Letter to his father, expressing his desire to study physics instead of mechanical engineering, as his father had wanted. His father agreed, on the condition that Bhabha pass the Mechanical Engineering Tripos with first class. He did that, and later passed the mathematical Tripos with First class too, as quoted in the "Homi Jehangir Bhabha" profile at the Vigyan Prasar Science Portal.
Kontext: I seriously say to you that business or job as an engineer is not the thing for me. It is totally foreign to my nature and radically opposed to my temperament and opinions. Physics is my line. I know I shall do great things here. For, each man can do best and excel in only that thing of which he is passionately fond, in which he believes, as I do, that he has the ability to do it, that he is in fact born and destined to do it... I am burning with a desire to do physics. I will and must do it sometime. It is my only ambition. I have no desire to be a "successful" man or the head of a big firm. There are intelligent people who like that and let them do it. … It is no use saying to Beethoven "You must be a scientist bybye for it is great thing" when he did not care two hoots for science; or to Socrates "Be an engineer; it is work of intelligent man." It is not in the nature of things. I therefore earnestly implore you to let me do physics.