Additive colors are interesting to young students. In this paper, we show a convenient and straightforward method to demonstrate this phenomenon by projector. It produces defocus blur and reduces imaging quality for color mixing. We believe this demonstration experiment can be performed in most classrooms.

When doing experimental work of image formation by mirrors and (thin) lenses, it turns out again and again that students often have partially incorrect preconceptions about how the light emerging from an object passes through a lens and how the image is formed on a screen or directly in the eye. To check students' prior knowledge and help get a…