In the figure given below,the black line (upper) indicates the action spectrum for photosynthesis and the lighter line (lower) indicates the absorption spectrum of chlorophyll-$a$. Answer the following:
$(a)$ What does the action spectrum indicate? How can we plot an action spectrum? Explain with an example.
$(b)$ How can we derive an absorption spectrum for any substance?
$(c)$ If chlorophyll-$a$ is responsible for the light reaction of photosynthesis,why do the action spectrum and absorption spectrum not overlap?

(N/A) An action spectrum indicates the rate of photosynthesis at different wavelengths of light. It can be plotted by measuring the rate of photosynthesis (e.g.,oxygen evolution or $CO_2$ uptake) at various specific wavelengths of light and plotting these rates against the corresponding wavelengths.
$(b)$ An absorption spectrum for any substance is derived by measuring the amount of light of different wavelengths absorbed by that substance using a spectrophotometer and plotting the absorbance values against the wavelengths.
$(c)$ The action spectrum and absorption spectrum do not overlap perfectly because chlorophyll-$a$ is the primary pigment,but other accessory pigments like chlorophyll-$b$,xanthophylls,and carotenoids also absorb light energy at different wavelengths and transfer this energy to chlorophyll-$a$. These accessory pigments broaden the range of light absorption and also protect chlorophyll-$a$ from photo-oxidation.