What is the correct chemical formula for $\beta$-carotene?

Chlorophyll $a$ is the primary pigment for the light reaction. What are accessory pigments? What is their role in photosynthesis?

Photosynthetically least efficient radiation is

Choose the correct statement.

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?

Carotenoids are made up of units of ......