 Light is a form of radiation that travels as a transverse wave. Light behaves in various different ways. It can be refracted and reflected. For more on this, see our video on ray diagrams. In this video, we're going to learn about how shadows are formed. It's important to remember that light travels in straight lines. Transparent materials allow light to pass through them. Translucent materials allow some light through, but not all. And opaque objects don't let any light pass through them, and so they cause shadows. A shadow is an area where there is no light. Shadows have two regions within them, the umbra and the penumbra. The umbra of a shadow is in the centre and is the darkest part of the shadow. The penumbra is an extended part of the shadow that is slightly lighter than the umbra. The size of the umbra and penumbra depend on the size of the object, and also the position of the object relative to the light source. Think of shadow puppets. If the light is closer to your hands, the shadow becomes bigger. What do you think happens to the size of the shadow, if the distance between the object and the light source stays the same, but the distance to the wall is increased? Pause the video and have a think. The size of the shadow, so both the umbra and penumbra, increases. During a solar eclipse, the moon passes between the sun and the earth. Light travelling in a straight line cannot pass through the opaque moon, and so a shadow is cast on part of the earth. As always, the shadow has an umbra and the penumbra region. So if you're standing in the umbra, on the part of the earth where the sun's light is totally blocked from reaching you, the sun will look like this in the sky. If you are ever lucky enough to witness this very rare event, make sure you don't look directly at the eclipse, as it can damage your eyesight.