Professor Stephen Hawking has celebrated his 75th birthday at a specially organised series of public lectures about gravity and black holes.
The physicist and cosmologist gave a talk reflecting on his life and scientific work at Sunday's symposium at Cambridge University.
It followed an afternoon of lectures from other distinguished scientists including Professor Brian Cox.
Prof. Hawking, who turned 75 in January, joked: "Actually, for those keeping count, I'm closer to 75-and-a-half. But that shouldn't get in the way of a good celebration."
In his talk, titled My Life In Physics, he recalled his diagnosis of amyotrophic lateral sclerosis (ALS) at the age of 21.
"At first I became depressed," he said. "I seemed to be getting worse really rapidly.
"There didn't seem any point working on my PhD because I didn't know I would live long enough to finish it.
"But then the condition developed more slowly and I began to make progress in my work.
"After my expectations had been reduced to zero, every new day became a bonus and I began to appreciate everything I did have. While there's life there's hope."
The event concluded with the full auditorium singing Happy Birthday to Prof. Hawking.