How light travels and why shadows form
Some objects, such as the Sun, a candle flame and a torch bulb, give out their own light and are called luminous. Most objects, like a book or the Moon, are non-luminous and are seen only because they reflect light into our eyes. Light travels in straight lines, which you can show by looking at a candle through three cardboard sheets with holes in a straight row.
Materials are grouped by how much light passes through them. Transparent materials such as clear glass let almost all light through, translucent materials such as butter paper let some through, and opaque materials such as wood block it completely. When an opaque object blocks light, a dark shadow forms on the other side. Shadows are always dark whatever the colour of the object, and their size changes as the object moves nearer to or farther from the light. A pinhole camera uses straight-line travel to make an upside-down image. Shiny surfaces like mirrors reflect light and change its direction.
Why it matters
Understanding light and shadows explains sundials that tell time, shadow puppets, eclipses and why we see objects only when light enters our eyes. Choosing transparent, translucent or opaque materials matters for windows, curtains and lampshades. Simple reflection with mirrors is used in periscopes and kaleidoscopes, and it leads to the study of reflection and refraction later.
Key terms
- Luminous object
- An object that gives out light of its own, such as the Sun or a lit bulb.
- Opaque
- Describes a material that does not allow any light to pass through it.
- Translucent
- Describes a material that lets some light through, so objects behind it look blurred.
- Shadow
- A dark area formed when an opaque object blocks the path of light.