Final answer:
The flash on Casey's camera needs to be set stronger when it is darker outside.
Step-by-step explanation:
The subject of this question is Physics. In physics, the behavior of light and the properties of a camera flash can be studied. When it is darker outside, Casey needs to set her flash stronger to compensate for the lack of natural light. This is because the flash provides an additional burst of light that illuminates the subject, making it clearer in the photo.