What does the light that came above the Spirit of Christmas Past's head represent?

The light
above the Ghost of Christmas Past's head represents revelation and self-discovery. It
illuminates Scrooge's past, reminding him of how Christmases used to be in his younger days and
how much he enjoyed celebrating them.

Scrooge's mind is clouded in darkness;
he no longer cares about Christmas and, indeed, has a profound contempt for those who do.
("Bah, humbug!") So it's particularly appropriate that the Ghost of Christmas Past is
able to light the way, giving Scrooge a glimpse of how things used to be.

This isn't a very pleasant experience for Scrooge. He'd so much prefer to remain in the
darkness as a Christmas-hating old skinflint. He doesn't wish to reminded of the past, whether
of good memoriessuch as Christmas partiesor badsuch as when his fiancee dumped him for being so
mean.

That's why Scrooge accuses the Ghost of Christmas Past of torturing him
by showing him all these old memories. Unable to take much more of his past, Scrooge tries to
cover up the light with his night-cap. He simply can't handle the truth of his
past.