Light sources make their own light, like the Sun, a light bulb or a candle flame. Most things we see are not light sources — we see them because light reflects (bounces) off them and travels into our eyes.

Light travels in straight lines. When an object blocks the light, a dark area forms behind it. This is a shadow. Shadows form on the side of the object away from the light.

Materials can be transparent (let almost all light through, like clear glass), translucent (let some light through, but you cannot see clearly, like tissue paper or a frosted bathroom window) or opaque (let no light through, like wood or cardboard). Opaque objects make the darkest shadows.

Smooth, shiny surfaces such as mirrors reflect light very well. That is why you can see your reflection in a mirror.