6. a) Some science fiction writers use a large sail to enable a space vehicle to move through space. They argue that sunlight will exert pressure on the sail causing

it to move away from the sun.Explain how the the quantum theory supports this statement. b) A lithium surface in a photo electric cell will emit electrons when the incident light is blue. A platinum surface will only emit electrons when the incident light is ultraviolet. i) which of the two metals has a higher work function? Why? ii) Which of the two metals has a higher threshold frequency? Why?