Light and shadows

When children draw pictures of the Sun, they often show rays radiating outwards – similar to the image below.

These light rays travel in a straight line at nearly 300,000 kilometres per second. Sunlight that travels towards the Earth takes just over 8 minutes to reach us. When the rays reach Earth, they hit whatever is in their path. If the object they hit is opaque, the light cannot pass through, and a shadow forms.

Simply speaking, a shadow is an absence of light. If light cannot get through an object, the surface on the other side of that object (for example, the ground or a wall) will have less light reaching it.

A shadow is not a reflection, even though it is often the same shape as the object.

Light sources and shadows

There are many sources of light – stars like our Sun, candle flames, light bulbs, glow-worms and computer screens produce light. All of this light travels in a straight line until it hits something. Sometimes, it travels a short distance – like when we switch on the lamp. Other times, light travels thousands of years – like the light from stars we see in the Milky Way.

It is easy to see our shadows when we are outdoors in the sunshine on a clear, bright sunny day, but do shadows form when an object blocks light from other sources? The answer is yes, but they may be difficult to see if the light source is not very bright (has a low light intensity). Shadows are also more definite (sharper) where there is contrast between the shadow and the lit surface, for example, a shadow on a white wall will be more easily seen.

The size of the light source can sharpen or blur the shadow. A small spotlight like a cellphone torch forms a more distinct shadow than an overhead room light, but the sharpness of the shadow changes when the torch moves away from the object.

Changing shapes and sizes

A shape of an object always determines the shape of its shadow. However, the size and shape of the shadow can change. These changes are caused by the position of the light source.

When we are outside on a sunny day, we can see how our shadows change throughout the day. The Sun’s position in the sky affects the length of the shadow. When the Sun is low on the horizon, the shadows are long. When the Sun is high in the sky, the shadows are much shorter. We can create the same effects indoors by changing the position of a torch as it shines on an object.

Although the shadow effects are the same, the reasons for the moving light source are very different. When we use a torch to make long and short shadows indoors, it is the light source that moves. When the Sun makes long and short shadows outdoors, it is the Earth, not the light source (Sun), that moves.

The spinning Earth

From our vantage point on Earth, it appears that the Sun moves across the sky during the day. We see the Sun appear to rise in the east and set in the west. Actually, the Earth is spinning (rotating on its axis) so it is our view of the Sun in the sky that changes during each 24-hour cycle of light and dark.

We see the sunrise when our location on Earth spins towards the light of the Sun. As the Earth continues to spin, we see the Sun higher in the sky. As the Earth spins away from the light, we see the sunset. The Earth continues to spin until we are in a shadow – our place on Earth is dark because the Sun’s light is blocked by the magnitude of our planet! We have several hours of night with our side of the Earth in darkness, and then as the Earth spins towards the Sun’s light, we see a sunrise. When New Zealand is in darkness during the night, the opposite side of the world is in sunlight.

Shadows change with the seasons

The tilt of the Earth’s axis affects the length of our shadows. During the summer, our location is tilted towards the Sun, so our midday shadows are very short. During the winter, our location is tilted away from the Sun, so our midday shadows are longer.