This is a great topic! I'm considering doing a light unit later this year with my kids (I'm in a small private school).
At first grade, light is primarily addressed in relationship to our sense of sight. When it's dark (we can't see), when there is light we can! Similarly, when we put things in the path of the light: mirrors, opaque objects, translucent objects light is blocked or not. What light is, how it's made, it's speed, is not addressed at this age.
My son's 2nd grade class went on a field trip to a cave. At one point the tour guide turned off the light. There was NO light anywhere. It was the blackest, darkest place I've experienced. But still our 'brains' tried to see something because we knew things were there! It was a great way to reinforce that we weren't actually 'seeing' things with our eyes (because there was no light at all), but the brain was making things up!