Question 10.9:

Why are potassium and cesium, rather than lithium used in photoelectric cells?

All the three, lithium, potassium, and cesium, are alkali metals. Still, K and Cs are used in the photoelectric cell and not Li.
This is because as compared to Cs and K, Li is smaller in size and therefore, requires high energy to lose an electron. While on the other hand, K and Cs have low ionization energy.
Hence, they can easily lose electrons. This property of K and Cs is utilized in photoelectric cells.