Black in the Spotlight

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Summary

6th-8th
Group Size
4-8 students
Active Time
75 minutes
Total Time
75 minutes
Area of Science
Physics
Materials Science
Key Concepts
Light reflection, absorption, and transmission; Light
Credits
Sabine De Brabandere, PhD, Science Buddies

Overview

How does light interact with matter? In this fun hands-on lesson, your students explore how different materials transmit, absorb and/or reflect light. They create their own experiments to demonstrate these phenomena and use a phones' built-in light sensor and a sensor app to add quantitative data to their arguments.

Learning Objectives

• Show examples of light absorption, reflection, transmission, or any combination of these phenomena.
• Use light intensity measurements to indicate whether a material absorbs, reflects, or transmits light.

NGSS Alignment

This lesson helps students prepare for these Next Generation Science Standards Performance Expectations:
• MS-PS4-2. Develop and use a model to describe that waves are reflected, absorbed, or transmitted through various materials.
This lesson focuses on these aspects of NGSS Three Dimensional Learning:

 Science & Engineering Practices Disciplinary Core Ideas Crosscutting Concepts Science & Engineering Practices Developing and Using Models. Develop and use a model to describe phenomena. Planning and Carrying Out Investigations. Plan an investigation individually and collaboratively, and in the design: identify independent and dependent variables and controls, what tools are needed to do the gathering, how measurements will be recorded, and how many data are needed to support a claim. Engaging in Argument from Evidence. Construct, use, and present oral and written arguments supported by empirical evidence and scientific reasoning to support or refute an explanation or a model for a phenomenon. Disciplinary Core Ideas PS4.B: Electromagnetic Radiation. When light shines on an object, it is reflected, absorbed, or transmitted through the object, depending on the object's material and the frequency (color) of the light. Crosscutting Concepts Structure and Function. Structures can be designed to serve particular functions by taking into account properties of different materials, and how materials can be shaped and used. Cause and Effect. Cause and effect relationships may be used to predict phenomena in natural systems.

Materials

For the class:

• Laser pointer, class I or II. The Laser Safety Guide provides information on laser classification and how to use a laser safely.
• White paper
• Mirror
• Smartphone with a sensor app such as phyphox, available for free on Google Play for Android devices (version 4.0 or newer) or from the App Store for iOS devices (iOS 9.0 or newer).
• Optional: scissors
• Optional: tape
• Optional: sheets of cardboard

For each team of 2 to 4 students:

• Flashlight
• Smartphone with a sensor app such as phyphox, available for free on Google Play for Android devices (version 4.0 or newer) or from the App Store for iOS devices (iOS 9.0 or newer).
Note: Phyphox does not support the light sensor on iOS devices. If you need the light sensor, you have to use Android devices for your experiment. Note that on some devices the light sensor is only updated when there is a coarse change of illuminance. This means that if the light intensity does not change or only changes slightly, the sensor appears to not record any data. The recording will continue once the light intensity changes again. If your experiments allows, it helps to wiggle the phone or the light source (e.g. flashlight) slightly to induce minimal reading fluctuations and keep the sensor active.

For each pair of teams:

Cardboard box filled with:
• Sheet of aluminum foil or baking pan
• Small mirror
• Sunglasses
• Flat pieces of plastic: transparent, translucent but not transparent, one white and one black. Examples: a translucent folder divider, a white lid of a yogurt container, a black plastic food container (like a to-go box) with a clear plastic lid
• Sheets of paper: white(1) and black(3)
• A sheet of tissue paper (white or black) or tracing paper
• Optional: Ruler

Background Information for Teachers

This section contains a quick review for teachers of the science and concepts covered in this lesson.

Why can we see some areas of the face in Figure 1 and not others? Is this face white, gray, or black? These are questions this lesson addresses.

Figure 1. Light reflected on a statue.

We can categorize materials by their appearance, like transparent, translucent, or opaque; dark or light colored; glossy or matte finish, etc. These classifications are based on how the materials transmit, absorb, and reflect light. These concepts are explained in more detail below.

When a material transmits light, it allows light to pass through (Figure 2, left), so it appears transparent or translucent. Notice that we cannot see materials that transmit all light; we see what is behind the material instead. The clear glass used in most windows is a good example.

When a material absorbs light, it captures the energy carried by the light (Figure 2, right), and transforms it into thermal energy. These materials look dark and tend to get warm when left exposed to light. Notice we also cannot see materials that absorb all light; we see the absence of light. This is shown in this super-black coating demonstration.

Figure 2. Illustrations of transmission of light (left) and absorption of light (right).

We can only see materials that reflect or bounce back light. The bouncing of light is similar to that of a bouncy ball in that the angle at which the light falls onto the reflecting surface is identical to the angle at which it bounces back (Figure 3, left). A mirror is a perfect example of reflection on a smooth material. Because it is so smooth, all light reflects at the same angle (Figure 3, middle) giving the material a shiny appearance. Materials that do not have a smooth surface, reflect light in a diffuse way (Figure 3, right). Bumps on their surface cause light to reflect in many directions. These materials appear matte.

Figure 3. Illustrations of how light reflects on surfaces.

No common material exists that transmits, absorbs, or reflects all light. For example, even though window glass looks clear to us, it still reflects a little bit of light. There is always a combination of two or all three phenomena. The ratio of light transmitted, absorbed, and reflected determines what the object looks like. Table 1 shows a list of common materials with an estimate of how much light they transmit, reflect, and absorb.

Material Transmit
(%)
Absorb
(%)
Reflect
(%)
Mirror 0 20-10 80-90
Black felt 0 95-85 5-15
White paper 0 30-20 70-80
Clear window glass 96-92 0 4-8
Table 1. Percentages of light transmitted, absorbed, and reflected for a few materials.

In this activity, students build examples showing transmission, reflection, and absorption of light. They measure light intensity with a phone equipped with a light sensor and a sensor app and use the data to prove the phenomenon occurs. The app provides access to data from the light sensor that is built into many smartphones and measures light intensity in lux.

This lesson can easily be extended by looking at how colors influence reflection, absorption, and transmission. White light consists of light of all colors in the rainbow, and each color interacts independently with material. You can see this happen when you place a red transparency in front of a white flashlight. You see a red beam of light. This shows how the red transparency transmits red light and absorbs all other colors. In a similar way, a red apple looks red under white light because it reflects red light and absorbs all other colors. Under green light, the same apple looks black, as the red apple absorbs green light. These examples show how adding color to this lesson can create rich and interesting experiences.

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