Building Science Concepts: Shadows

Several basic science concepts are needed to understand shadows.

The New Zealand Ministry of Education’s Building Science Concepts (BSC) series presents sets of interlinking concepts that build stage by stage towards 'big ideas' in science. A big idea shows how a fully developed understanding of the concept might look, but recognises that such an understanding might not be achieved until New Zealand Curriculum level 7 or 8.

This resource is a partial replication of Building Science Concepts Book 9 Shadows: Effects of the Absence of Light. The background information on this page, combined with the information in the interactive, covers the science notes provided in the original BSC book. The overarching science concepts (big ideas) and how they may be scaffolded in sequence are illustrated in the interactive below.

The background information in this article and in the interactive are to check and clarify teacher understanding of the science concepts. It is not intended as resource material for students working at levels 1 and 2.

Visible light

Light is one of many forms of electromagnetic radiation. These forms make up what is known as the electromagnetic spectrum. They range in their wavelength and frequency from long, low-frequency radio waves to short, high-frequency gamma and cosmic rays. Human eyes are able to see just one part of that spectrum – what we call visible light.

Parts of the electromagnetic spectrum

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Parts of the electromagnetic spectrum are named according to wavelength and energy. Radio waves can be over a kilometre, X-rays shorter than a nanometre (a billionth of a metre).

Rights: The University of Waikato

Sun and shadow – day and night

Earth’s largest source of light is the Sun. As the Earth travels on its orbit around the Sun, one side of the planet is always illuminated by this source of light and the other side is blocked from it. But the Earth is also spinning on its axis, making a complete turn every 24 hours. During that time, the planet as a whole experiences approximately 12 hours of sunlight (day) and 12 hours without sunlight (night).

From our position on the Earth’s surface, we experience this spin as gradually moving into the path of the Sun’s light and then gradually being blocked from that light by the Earth’s bulk.

Moon shadows and eclipses

Moonlight is sunlight that is reflected by the Moon. The Moon, like the Earth, is always in the full light of the Sun and always reflects the same amount of that light. But the shape of the moonlight and the Moon shadow that we can see from Earth depends on the angle at which the Sun’s light hits the Moon and is reflected down to us. We interpret the change in angles as phases of the Moon.

Solar and lunar eclipses are also examples of shadow formation. They happen when the Earth, the Moon and the Sun are positioned in a direct line. A solar eclipse is comparatively rare. it occurs when the moon passes between the Earth and the Sun, blocking the Sun’s light from reaching the Earth. A lunar eclipse occurs when the Earth passes between the Sun and the Moon, blocking the Sun’s light from reaching the Moon.

Lunar eclipse

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Using different camera exposures, 1/80, 2/5 and 2 seconds, the two regions of a lunar eclipse can be seen. The outer penumbral shadow is where the sunlight is dimmed and the inner umbral shadow is where the much dimmer sunlight can be seen as a red colour as it reflects through the Earth’s atmosphere.

Rights: Tomruen CC BY-SA 4.0

Alternative conceptions

Children often have ideas about science concepts that are strongly held, but not scientifically accurate. Some common alternative ideas related to light and shadow are:

• light is instantaneous (it appears to be this way because it travels so incredibly fast - a million metres per second!)

• shadows are 'things' (rather than shapes caused by the absence of light)

• the Sun moves across the sky (it actually stays still; the movement that we see is caused by the Earth rotating around its own axis)

• longer shadows are caused by the Sun being further away (the length of a shadow is actually caused by the angle between the Sun and the object blocking the light rays).

The article Alternative conceptions about light and shadows provides additional information about this topic, including how to scaffold change.

Shadows and the science capabilities

The phenomenon of shadow is readily observable and offers opportunities for investigating the properties of light, substances and the passage of light, and the movement of the Sun, Earth and Moon. Through observation and measurement, students can gather data and make meaning of it via interpretation and discussion. Encouraging students to use evidence when supporting their explanations can adjust their thinking and help to move them away from alternative conceptions. A third capability to consider using is ‘Interpret representations’. Using images of shadows – and discussing the concepts they represent – may help to consolidate scientific thinking.