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What You See Is Not What You Get: The Difference Between Sunlight and Ultraviolet Radiation

Scientists have known that there is a difference between the sun’s visible radiation and invisible ultraviolet radiation from the sun. Ultraviolet radiation is separated into 3 types: UVA, UVB, and UVC. The scientists in this study were interested in trees growing in urban areas. They wanted to know whether the shade that we can see under urban trees protects people from UVB radiation.

Note: In earlier Natural Inquirer articles like this one, the Discussion section is called "Implications."

What You See Is Not What You Get: The Difference Between Sunlight and Ultraviolet Radiation

Meet the Scientists

Richard Grant

Meteorologist

Gordon Heisler

Meteorologist

Thinking About Science

Scientists use many ways to discover new knowledge. Sometimes collecting existing information from many sources and putting it together in one report is a valuable addition to science. This is similar to what you do when you write a paper using information from the library and the Internet. In this study, the scientists collected information from past research and added it to their own research findings. In this research, the scientists wanted to know how much is already known about ultraviolet radiation in urban areas.

Thinking About the Environment

All life on Earth needs the sun because it is the original source of all of our energy and food. Sometimes, however, we can get too much of a good thing. Take ultraviolet radiation, for example. Ultraviolet radiation, or UV radiation, comes from the sun and is invisible. UV radiation is a part of the electromagnetic spectrum (figure 1).

Graphic of electromagnetic spectrum — **Figure 1**. Electromagnetic Spectrum

What makes UV radiation good? UV radiation causes the skin to create Vitamin D, which helps people to absorb calcium. A little bit of sunlight can also protect people from some kinds of nonskin cancers. Too much UV radiation, however, can be hazardous to human health. If you are going to be in the sun for a long time, use sunscreen or cover your skin. Some of the negative impacts of too much UV radiation include sunburn, eye damage, and skin cancer. Because too much UV radiation may damage a person’s immune system, other cancers and diseases may be related to having too much sun exposure.

Introduction

Scientists have known that there is a difference between the sun’s visible radiation and invisible ultraviolet radiation from the sun. (See figure 1.) Ultraviolet radiation is separated into 3 types: UVA, UVB, and UVC. UVB radiation is best known for its ability to cause damage to human health. At the short end of the UVB wavelength, where UVB meets UVC, almost all of the radiation is absorbed by ozone in the atmosphere. At the longer end of the wavelength, UVB meets UVA radiation. Ozone does not absorb very much of the radiation at this end of the UVB spectrum (figure 2).

Graphic of UVA, UVB and UVC — **Figure 2**. UVA, UVB, and UVC radiation and their relationship to ozone in the atmosphere.

When it is hot or very sunny, people often seek the shelter of a tree’s shade to protect them from the sun. This is because the tree’s leaves provide shade from the sun’s visible radiation. Although leaves protect people from the sun’s visible radiation, they may not protect people from invisible UVB radiation. The scientists in this study were interested in trees growing in urban areas. They wanted to know whether the shade that we can see under urban trees protects people from UVB radiation. Ultimately, it is shade from the UVB radiation that is the most important for human health.

Reflection Section

Method

The scientists read over 100 research papers that were related in some way to the question about the sun’s radiation on urban trees. Urban trees are the trees that grow where people live, work, and play. The scientists also had the results from their own previous research. The scientists were most interested in the optical properties of leaves. They read research papers that reported how much of the sun’s visible radiation and UVB radiation passes through leaves. They read research about how the leaves from different tree species reflect visible radiation and UVB radiation. They read research about how the quantity of visible radiation compares with the quantity of UVB radiation in the sunny areas near urban trees. They included the information they had collected themselves about the amount of radiation reaching the area under urban trees. Then, they put all of the information together into one report.

Reflection Section

Findings

The scientists found that while leaves allow some of the sun’s visible radiation to pass through them, tree crowns do not allow much light to pass to the ground. This is because most of the sun’s rays encounter many leaves as they attempt to get through the crown. Therefore, the scientists focused their attention on the reflectivity of leaves (figure 3). They found that the reflectivity of visible radiation is much higher than invisible UVB radiation. For example, about 10 to 30 percent of the sun’s visible radiation is reflected off of leaves. About 3 to 6 percent of the sun’s UVB radiation is reflected off of leaves.

Graphic of a leaf showing how it reflects UV radiation — **Figure 3**. The reflection of the sun’s radiation off of a leaf.

In dense forests you can usually only see a small part of the sky from the forest floor. In these forests, only a little visible radiation and UVB radiation reach the forest floor. This is because there are many layers of leaves shielding the forest floor from the sky.

The situation is very different in urban forests. In urban forests, trees either are planted alone or with just a few others (figure 4).

A street tree planted where the sidewalk as been cutout

Trees planted in the median between two roads

Figure 4. In an urban forest, people can see the sky beyond the edge of the tree crown.

In urban forests, people can usually see the sky beyond the edge of the tree crown. The scientists used special equipment to measure the amount of radiation reaching the ground, both in the open and under urban trees (figures 5 and 6). UVB radiation is widely scattered across the sky. If a person can see much of the sky, the UVB radiation reaches them even when they are under a tree.

Special sensor equipment — **Figure 5**. Scientists use special sensor equipment placed under a tree or on the ground to measure the amount of the sun’s radiation reaching the ground. Notice the clear bubble on the top of the equipment.

A birdseye image — **Figure 6**. If the equipment could see, this is an example of what it would see.

The scientists reported some of their own research, in which they measured the amount of radiation reaching six types of areas (table 1).

The six areas were:

Sunlit areas under a tree in summer (with leaves)
Shady areas under a tree in summer (with leaves)
Sunlit areas under a tree in winter (no leaves)
Shady areas under a tree in winter (no leaves)
Sunlit areas under a tree in winter with a building nearby (no leaves)
Shady areas under a tree in winter with a building nearby (no leaves)

Table 1. Average percent reductions in the sun’s visible radiation and invisible UVB radiation below a street tree canopy.
Area Under Tree Canopy	Percent Reduction in UVB Radiation	Percent Reduction in Visible Radiation
1. Sunlit Area–With Leaves	39	3
2. Shady Area–With Leaves	63	84
3. Sunlit Area–No Leaves	40	6
4. Shady Area–No Leaves	56	73
5. Sunlit Area and Building–No Leaves	59	5
6. Shady Area and Building–No Leaves	70	47

Look closely at table 1. If our eyes were able to see UVB radiation, the shadows cast by UVB radiation would be different than the shadows cast by visible radiation. Compare the numbers in areas 1 and 2 in each column. Then look at the numbers in areas 3 and 4 in each column. Now look at the numbers in areas 5 and 6 in each column. You can see that the reduction in UVB radiation was more similar in sunny and shady areas than the reduction in visible radiation in sunny and shady areas. In sunny areas, not much visible radiation is reduced. In shady areas, a lot of the visible radiation is reduced. The radiation that we can see is not like the UVB radiation that we cannot see. The UVB radiation is scattered across the sky. When you can see a part of the sky from where you are standing, you are receiving some UVB radiation.

The scientists also found that in sunny areas close to urban trees, a greater percentage of UVB radiation was reduced even though the visible radiation was not reduced (figure 7). This is because when people stand close to a tree, part of the sky is blocked by the tree. When a part of the sky is blocked, some of the UVB radiation is blocked as well.

Graphic of a person pointing at a tree with the sun high in the shy creating a shadow from the tree — **Figure 7**. Sunny areas near urban trees experienced a greater reduction in UVB radiation than visible radiation.

Reflection Section

Thinking About Ecology

No matter what you look at in nature, it is a part of a system. Systems can exist at a number of different levels, from small and simple to large and complex. Your body is a good example. You have different organs that are systems composed of cells. Your whole body is a larger, more complex system that is composed of different organs. As systems get larger and more complex, new properties appear that were not evident at the smaller and simpler levels. In this study on UVB radiation, the crowns of trees demonstrate this idea. Tree crowns are made up of many individual leaves growing on branches. An individual leaf is one type of system. When hundreds or thousands of leaves make up a tree crown, new things are possible. Now, visible radiation and UVB radiation from the sun can be partially blocked. The temperature under the tree is now lower in the warm summer months. Birds now have a place to build nests. As systems get larger and more complex, new things are possible that were not possible before.

Implications

If you are in a densely forested area and cannot see part of the sky above you, you are probably protected from UVB radiation. If you are standing under an urban tree or a small group of trees and you can see part of the sky beyond the edge of the tree crown, you still receive some UVB radiation. The amount of UVB radiation you receive depends on how much of the sky you can see. Larger, older trees provide more protection from UVB than smaller, younger trees. If you are in the sun but just outside of the shade of a large tree, you receive a smaller amount of UVB than if you were standing in the sun farther away from the tree’s shade.

Reflection Section

From Heisler, G. M. & Grant, R. H. (2000). Ultraviolet radiation, human health, and the urban forest. USDA Forest Service General Technical Report NE-268, Newtown Square, PA.

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Urban Forest - Vol. 6 No. 1

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What Is a Natural Inquirer Journal?

Three Natural Inquirer journal covers with NI bee

A Natural Inquirer journal is a collection of 4-8 articles on a related science topic. Journals are written for a middle school audience, but they can also be adapted for both high school students and advanced upper elementary students. Some journals are particularly suited to high school students; you can find our grade level recommendations in the tags on the product page or by filtering journals by grade level.

Journals include:

Four to eight articles based on published, peer-reviewed research papers; the articles keep the research paper format (see more below) but are written in language students can understand.
A FACTivity for each article, which is an activity to complete after reading the article. The FACTivity helps reinforce major science concepts from the article. These activities are designed to be easy to implement, with few material requirements and options for adapting them for your audience or available resources. Some articles in a journal may have two FACTivities.
A short “Welcome to the journal” article about key background information and science concepts that unify the articles included in the journal
A glossary of new terms for each article and the introductory materials.
A list of related Natural Inquirer publications for each article as well as outside references.
Standards correlations, including Next Generation Science Standards, addressed in the articles and the FACTivities.

Journals may also include additional essays (called spotlights), other activities (like crossword puzzles or vocabulary challenges), and more.

A screenshot of the article resource tile for "A Flame Changer" showing the three different format option buttons.

Reading Modes

Journals are available in three different formats:

Hard copies can be ordered from the website and shipped, all free of charge.
PDF versions of the printed journal can be downloaded free on the website. The PDF version directly replicates the content and layout of the printed version. You can also download individual articles as pdfs.
The “Read Distraction Free” option allows the individual articles to open in their own window, without the rest of the website being visible. These articles can be found under the “Articles” tab. This version allows readers to scroll to particular sections of the article using the sidebar menu on the left side of the screen. This version also has interactive Reflection Sections and Number Crunches. Students can enter their answers, submit them, and then receive the correct answers to double-check their work. Submitted answers are not saved on the website and will disappear once the window is closed.

What's in a Natural Inquirer Article?

Here, we'll go into more detail about the parts of a Natural Inquirer article and give you some ideas about how they can be used.

Meet the Scientists
This section introduces the scientists (and others) who worked on the study. In their own words, they each share a memorable science experience, a favorite research project, or something they learned during the course of their education or research.

Use this section to:
- Introduce kids to the variety of people who work in science
- Introduce kids to the variety of scientific fields and give brief descriptions of science-related jobs
- Explore ways that people interact with science every day
Next Generation Science Standards (NGSS) applications:
- Science and Engineering Practices
- Crosscutting Concepts: Influence of Science, Engineering, and Technology on Society and the Natural World
Note that specific standards for this particular journal are linked on this educator guide tab.

Other resources:

Many of the scientists and engineers featured in this section are also featured on our collector cards. Learn more about their work, how they got interested in their fields, and interesting projects they worked on. Cards can be printed as posters, too.
Thinking About Science
This section briefly describes a concept about science or scientific research. This overview can touch on topics like
- study type (longitudinal study, quantitative vs. qualitative data),
- behaviors of scientists (conducting literature reviews, collaborating with other specialists, replicating earlier studies),
- the practice of science (the scientific method, engineering design, data collection, randomization, controls and variables),
- or other aspects of science (bias, correlation vs. causation).
Use this section to:
- Reinforce steps in the scientific method and the process of science
- Encourage students to think about the practice of science and what it can and cannot tell us
- Consider the many types of scientific study and what information each type can provide
Next Generation Science Standards applications:
- Science and Engineering Practices
- Life Science Disciplinary Core Ideas (depending on topic)
- Most Crosscutting Concepts (depending on topic)
Note that specific standards for this particular journal are linked on this educator guide tab.

Other resources:

You can use key words to search for other or related scientific topics on our website (e.g. “longitudinal study,” “bias,” or “sampling”).
Thinking About the Environment
This section provides a brief overview of a topic or concept in environmental/life science. The topic or concept is directly related to the research study that follows. Examples of topics include the carbon cycle, the water cycle, habitat fragmentation, phenology, biodiversity, and ecosystem services.

Use this section to:
- Provide important background information to help students understand the research study
- Serve as a quick reference during reading or class instruction
- Connect the research article with other activities or media on the same topic
Next Generation Science Standards applications:
- Life Science and some Earth Science Disciplinary Core Ideas (depending on topic)
- Most Crosscutting Concepts (depending on topic)
Note that specific standards for this particular journal are linked on this educator guide tab.

Other resources:

You can use key words to search for more resources on life or earth science topics on our website (e.g. “habitat,” “carbon,” or “genetics”).
Introduction
This section begins the scientific article format. Much like the published, peer-reviewed study this article is based on, the introduction provides background information for the study – what is currently known and what remains unknown. The introduction culminates in the question(s) the study hopes to answer.

The introduction is also the first section with a Reflection Section. This section includes two or three questions to help kids reflect on what they’ve just learned in the Introduction. If they are using the online distraction-free reading mode, they can answer these questions directly on the website.

Use this section to:
- Review important background information that kids need to understand the study
- Connect the study to the concepts addressed in the Thinking About Science and Thinking About the Environment sections
- Understand research questions and hypotheses, including generating their own hypotheses given what they already know
Next Generation Science Standards applications:
- Life Science and some Earth Science Disciplinary Core Ideas (depending on topic)
- Most Crosscutting Concepts (depending on topic)
Note that specific standards for this particular journal are linked on this educator guide tab.

Other resources:

Use one of the guided reading lesson plans to help kids follow the format of a scientific paper.
Methods
This section is the nuts and bolts of the study design – the who, what, when, where, why, and how of the research. Contained within the Methods section are usually maps of the study location or the set-up of study plots, as well as details about what data was collected and how.

The Methods section also ends with a Reflection Section – two or three questions to help students think through what they just read. These questions are interactive on the distraction-free reading mode.

Use this section to:
- Show students how experiments and studies are designed and carried out
- Explore sampling methods and randomization
- Introduce various data collection tools (e.g. camera traps, surveys, insect collection tools, weather stations, etc.)
- Explain bias and how studies are designed to remove bias
- Help students gain experience with map reading
Next Generation Science Standards applications:
- Life Science and some Earth Science Disciplinary Core Ideas (depending on topic)
- Most Crosscutting Concepts (depending on topic)
Note that specific standards for this particular journal are linked on this educator guide tab.

Other resources:

Many Methods and Findings sections contain Number Crunches, which are simple math exercises designed to help students interact with the data from the study.
Findings
This section summarizes the data collected during the study. The Findings section usually includes data tables or graphs and highlights the significant data points from the study. This section often mentions statistical analysis or the use of computer programs to model or analyze the data, though these methods are only discussed generally.

The Findings section also ends with a Reflection Section – two or three questions to help students think through what they just read. These questions are interactive on the distraction-free reading mode.

Use this section to:
- Have students practice reading and interpreting graphs and tables
- Compare results between variables and controls
- Explain the concept of statistical significance
- Discuss how no data or negative results still provide valuable information
Next Generation Science Standards applications:
- Life Science and some Earth Science Disciplinary Core Ideas (depending on topic)
- Most Crosscutting Concepts (depending on topic)
Note that specific standards for this particular journal are linked on this educator guide tab.

Other resources:

Search the website for “map” or “graph” to find activities where students can practice making and reading maps and graphs.
Discussion
This section concludes each article. In it, we summarize the main findings of the scientists’ study. Additionally, we present the scientists’ ideas about the limitations of their study, the big-picture impacts of their research, and the scientists’ plans for future study or action.

The Discussion section ends with a Reflection Section – two or three questions to help students think through what they just read, especially general take-aways from the study. These questions are interactive on the distraction-free reading mode.

Use this section to:
- Discuss what conclusions can and cannot be drawn from the available data
- Explain the difference between correlation and causation
- Explore study limitations and opportunities for further study
- Brainstorm ways the study findings could be applied to real-world situations
Next Generation Science Standards applications:
- Life Science and some Earth Science Disciplinary Core Ideas (depending on topic)
- Most Crosscutting Concepts (depending on topic)
Note that specific standards for this particular journal are linked on this educator guide tab.

Other resources:

Use the “Designing Your Own Study” resource page for videos of scientists discussing their own research studies. The page also includes educator resources to help students plan their own scientific studies.

Additional Resources on the Website

A screenshot of the product tabs for an NI monograph On the website, we pair each journal with a variety of other resources, as well. Use the tabs on the product page to browse through the following:

Related activities, including the FACTivity for each article
An “About” essay that gives some larger context for the research the scientists conducted or more information about the science topic from the journal
A glossary of all boldfaced terms from the journal
A “Scientists and Collaborators” page that lists the people involved in the studies in the journal; click on a researcher to reach their bio page and see what other articles they might be featured in
A “Related Content” page that lists both Natural Inquirer resources about similar topics and also outside reference materials

Article Selection and Review

Natural Inquirer partners with the USDA Forest Service, so we source research studies by Forest Service scientists that have been peer-reviewed and published in reputable journals. Some of our articles have also been created in collaboration with scientists from other Federal agencies, such as U.S. Geological Survey and the United Nations Food and Agriculture Organization, universities, and other non-profits.

All journal articles are reviewed by scientists who conducted the original research study to verify scientific accuracy. Journals are also reviewed by student editorial review boards of middle or high school students before publication. Additionally, all journals are reviewed by the Forest Service and the U.S. Department of Agriculture before publication.

A screenshot of the citation for "Lights, Camera, Tracks" Every journal article includes a citation of its source study. Many educators pair the original research paper with our article to help more advanced students learn how to read formal research papers. The journal article then serves as adapted primary literature, bridging the two articles.

Lessons

In this lesson plan, students will assess the biological diversity of trees on their school grounds or another designated area. Students will be able to: observe identify, measure, transform, and...
Lesson Plan – Urban Forests
In this lesson plan, students will assess the biological diversity of trees on their school grounds or another designated area. Students will be able to: observe identify, measure, transform, and...
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- Download Lesson Plan (PDF)
- Explore Lesson Plan
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Urban Forest - Vol. 6 No. 1
In this lesson, students will create a storyboard to graphically represent the main points from the research article they read. This lesson plan can be used with any Natural Inquirer...
Lesson Plan – Storyboard
In this lesson, students will create a storyboard to graphically represent the main points from the research article they read. This lesson plan can be used with any Natural Inquirer...
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Freshwater - Vol. 18 No. 1
In this lesson, students will increase their reading comprehension, critical thinking skills, and summarization and explanation skills. Students will role-play as members of the Natural Inquirer Twitter Team (now X...
Lesson Plan – Twitter Team
In this lesson, students will increase their reading comprehension, critical thinking skills, and summarization and explanation skills. Students will role-play as members of the Natural Inquirer Twitter Team (now X...
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Hawaii Pacific Islands - Vol. 1 No. 16

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FACTivity – What You See Is Not What You Get

Alternate FACTivity – What You See Is Not What You Get

Standards addressed in this Article:

What Is a Natural Inquirer Journal?

Reading Modes

What's in a Natural Inquirer Article?

Meet the Scientists

Thinking About Science

Thinking About the Environment

Introduction

Methods

Findings

Discussion

Additional Resources on the Website

Article Selection and Review

Lessons

Lesson Plan – Urban Forests

Lesson Plan – Storyboard

Lesson Plan – Twitter Team

Education Files

Glossary

average

canopy

crown

immune system

optical

quantity

reflectivity

relationship

species

ultraviolet radiation

Richard Grant

Gordon Heisler