What type of material cannot be seen through when lit from the front, but can be seen through when lit from behind?

The correct answer is gauze. Gauze is a type of material that is semi-transparent and has a unique property when it comes to lighting. When lit from the front, it appears opaque, making it difficult to see through. This characteristic allows it to be used effectively in theatrical lighting to create specific visual effects and conceal behind it. However, when light shines from behind the gauze, it becomes more transparent, allowing what is behind it to be visible. This function is often utilized in stage design to create layers and depth in a production.

The other materials mentioned have different properties. Scrim, while similar to gauze, has a more defined purpose and behavior with light, often used to create illusions. Blackout cloth is designed to block all light, making it completely opaque from either side. Muslin, on the other hand, is typically used for backdrops and can have varying transparency but does not share the same specific properties regarding the effects of front versus back lighting as gauze does.