Emission and reflection nebulae differ primarily in their interactions with light. Emission nebulae emit their own light due to the excitation of gas (often hydrogen) by nearby hot young stars, causing the gas to emit light in specific wavelengths. In contrast, reflection nebulae do not emit their own light; instead, they reflect the light from nearby stars, which is typically scattered by dust within the nebula. Therefore, the correct response is:
interactions with light.