In Chapter 10 of The Kite Runner, what first gives light in the tanker and why does it matter?

Baba and Amir made their escape from Kabul in an old,
tarpaulin-covered Russian truck. They survived an altercation with a Russian guard who
threatened to rape one woman and, later, to kill Baba. After spending the night in a
dark, dank basement in Jalalabad, they discovered that the second truck in which they
hoped to escape had broken down. The chose another option--riding in the empty tank of a
fuel truck. It proved to be a terrible ride to Peshawar, travelling in total darkness
and breathing the fumes of gasoline. Amir was happy to at last see a small sliver of
light: It came from the fluorescent green hands of Baba's wristwatch. Amir kept his eyes
on the watch, which helped him to take his mind off their awful
journey.