SPEAK OUR LANGUAGE

• CELL – Core Experience Learning Lab
• SDR – Scientist Data Record

ASK WHY

Great scientists question the world around them. We encourage our LabLearner students to do the same. In anticipation of this, we explain the importance of learning the concepts in the Ask Why section within the CELL. Our hope is that these explanations help students understand why science matters.

BRANCH OUT

Each Investigation introduces students to a different branch of science or STEM (Science, Technology, Engineering, Mathematics) career that utilizes the scientific concepts of the CELL. These real-world connections will help students see the relevance of what they are learning. STEM connections are also integrated into each Performance Assessment.

GET FOCUSED

The Focus Questions in each Investigation are designed to help teachers and students focus on the important concepts. By the end of the CELL, students should be able to answer the following questions:

Investigation 1:

• What is the relationship between the absorption and transmission of light through transparent substances? There is an inverse relationship between the amount of light that is transmitted versus that which is absorbed as light interacts with a transparent liquid.

Investigation 2:

• How does light wave interact with objects that reflect light? When a light wave interacts with an object that reflects light, the angle at which the wave is reflected off the object (the angle of reflection) is equal to the angle at which it encounters the object (the angle of incidence).

Investigation 3:

• How does wavelength affect the perception of light? We perceive the object or medium as being a particular color because those wavelengths are the ones detected by our eyes.

Investigation 4:

• How does a change in mediums affect the wavelength of light? When light passes from one medium to another, the speed of the wavelengths change. This causes the light wave to change directions. This process is called refraction.

Note: These are succinct responses to the Focus Questions and are placed here for easy reference. Fully developed responses to the Focus Questions can be found on each PostLab page.

Note: Some questions may be revisited as the CELL progresses. As students acquire additional knowledge, their responses should reflect this.

LEARN THE LabLearner LINGO

The following list includes Key Terms that are introduced within the CELL. These terms should be used, as appropriate, by teachers and students during everyday classroom discourse.

Note: Additional words may be bolded within the Backgrounds. These words are not Key Terms and are strictly emphasized for exposure at this time.

Investigation 1:

• Light wave: a wave of energy that has crests and troughs that repeat
• Wavelength: the distance between two adjacent crests or two troughs of a transverse wave
• Nanometer: metric unit of measurement, 1,000,000,000 nanometers is equivalent to one meter
• Absorbance: to take light in and not reflect or refract it
• Transmittance: the ability of light to pass through a medium

Investigation 2:

• Reflection: the bouncing of light off objects
• Law of Reflection: a physics principle that states that the angle of reflection of light equals the incident angle of light
• Angle of incidence: the angle at which light encounters a substance
• Angle of reflection: the angel at which light is reflected from a substance

Investigation 3:

• Transparent: the property of matter that allows light to pass through it
• Opaque: the property of matter that prevents light from passing through it

Investigation 4:

• Refraction: the bending of light waves as they travel between media with different indices of refraction
• Index of refraction: a measure of the degree to which a medium refracts light; the speed of light in a vacuum divided by the speed of light in a medium

BE PREPARED

An overview of the materials for each lab is placed here for easy reference. Specific teacher preparation for the labs is placed at the beginning of each Lab page.

EXTEND YOUR THINKING

The following information is included so that teachers have additional background knowledge pertaining to the concepts introduced in the CELL. Teachers may choose to use this information enrich students during instruction, but this is optional and not necessary for the intended students’ learning outcomes.

Visible light, though easy to observe, constitutes only a small portion of a larger electromagnetic spectrum of energy produced by the Sun and other sources. Several theories exist as to the nature of this energy, one of which describes electromagnetic energy and visible light as a wave.

When describing the wave nature of electromagnetic energy, two properties of waves, wavelength and frequency, are often considered essential for understanding how waves interact with different objects or media. The wavelength of a wave is defined as the distance between two adjacent crests or troughs of the wave whereas the frequency of a wave is defined as the number of crests or troughs that pass a given point every second. Both of these properties of a wave are related to the speed of the light wave.

Here v is the speed of light (in the vacuum, this is defined as c), λ is the wavelength of the light and f is the frequency. The speed of light in any medium is always a constant. Thus when a wave has a large distance between crests or a long wavelength, the number of crests that occur every second or its frequency is low. Conversely, when a wave has a small wavelength, it has a higher frequency.

Differences in electromagnetic energy result from differences in the wavelength or frequency of waves. These differences produce what is referred to as the electromagnetic spectrum, or energy released by the sun or other sources. The electromagnetic spectrum consists of waves of different wavelengths and therefore different properties. The wavelengths in the electromagnetic spectrum range from greater than 10⁵ m (long wavelength) to less than 10^-15 m (short wavelengths). The portion of the spectrum that we can see is termed visible light. Although visible light is the only range of wavelengths perceived by the human eye, all of the effects of the electromagnetic spectrum are real and significant. For example, the ultraviolet region of the spectrum is centered at a wavelength of approximately 10^-8 m. Although this region of the spectrum is not visible, it is responsible for producing sunburn in unprotected skin. The infrared region of the spectrum is centered at a wavelength of approximately 10^-4 m and is sensed by our bodies as heat.

Within this Core Experience Learning Lab, students will focus their attention and investigation on the visible light portion of the electromagnetic spectrum. Just as the electromagnetic spectrum represents energy with a range of wavelengths, what we refer to as visible light also represents energy with a range of wavelengths. This range of energy wavelengths is referred to as the visible spectrum and includes waves with wavelengths that range from 4 x 10 ^-7 m (400 nm) to 7 x 10 ^-7 m ( 700 nm). Visible light from the Sun and other sources is often perceived by the human eye as white light. When the visible light is dispersed by a glass prism it can be observed to consist of the continuous spectrum of waves that differ in their perceived color. We perceive these different wavelengths of energy as different colors and shades of colors. Although the visible spectrum seamlessly changes from one color to the next as the wavelengths of the waves change, the colors that are typically identified are simply those that are easily recognized as discrete colors by the human eye.

As light interacts with different media, it can be refracted or bent, reflected or bounced back, absorbed, transmitted, scattered, or diffracted. The visible effects of its interaction often depend upon the combination of these phenomena. This Core Experience Learning Lab focuses on the absorption, transmission, and refraction of light through transparent media and the reflection and absorption of light by opaque objects.

Certain wavelengths of light can be absorbed and transmitted by transparent materials. The extent to which each wavelength is absorbed or transmitted through a transparent substance can be determined by using a piece of equipment called a spectrophotometer. Simply put, a spectrophotometer uses a light bulb to produce white light. A reflective diffraction grating and a slit within the apparatus can be used to isolate specific wavelengths of light from that white light. Sensors within the spectrophotometer analyze the intensity of each wavelength of light emitted from the source. When a sample of transparent liquid is placed between the light source and sensors, the sensors again determine the intensity of light. By comparing the intensity of light that reaches the sensors between the two conditions, the amount of light transmitted or absorbed by the sample can be determined.

Some transparent materials, such as clear window glass, are transparent to all wavelengths of visible light. That is, the glass transmits essentially all visible wavelengths of light, absorbing only a miniscule amount. Other transparent materials are transparent to only certain wavelengths of light in the visible spectrum. Colored glass, for example, allows only a particular wavelength of the visible spectrum to pass through it, absorbing all of the other wavelengths of light. For example, a traffic light is illuminated from behind by a light bulb that produces all the wavelengths of the visible spectrum. When the red glass of the traffic light is lit from behind by the bulb it appears red because the pigments incorporated into the glass absorb all the wavelengths of visible light except the wavelengths around 700 nm that appear as red light and which are transmitted. Differences in the absorption and transmission of light also explain the differences we perceive as the shade of a color. In general, the greater the absorption of a particular wavelength region of transmitted light is, the darker the shade of the substance will appear.

A second way in which light interacts with transparent media is refraction. Refraction is described as the bending of light waves. Refraction of light occurs whenever light travels from one transparent medium into another. Because light waves travel at different speeds through different media, the light waves are bent as they move from one medium to another. For example, light travels close to 299,700,000 meters/sec in the air, but slows to about 225, 400,000 meters/sec in water. Light waves slow as they enter the water and bend or refract. As a result, if we were to view an object at the bottom of a pool or lake through the water, it would appear different than if it were viewed in air, outside of the water.

The diagram below shows a light ray as it moves from air into water, a different medium. The angle at which the light ray enters the water is called the angle of incidence. The angle at which the light ray is refracted in the water is called the angle of refraction. The dotted line in the diagram is a reference line that is drawn perpendicular to the surface of the water. The angle of incidence is the angle between the perpendicular line and the ray as it enters the water. The angle of refraction is the angle between the perpendicular line and the light ray in the water.

Different transparent media or substances refract or bend light to different degrees. A measure of the degree to which a medium refracts light is indicated by a value called the index of refraction, the speed of light in a vacuum divided by the speed of light in the medium. The index of refraction of air is 1.00029 indicating that light travels almost as fast in air as it does in a vacuum. The extent to which light is refracted as it passes from one medium to another depends upon the differences in the indices of refraction of the two media. As light travels from a medium with a lower index of refraction to one with a higher index of refraction, the greater the difference between the two indices of refraction, the more light will be refracted or bent. The relationship between the angles at which light enters the two media and the indices of refraction of each medium is described by Snell’s Law:

Application of Snell’s Law permits the extent of light refraction between two media to be determined. In general, when two media are compared the relationship between refraction and the index of refraction can be stated as follows: the greater the index of refraction of a medium, the greater the refraction of light.

When the interaction of light with transparent materials is compared to that of opaque materials, we observe that light is absorbed but not transmitted or refracted by opaque materials. In addition, light is also reflected off of opaque objects. In terms of perception of color, it is the relationship between the absorption and reflection of different wavelengths determine color identification. An object appears as a certain color because of the wavelengths of light reflected off of the object. For example, a blue book appear blue because the pigments incorporated into the cover absorb all wavelengths of visible light except the 400 nm wavelength of blue light.

The way in which light is reflected off of objects, however, is best described by the Law of Reflection. The Law of Reflection states that the angle at which light is reflected off of an object is equal to the angle at which it encounters the object. In other words the angle of reflection of light equals the angle of incidence.

As students perform experiments within this Core Experience Learning Lab, they will encounter four different ways in which light interacts with transparent and opaque substances. Understanding these interactions provides a basis for future investigations into the nature of light and for understanding many of the natural phenomena within our world.