Her magic condenses all the light in the area into rays that goes from point A to B. The darkness is a helpful side effect of concentrating the light. Outside the rays would be visible unless she added other magic to darken the area. Something that should be well within the realm of her capability as a magician that excels in light-based magic.
At least im pretty sure that's how it works.
But it was said that:
The attack appears like a shower of lasers, but the power of Meteor Line has no connection with the energy of the light. Even the amount of light is irrelevant.