Short answers, pulled from the story.
The ASV Mk. II radar set had a minimum detection range of about 1,500 yards. This distance proved fatal for night operations over the Atlantic Ocean because the target vanished from the radar screen before the pilot could see it with naked eyes.
Wing Commander Humphrey de Verd Leigh devised the solution after chatting with returning aircrew members. He developed the system on his own without official sanction and presented a modified Vickers Wellington DWI in March 1941 that proved the concept sound.
Two distinct types of Leigh Light entered operational use during the conflict. The Turret type fitted on Wellington aircraft featured a 30-inch searchlight mounted in a retractable under-turret while the Nacelle type fitted on Catalinas and Liberators used a 40-inch searchlight slung from bomb lugs on the wing.
The first submarine successfully sighted was the Italian submarine Torelli on the night of the 3rd of June 1942. The first confirmed kill occurred on the 5th of July 1942 when a Vickers Wellington of 172 Squadron sank an enemy vessel.
Germany introduced the Metox radar warning receiver to counter the combination of ASV and Leigh Light systems. This device provided submarine crews early warning that an aircraft using radar was approaching because it detected emissions at greater ranges than the radar detected vessels.