Fall Colors

This activity allows students to scientifically analyze leaf color changes in fall. It makes the link between visual observation of plant biology and spectral analysis of colors.

Every autumn, temperate forests stage one of nature's most spectacular color shows. Leaves that were uniformly green all summer transform into brilliant displays of yellow, orange, red, and brown. This transformation is not random but follows a precise biochemical sequence driven by the breakdown of chlorophyll, the green pigment that powers photosynthesis. As days shorten and temperatures drop, trees stop producing chlorophyll, and the green color fades to reveal yellow and orange pigments (carotenoids) that were present all along but masked by the dominant chlorophyll. Some species also produce new red pigments (anthocyanins) in response to sunlight and cool temperatures. Using the FizziQ colorimeter, students can quantify these color changes by measuring the red, green, and blue (RGB) components of light reflected by leaves at different stages of autumn coloring, connecting the visual beauty of the season to the underlying physics of light and the biology of plant pigments.

Learning objectives:

The student uses the FizziQ colorimeter to analyze the colorimetric composition of leaves from the same tree at different stages of autumn coloring. By measuring the proportions of red, green and blue in each leaf, the student observes the decrease in chlorophyll (green component) and the increase in carotenoids (yellow to red components) to understand the biochemical mechanism of color change.

Level:

Middle school

FizziQ

Author:

Duration (minutes) :

30

What students will do :

- Measure the RGB color components of leaves at different stages of autumn coloring using the FizziQ colorimeter
- Track the decrease in green component and increase in red component as chlorophyll degrades
- Understand the role of chlorophyll, carotenoids, and anthocyanins in leaf coloration
- Create a quantitative record of the autumn color transition
- Connect the physics of light absorption and reflection to plant biology

Scientific concepts:

- Plant pigments
- Colorimetry
- Photoreceptors
- Chlorophyll and carotenoids
- Leaf life cycle
- Pigment degradation

Sensors:

- Camera (used as a colorimeter for RGB analysis)

What is required:

- Smartphone with the FizziQ application
- Leaves of the same tree at different stages of coloring (green yellow red brown)
- White support for placing the sheets
- FizziQ experience notebook

Experimental procedure:

Collect leaves from the same tree species at different stages of autumn coloring: fully green, yellow-green, yellow, orange, red, and brown. Aim for at least 5-6 leaves spanning the transition.
Open FizziQ and select the Colorimeter tool, which uses the smartphone camera to measure RGB values.
Place the first leaf (fully green) on a white background in good, even lighting. Avoid direct sunlight which can cause glare.
Point the camera at the leaf and record the Red (R), Green (G), and Blue (B) values displayed by FizziQ.
Repeat the measurement for each leaf, progressing from green to brown. Take three readings per leaf (at different spots) to account for color variation across the surface.
Record all data in a table in your FizziQ notebook with columns: leaf stage, R value, G value, B value.
Plot the R, G, and B values versus leaf stage (from green to brown) on the same graph.
Observe that the G component decreases as the leaf changes from green to yellow, reflecting chlorophyll degradation.
Note whether the R component increases for leaves that turn red, indicating anthocyanin production.
Calculate the R/G ratio for each leaf. This ratio should increase progressively from green to red leaves.
If possible, compare leaves from different tree species and note which produce red pigments (e.g., maples) versus those that only turn yellow (e.g., birches).
Relate your observations to the underlying biochemistry: chlorophyll absorbs red and blue light (reflecting green), while carotenoids absorb blue (reflecting yellow-orange) and anthocyanins absorb green (reflecting red).

Expected results:

For a green leaf, the RGB values should show a dominant green component (G > R > B, typically G around 120-160, R around 60-100, B around 40-80 on a 0-255 scale). As the leaf yellows, the green component decreases while red remains stable or increases slightly, and the R/G ratio approaches 1. For orange and red leaves, the red component dominates (R > G > B). Brown leaves typically show low values of all three components with R slightly dominant. The R/G ratio should progress from about 0.5-0.7 (green) through 1.0 (yellow) to 1.5-2.5 (red). Students should observe that the transition is not linear and that some leaves show patchy or mottled coloring, reflecting the localized nature of pigment production and degradation.

Scientific questions:

- Why do leaves appear green in summer? Which wavelengths does chlorophyll absorb and which does it reflect?
- Why were the yellow pigments invisible during summer even though they were present?
- Why do some tree species produce red autumn colors while others only turn yellow?
- How does temperature affect the rate of chlorophyll degradation and anthocyanin production?
- Could you use the colorimeter to determine the relative concentrations of different pigments in a leaf?
- Why do fallen brown leaves eventually become uniformly dark? What happens to the remaining pigments?

Scientific explanations:

The FizziQ colorimeter uses the smartphone camera equipped with a Bayer filter to break down the light reflected by an object into its red, green and blue (RGB) components. This technology allows quantitative analysis of leaf color changes.

In summer, leaves appear green because they contain a high concentration of chlorophyll, a pigment that primarily absorbs red and blue wavelengths and reflects green. With the arrival of autumn, the reduction in day length and the drop in temperatures trigger biochemical processes: the tree stops producing chlorophyll and begins to degrade it to recover nutrients before the leaves fall.

As chlorophyll disappears (visible in the decrease in the green component), carotenoids and anthocyanins, present all year round but masked by chlorophyll, become visible (increase in the red and yellow components). Carotenoids (yellow-orange) are accessory pigments that participate in photosynthesis, while anthocyanins (red-purple) are synthesized in the fall and play a protective role against UV rays.

The variation in the proportions of these pigments explains the diversity of autumn colors.

Extension activities:

Frequently asked questions:

Q: The RGB values vary a lot even within a single leaf. Which values should I use?
R: Take three readings at different spots on the leaf and use the average. Avoid measuring near the veins or edges, which often have different pigment concentrations. Consistency in lighting is also important: use the same distance and angle for all measurements.

Q: My green leaf shows higher R values than expected. Is the sensor wrong?
R: Green leaves do reflect some red light; they are not purely green. A healthy green leaf might have RGB values like (80, 140, 50), showing significant red content. The key is that G is dominant, not that R is zero.

Q: Does the white background affect the measurement?
R: Yes. The background should be white and matte to avoid adding colored reflections to the measurement. A sheet of white printer paper works well.

Q: Can I do this experiment in spring or summer?
R: The experiment is designed for autumn, when the color transition occurs naturally. However, you could study stressed leaves (drought, nutrient deficiency) at any time, as these conditions also cause chlorophyll loss and color changes.