Mrs. Ramsay continues to think about how children "never forget" and it’s therefore important to watch what you say.
Despite all of Mrs. Ramsay’s thinking, if we were impartial observers, this is what we would have seen:
Mrs. Ramsay knits, watching the Lighthouse.
Mrs. Ramsay stops knitting.
Mr. Ramsay looks at Mrs. Ramsay.
Mrs. Ramsay takes a green shawl and goes to her husband.
We know the whole sequence misses some explosions and bikini-clad babes, but this is "action" in a Woolf novel. Seriously.
What happens beneath the surface:
Mrs. Ramsay finally has a breather, i.e., a moment to herself. She has no one to check up on, take care of, attend to, etc., so her real self is let loose. And just what does that real self do? Well, it becomes hypnotized by the strokes of light coming from the Lighthouse. Imagining that the beam of light is stroking her brain, she feels momentarily ecstatic.