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# Introduction

#### Light: Introduction

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. More specifically, they are typically described as sinusoidal waves. A model sine wave is shown below:

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.

The speed of light in any medium is always a constant. Thus when a wave has a large distance between crests of a long wavelength, the number of crests that occur every second or its frequency is low. Conversely, when a wave has a short 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. 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 (100 – 400 nm on the illustration below). Although this region of the spectrum is not visible, it is responsible for producing sunburn on unprotected skin. The infrared region of the spectrum is centered at a wavelength of approximately 10,000 nm and is sensed by our bodies as heat.

Within this CELL, you will focus your 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 (below) and includes waves with wavelengths that range from about 400 nm to about 700 nm.

Visible light from the Sun and other sources is 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.

Interaction of Light with Other Objects

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 depends upon the combination of these phenomena. This CELL 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, like the one pictured here. 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. You will use the spectrophotometer in the lab and become quite familiar with the instrument.

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 minuscule 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 through the glass. 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. The 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 (at a right angle) 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:

The Greek letter theta (𝜭) simply indicates an angle, while the subscripts i and r indicate the angle of incidence and refraction, respectively.  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. You will perform refraction experiments and use Snell’s Law in the lab.

Law of Reflection

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 to 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 appears blue because the pigments (certain molecules) 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 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. In mathematical terms, this is expressed as follows:

Once again, the Greek letter theta (𝜭) indicates an angle. However, the subscripts i and r in the case of the Law of Reflection indicate the angle of incidence and reflection, respectively.

The Law of Reflection comes into play all of the time in everyday life, sometimes in the most beautiful and dramatic ways, as in the way light is reflected off the surface of water in nature.

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Light reflection can sometimes play tricks on us. For example, how many girls do you see in the photograph below (don’t scroll down yet for the answer)? It was taken and posted on Instagram by the Swiss photographer Tiziana Vergari.

Obviously, there is at least one mirror involved in capturing this image. Can you figure out how many mirrors there must be and where they are located in relation to the girls? Also, can you guess where the camera would have to be to get this picture? You might try this with your family or friends and see what they guess. On Instagram, serious guesses ranged from thirteen to only two girls. Actually, there are only two girls in the picture! The schematic drawing below represents the approximate setup of the shot.

As you perform experiments within this CELL, you 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. As you learn new information in this CELL, always try to relate it to observations you see in the world around you. When you do this, the science becomes more interesting and understandable!

#### Home Science: Experiment with Hand Lens

As discussed above, sunlight consists of an entire spectrum of wavelengths and colors. Visible light, however, appears to be white. It seems hard to believe that adding all of the wavelengths together produces a white light. Shown here is a picture of the three primary colors of light used in electronics. Notice that where all three primary light waves overlap, white light results.  If you are not convinced of this, you can do an experiment for yourself to prove it. All that you need to do the experiment below is a hand lens (the higher the magnification the better, a jeweler’s loop is best) and a television or computer screen. We used a simple cell phone to photograph the images that are shown below.

When the primary light colors are viewed on a computer screen, probably similar to the one you are viewing right now, they appear to our naked eye as bright, solid colors. However, see what happens when you get close to the screen surface with the hand lens. At higher magnification, you will be able to see the small pixels that make up the image. A full-HD screen, for example, is 1,920 by 1,080 pixels = 2,073,600 pixels. The more pixels, the higher the resolution.

Notice that when you magnify the solid blue, red, or green, you see primarily that color of pixels. However, when looking at the white section of the overlapping circles (lower right), you see all three of the colors, blue, red, and green! What do you see when you look at the areas where only two primary colors overlap? You can look for yourself and then learn more by reading the right-hand column of this page under Color: Additive and Subtractive Color Mixing.

Below is an additional simple demonstration where the same technique is applied to natural mixtures of colors. Moving from the upper left to lower right, the same spot on the monitor displaying an image of a dish soap bubble in the Sun. All of the colors on the screen are a combination of the three basic light colors of blue, red, and green.

# CONTENT

• Fun Facts
• Learn the Lingo
• Get Focused

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## Newton’s Prisms

Using a triangular prism of glass, Sir Isaac Newton separated pure sunlight into its component wavelengths of color. The prism works by refracting light that enters it. Refraction is a central concept that you will learn a lot about in this CELL. Light is refracted when it moves from one medium, air for example, into another, glasses in the case of Newton’s prism.

You will see in your experiments that refraction bends light rays. In the most spectacular way imaginable, Newton found that not all wavelengths of light are refracted (bent) to the same extent. Consequently, bright, “white” sunlight dramatically separated into bands of separate colors. Today, we know the precise wavelengths of all of the colors in the visible spectrum of light from the Sun.

The scientific implications of the finding that sunlight is composed of a “rainbow” of different colors were enormous. Not only were the colors separated, Newton noticed that they were always separated and arranged in the same order of colors. Reds, oranges, yellows, greens, blues, and violets were separated from each other because each has unique wavelengths. Red has the longest wavelength and violet has the shortest. The other colors are intermediate in wavelengths between these two ends of the visible range. The prism is able to so nicely separated the light colors because the glass refracts each wavelength of light to a different degree.

As you will see throughout this CELL, the knowledge and understanding of wavelengths of visible light has impacted science in a variety of ways. From telescopes to microscopes, scientists have used refraction in designing instruments that let them see what was formerly entirely invisible.

Was Newton the first to witness a light spectrum? Not really, anyone that has ever seen a rainbow has witnessed a prism-like color spectrum caused by the refraction of sunlight by suspended water droplets in the atmosphere. You can probably think of many instances where the interplay of light and other materials produces spectacular results. Below are just two examples of how light can be made to separate into its component wavelengths and colors to our delight. The top video shows white light refracting through the facets of a diamond. The second video is much less expensive but hardly less beautiful. It shows the surface of a common dish soap bubble refracting light through its surface.

## Color: Additive and Subtractive Color Mixing

Additive color is how we see light itself. Colored light wavelengths are directly detected by our eyes and interpreted by our brains. In the short experiment near the bottom of the left column of this page (Home Science: Experiment with Hand Lens), we discussed additive color mixing. Why is it called additive? It is because as you add light of various wavelengths together, the wavelengths “add” up and we see a combination of the wavelengths. When we add all of the wavelengths from the entire visible spectrum together, our eye is exposed to all of them and we see white light. This is the way that all of the colors on a computer screen are created.

Subtractive color, on the other hand, permits us to perceive only wavelengths that are reflected off other objects. Color mixing of paints is subtractive. This is because each paint color absorbs all wavelengths of light except those that it reflects. That’s why we can see the color – because light waves are reflected from the paint surface to our eye. However, all of the other wavelengths of light that strike the object are absorbed and therefore not reflected and available for us to see.

Take the color red, for example. We see the color red because red wavelengths (in the 700 nm range) are reflected from the paint. All other wavelengths in the visible range are absorbed. Now, mix in a second color, blue for example. Blue paint reflects blue wavelengths (in the 470 nm range) but absorbs all of the other wavelengths in the visible spectrum. That means that blue absorbs red wavelengths as well. Therefore, when red and blue are mixed together, only red and blue wavelengths are reflected and perceived by our eye. All of the other colors are subtracted. That is why, when we add all of the three primary colors together (red, blue, and yellow), the mixture turns a muddy black. Combined, the primary colors act to absorb almost all wavelengths in the visible spectrum, leaving little reflected wavelengths for us to perceive. As a result, we see black, the absence of light!

#### LEARN THE LABLEARNER LINGO

The following list includes Key Terms that are introduced within the Backgrounds of 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 Background(s). These words are not Key Terms and are strictly emphasized for exposure at this time.

##### Investigation 1:
• Chemical reaction: occurs when chemical reactants are converted into chemical products
• Reactant: a chemical compound that is consumed in a chemical reaction
• Product: a chemical compound that is produced in a chemical reaction
• Formula: a description of a chemical compound using the letter designations of the elements
• Chemical equation: Describes a chemical reaction by indicating the formulas of all reactants and products
• Yield: the amount of a product produced by a chemical reaction compared to the amount of reactant
• Calorimeter: an insulated container that prevents a chemical reaction from gaining heat from its surroundings or losing heat to its surroundings
##### Investigation 2:
• Chemical bonds: the forces between atoms that hold those atoms together to form compounds
• Atoms: the smallest particle of matter that still retains the properties of an element
• React: when reactants interact to form products – It always requires the breaking and reforming of chemical bonds
• Matter: anything with mass and volume
• Law of Conservation of Matter: matter cannot be created nor destroyed
• Consumed: when a reactant is converted to a product
• Unreacted: when a reactant is not converted to a product
##### Investigation 3:
• Rate: the quantity of reactants consumed or the quantity of products produced during a specific time period
• Wavelength: the distance between two adjacent crests or two troughs of a transverse wave of electromagnetic radiation
• Nanometer: wavelength is usually expressed in nanometers – A nanometer is a metric unit of length that equals 10-9 meters. 1,000,000,000 nanometers is equivalent to one meter.
• Spectrophotometer: an instrument that can quantitate the amount of light of a specific wavelength that is absorbed by a chemical compound
• Absorbance: the ability of a chemical compound to take in electromagnetic radiation of a specific wavelength – The energy of a specific wavelength, when absorbed, is dissipated in the electrons of the chemical compound.
• Catalyst: a chemical that accelerates a chemical reaction without being consumed in the reaction

#### 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?
##### Investigation 2:
• How does light wave interact with objects that reflect light?

##### Investigation 3:
• How does wavelength affect the perception of light?
##### Investigation 4:
• How does a change in mediums affect the wavelength of light?