This is the new updated edition of the first book in the bestselling Uncovering Student Ideas in Science series. Like the first edition of volume 1, this book helps pinpoint what your students know (or think they know) so you can monitor their learning and adjust your teaching accordingly. Loaded with classroom-friendly features you can use immediately, the book includes 25 “probes”—brief, easily administered formative assessments designed to understand your students’ thinking about 60 core science concepts.

Access this probe as a Google form: English

Download this probe as an editable PDF: English

The purpose of this assessment probe is to elicit students’ ideas about light and shadows. The probe is designed to find out students’ ideas about how shadows change throughout the day.

Friendly Talk

Earth-Sun system, shadows

The best response is Fabian’s: My shadow will keep getting shorter until noon and then it will start getting longer. Shadows are longest right after sunrise and right before sunset. The angle at which sunlight strikes Earth’s surface changes as the Sun appears to move across the sky due to the rotation of Earth. In the early morning, the Sun is low on the horizon and the angle between the Sun and Earth’s surface is small. The shadow that results from blocking the Sun’s rays is long. As the angle between Earth’s surface and the Sun’s rays that strike Earth’s surface increases throughout the morning, the shadow gets smaller. Noon is defined as the time when the Sun is highest in the sky. At noon, the size of a shadow is shortest and will begin increasing. Throughout the afternoon, the shadow gradually grows longer and its position is now on the other side of the object. It reaches its longest length just before sundown when the angle between the Sun’s rays and Earth’s surface is small and the Sun is once again low on the horizon. The sequence of shadow length is such that it starts out very long on one side of the object, gradually shortens and then gradually lengthens on the opposite side of the object until night, when there is no more sunlight to cast a shadow.

Elementary Students

Observing changes in shadows is a common activity for elementary school students. It is a good way to build inquiry skills and identify patterns. In the early grades, the changing length of shadows is primarily observational. In the later elementary grades, students begin to relate the changing size and position of the shadow to the position of the Sun in the sky in relation to Earth’s surface in order to explain how shadows change.

Middle School Students

Middle school students can extend their investigations of shadows to look for seasonal patterns. At this level, students understand that the position of the daytime Sun is related to Earth’s rotation and results in the length and position of shadows. Students also develop the notion that the shortest shadow on any given day always points due north and that the length of the shadow at noon varies with the seasons. Using models, middle school and high school students can investigate how variations in length of a shadow at noon relates to Earth’s 23.5-degree tilt as it revolves around the Sun. This also helps build students’ understanding of why we have seasons and why seasons in the Northern and Southern Hemispheres are opposite. Students can combine their knowledge of shadow formation with technological design by constructing sundials or examining how ancient cultures used shadows to mark certain days of the year. Students explore other ideas about shadows from light sources other than the Sun.

High School Students

At this level, students use ideas about light reflection to further investigate how shadows are formed, including comparing shadows formed by point sources versus extended sources of light. Students make quantitative comparisons between the size of a shadow and the distance of an object from a light source. However, even though students’ shadows on a sunny day are a familiar phenomenon, some high school students still have difficulty picturing how a shadow changes throughout the day.

Make sure students understand that the probe is asking about what happens to the length of the shadow from the time the Sun rises to the time the Sun sets. Consider having students draw a sequence of pictures showing the shadow in relation to the Sun during different parts of the day.

• Some researchers have found that children expect the shadow of an object to be the same shape as the object. Their predictions about shadows often refer to a shadow as a “reflection” or as a “dark reflection” on a screen (Driver et al. 1994).
• Students seem to have more success in locating where an object’s shadow will fall in relation to a light source if the object is a person. They have more difficulty anticipating where a shadow will fall if it is a nonhuman object, such as a tree (Driver et al. 1994).
• Students who were able to anticipate where a shadow would fall and explain their ideas in terms of relative position of the light source, object, and the object’s shadow did so without including an explanation of the straight path of light in their explanation (Driver et al. 1994).

American Association for the Advancement of Science (AAAS). 1993. Benchmarks for science literacy. New York: Oxford University Press.

American Association for the Advancement of Science (AAAS). 2001. Atlas of science literacy. Vol. 1, “changes in the Earth’s surface,” 50–51. Washington, DC: AAAS.

Barrows, L. 2007. Bringing light onto shadows. Science and Children 44 (9): 43–45.

Keeley, P. 2005. Science curriculum topic study: Bridging the gap between standards and practice. Thousand Oaks, CA: Corwin Press.

National Research Council (NRC). 1996. National science education standards. Washington, DC: National Academy Press.

• This probe lends itself to an inquiry investigation. After students commit to an idea on the probe, have them test their idea by going outside throughout the day and observing the lengths of their shadows. Students should be asked to make predictions about how the length and direction of the shadow will change over the next hour or so and be able to test their predictions.
• Make sure students understand what a shadow is before asking them to explain how shadows change.
• Have students model what happens to a shadow when the position of a light source, and thus the angle at which light strikes an object, changes. Provide students with flashlights and an upright object to test their ideas and record observations. Help students link their flashlight findings to the position of the Sun throughout the day.
• Consider having students draw a sequence of pictures, like a storyboard, to show the relationship between a shadow’s length and position and the position of the Sun throughout the day.
• Extend shadow investigations across the year to show that there are seasonal patterns as well as daily patterns.
• Have students investigate how ancient cultures used their knowledge of shadows to mark seasonal events and tell time.
• Consider having students participate in NSTA’s “Astronomy with a Stick” project. Information on this project can be found at www3.nsta.org/awsday.
• Have students use mathematics to figure out how high the Sun is at noon during each of the four seasons to show that the height varies throughout the year, which accounts for why shadow size at noon varies throughout the year. On the first day of spring and fall, the Sun is 90 degrees minus your latitude above the horizon at noon (for example, 90 degrees – 40 degrees = 50 degrees for New Jersey). For noon on the first day of summer, add 23.5 degrees (the angle of Earth’s tilt) to that (50 degrees + 23.5 = 73.5 degrees for New Jersey) and for noon on the first day of winter subtract 23.5 degrees from that (50 degrees – 23.5 = 26.5 degrees for New Jersey). Have students try this for their locations and compare the measurements of their shortest shadow during the first day of fall, winter, spring, and summer. Students may be quite surprised to see that in some locations, such as New Jersey, the noon shadow on the first day of winter can be quite long.