oxygen is released as a by-product during this process. The chlorophyll present in the leaves traps light energy from the sun.
The overall reaction of photosynthesis as:
$6C{O_2} + 12{H_2}O\xrightarrow[{Chlorophyll}]{{Sunlight}}{C_6}{H_{12}}{O_6} + 6{H_2}O + 6{O_2}$
The rate of photosynthesis is directly proportional to the intensity of light. Both the photosystems PS I and PS II absorb light of wavelengths in the red region 680nm and 700nm , respectively, the red light is most effective in photosynthesis. Carotenoids absorb the blue region's light, later it is passed on to the chlorophyll, whereas the chlorophyll absorbs the light in the red region. Since plants are green, that means it reflects green light. Chlorophyll is the light-harvesting molecule that’s why it is essential for photosynthesis. Chlorophyll is a poor absorber of the green region of the spectrum as they are reflected by the plant and imparting green colour to the plant.
Types of chlorophyll in the photosystems: chlorophyll a and b. Absorption frequencies for photosynthesis:

Chlorophyll a = 400nm to 500nm
Chlorophyll b = 600nm to 700nm
So, the least effective frequencies in photosynthesis are all of them except the ones that are absorbed by chlorophyll and carotenoids.

So, the correct answer is (C) Green.

Note: Photosynthesis is the process in which light energy is converted into chemical energy in the form of carbohydrates. The term photosynthesis is derived from the Greek words, photo for ‘light’ and synthesis meaning ‘putting together’. Oxygen is released, as a waste product. The Process of Photosynthesis takes place when a green plant fixes carbon dioxide and water into carbohydrates with the help of photosynthesis. Chlorophyll absorbs the light energy while stomata help in exchange of carbon dioxide and oxygen. Glucose is synthesised during photosynthesis, is used by plants as a source of energy for numerous works like the formation of tissue, respiration, etc. Later on, glucose combines to form the starch and cellulose. The cellulose forms the structural material in plant cell walls while starch is used as a food reservoir for metabolic activity. Light colour plays a major role during photosynthesis as photosynthesis occurs more in blue and red-light rays and least in green light. Blue is best absorbed so the highest rate of photosynthesis takes place, followed by the red light. Greenlight is reflected so cannot be used in photosynthesis.
Factors affecting Photosynthesis:
> Light Intensity
> Concentration of carbon dioxide
> Temperature
> Water