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The Properties of Light and Color What we see and how we see it! After reading this chapter summary of the properties of light and color, we hope that you will understand the following objectives: 1. Why light is important in our world. 2. Describe the difference between luminous and non-luminous objects. 3. Represent light as a ray diagram. 4. Explain how we see light. 5. Define and classify objects as transparent, translucent, and opaque. 6. How a light source affects the type of shadow created. 7. Define and explain umbra and penumbra. 8. Predict how a pinhole will work. 9. Identify the primary colors of light and pigment. 10. Explain how primary colors combine to form all colors. 11. White light is all colors combined and black is the absence of light. 12. Explain how we see colored materials. 13. Use the Electromagnetic Spectrum to identify the most and least energetic waves. The following experiments should be conducted in order to reinforce your understanding of light and color: 1. The Laser Lab: Experimenting with lasers in several situations help you to understand the properties of light and how light behaves. Comparing light bulbs in flashlights to laser pointers and how they differ in different situations. You learn how to draw ray diagrams for light, and learn that light is only visible when it reflects off another object. By drawing these ray diagrams you learn how light spreads out in all directions and that light travels in a straight line. One experiment with laser pointers is spraying water into the air, and shining the laser into the mist. You’re able to see bits and pieces of the red beam because it’s reflecting off of the water in the air. This teaches you that light is not visible without material to reflect off of. The one major safety precaution that we focused on in this lab was safety with lasers, and ensuring that we wouldn’t shine a potentially blinding laser into somebody’s eye, the rule being to never shine a laser in the direction of an individual. 2. Pure water vs. Cloudy water This experiment explores the idea of light going through interference. By exploring how light acts when it interacts with pure water vs. cloudy water, we are able to understand how light behaves in these situations. When interacting with pure water, light passes through with extremely limited alterations, while when interacting with cloudy water, some of the light beams reflect off the dust and cloudiness from within the water, with less light passing through to the other side. This experiment’s main safety precaution is that the students must be careful and not drop the light (whether it is a flashlight or a bulb) in the water, with the danger of electrocution occurring. 3. Light and Slits In this experiment, students learn more properties of light, and how shadows behave. Using slits, we can see how light can be filtered by slits, and how slits can restrict the amount of light to pass onto a surface. While a point light source would normally send light rays in all directions, creating a large spot of light on a surface, the slit allows only a small fraction of the light from the point source to hit the surface, creating a small dot of light. Slits also impact the amount of light that went through from an extended source. You learn that slits invert the shape of the filament from an extended light source. 4. A Light on the Wall When you experiment with light on walls, you learn how light spreads out in all directions. When you use a point source, it’s not just a point of light that shines off the wall; it’s an expanded illuminated circle. Also, in the circle the center of the circle is the brightest part and as you move away from the center the wall becomes less and less bright. 5. Color filters When experimenting with color filters, we learned about properties of light and color, such as the primary colors of light and primary colors of pigment. We experimented with the three different colored light bulbs (red, blue, and green) and held up corresponding colors to see what we would view through them. We discovered many different properties and how light acts with filters, and we learned how different colors act in different situations. Some safety precautions consisted of making sure you don’t drop a light bulb and not looking directly into the light. --------------------------------------------------------------------------------------------Every-day experiences that one can observe properties of light from are: 1. Green color reflected from grass and leaves into our eyes. 2. Light rays from the sun illuminating the Earth. 3. Lengths of shadows, and different shades of shadows. 4. Prisms and how they split up light to form rainbows. 5. Lamps illuminating an entire room. 6. Clouds blocking sunlight from passing onto parts of the earth. LIGHT – GETTING UP CLOSE AND PERSONAL One of the most important skills necessary in order to become proficient in understanding light is being able to draw ray diagrams, and model how light travels to understand how we see colors. Ray diagrams can be used to show various aspects of light. One of the most important is how they depict how we see light. We can use ray diagrams to show how light rays go in a straight light from a light source to our eye, allowing us to see the object. With these ray diagrams, we can model how light travels, whether it is reflecting off a surface into our eye, or coming straight from a luminous object, ray diagrams show us how we see light by showing how light moves. We use ray diagrams to show the motion of light because when drawing ray diagrams, you always draw rays in a straight direction with an arrowhead at the end, explaining that the light is going on continuously. By using ray diagrams we can also show how we see various colors, by drawing a ray and showing how it splits into different colors (from white light) into different colors after interacting with an object with colors of pigment. Rays can also be used with prisms, to show how light splits up into many different colors due to many different frequencies. Ray diagrams are drawn by starting from a point, whether it’s on a point source or an extended light source, and drawing rays extending in all directions from the light source, since light travels in all directions. Always ensure that your rays are straight and they always have arrowheads on the end of them, as well as making sure that they are extending in all directions. Pinholes are extremely important objects that can be used to further understand light. Pinholes allow only a certain amount of light to travel through it and reflect off of the wall behind it. A pinhole doesn’t allow a lot of light through it, but, rather just a small amount. When shining a light source onto a pinhole, the light reflected off will be an expanded version of the light source, but inverted. For example, if you shine a light bulb with an “L” shaped filament, the shape of light reflecting off of the wall behind the screen after it filters the light will be the shape of an “L” but rotated 180 degrees. There are two different kinds of light sources, point sources and extended sources. When using a point source, all light comes from a single point, rather than when using an extended source which exerts light from extended points on the extended source. Shadows created from a point source have just one shade of shadow with the shadow being of complete darkness, while extended light sources cause shadows to form with umbrae and penumbrae. Umbrae and penumbrae are formed when an extended light source shines on an object while there is another object in between the two. The umbra is the area of complete darkness, or the absence of light, where absolutely no rays from the light source are able to reach. The penumbra is formed by the areas of shadow, but not complete darkness or complete exposure to light. Penumbrae are formed when only part of the light source is capable of reaching the area. Whenever the entire light source is unable to shine, but at least a small trace of light is capable of reflecting off, the area is called a penumbra. Also, obviously, wherever the entire light source is able to shine on, with no interference, the area is a region with no shadows. Objects on the world can be categorized in many different ways. Different categories consist of luminous, non-luminous, transparent, translucent, opaque, specular, and diffuse. Luminous objects omit light while non-luminous objects don’t, and just reflect light. An example of a luminous object is molten rock, or simply a light bulb, while an example of a non-luminous object is a desk, or a plate, which doesn’t generate its own light, but reflects light, no matter how shiny it may seem. Transparent objects allow all rays of light to pass through it, all direct and indirect rays. Translucent objects only allow some light rays to pass through it, allowing viewers to see the general shape of the object, while opaque objects allow absolutely zero light rays through it. Examples of transparent, translucent, and opaque objects are a window, wax paper, and a rock, respectively. Finally, specular objects are mirror-like objects that are mainly flat and allow light rays to reflect smoothly off it, creating a clear image, while diffuse objects are microscopically rough, and reflect light rays in all directions. The primary colors of light are red, green, and blue, while the primary colors of pigment are magenta, yellow, and cyan. The secondary colors of light and pigment are the same as the primary colors of the opposite, so the secondary colors of light are the primary colors of pigment, and vice-versa. The primary colors can be used to create all different shades of color, depending on the variety of intensities and concentrations of color. More red and less blue or more green and less red can allow light to create all the different colors of the spectrum. We are able to see colored objects when white light shines on them because the object absorbs certain colors while the other colors are reflected off of it. For example, a cyan book would absorb the red light and reflect the blue and green light, allowing the viewer to see cyan. Color filters play an important role in what colors we see. A filter of any color allows only rays of light of the same colors through it, and doesn’t allow any other colors through. If you were to hold up a red filter, all white objects would seem red, since the filter would absorb the green and blue light, while all red objects would stay red, and all other colored objects would seem black, since the red filter would absorb the green and blue light, while only red light would be allowed to pass, however there would be no red light so the object would seem black due to the absence of light. The electromagnetic spectrum is a tool that scientists use to compare different colors of light, using their frequencies to set them apart. You can tell how the spectrum is set up simply by using a prism and shining white light through it, and seeing how the colors appear when they refract and go through the prism. Red is the fastest, or most active, out of the colors in the spectrum, so it is bent the most, while violet is the slowest, or least active, of the colors in the spectrum, so it bends the least. To interpret ray diagrams there are a few key things you must take into consideration. First you must look to see that light is traveling in all directions. Second you must see if the light rays are traveling in a straight direction. And third you must determine where light is reflected, and where light doesn’t reach. By determining these factors, you can understand what we see in the world and how we see it. When faced with problems that have to do with ray diagrams, first think the problem through. After coming up with a simple plan of how to go about the problem-solving, construct a ray diagram of what you think is happening in the system. Use rays and pictures to determine the answer to what the question is asking you, because all that you could ever be asked can be determined by drawing a ray diagram. Remember, a ray diagram is a scientist’s best friend, because even though human error exists, ray diagrams are too simple and direct to mess up! Referring to the beginning objectives of this chapter, we shall review what we have learned after going over several different aspects and properties of light. We’ve learned: 1. Why light is important in our world. Light is important in our world today because without it, we wouldn’t be able to see anything or even exist on earth. 2. Describe the difference between luminous and non-luminous objects. As previously mentioned, luminous objects omit light while non-luminous objects do not. 3. Represent light as a ray diagram. We’ve constantly reviewed how to represent light as a ray diagram, the following questions will also help you to understand and view how ray diagrams work. 4. Explain how we see light. We’ve explained how light reflects off objects and into our eyes allowing humans to see. 5. Define and classify objects as transparent, translucent, and opaque. We’ve defined transparent objects to allow all light rays through, translucent objects to allow some light rays through, and opaque objects to allow no light rays through. 6. How a light source affects the type of shadow created. We’ve learned that a point source creates one shade of shadow, with complete absence of light or complete absence of shadow, while extended light sources create umbrae and penumbrae. 7. Define and explain umbra and penumbra. We’ve defined an umbra as an area of complete absence of light and a penumbra as an area of some light and some shadow. 8. Predict how a pinhole will work. We’ve understood how pinholes flip the light shining through it to rotate it 180 degrees, so either the dot from a point source or the filament shape from an extended source will be rotated and shown on the wall behind the pinhole. 9. Identify the primary colors of light and pigment. We’ve identified the primary colors of light as red, blue, and green, while primary colors of pigment are magenta, cyan, and yellow. 10. Explain how primary colors combine to form all colors. We’ve understood how the primary colors of light are the primary colors of pigment combined in certain combinations, and vice-versa, while different concentrations of specific colors influence the colors created. 11. White light is all colors combined and black is the absence of light. We’ve explained how white light is the combination of all colors and how it can be split up into several different primary colors of light while we see black when no light is reflecting off an object or all the light is absorbed. 12. Explain how we see colored materials. We’ve understood that we see colored materials when certain objects absorb certain colors and reflect others, which dictate which colors we see after the light rays reflect off the material. 13. Use the Electromagnetic Spectrum to identify the most and least energetic waves. We’ve identified the rainbow as an easy way to remember the Electromagnetic Spectrum as well as prisms and we’ve identified that red is the most energetic and violet is the least energetic. This concludes the written proportion of the review of the properties of light and color. The upcoming review section will help refresh your mind allow you to interact with these topics yourself. Questions will all be provided followed by all the answers. 1. Draw a ray diagram from the point source onto wall. 2. Draw a ray diagram from the extended source onto the wall. Label the shadows 3. Draw a ray diagram from the extended source onto the wall. Label the shadows 4. What happens as the light travels from the point source through the pinhole? 5. Is this an accurate illustration? What is happening to the shape of the filament? 6. What color is the blue ball through the red filter? 7. What color is the yellow ball through the green filter? 8. What color is the blue ball through the blue filter? 9. Explain why there are the different areas of shadow. 10. What is being illustrated? Why is this event occurring? 11. What would you consider the sun due to this ray diagram? 12. Which bulbs can each person see? 13. 14. Why shouldn’t you do so? 15. 16. ANSWERS!!! 1. 2. Top to bottom: penumbra, umbra, and penumbra 3. Top to bottom: penumbra, umbra, and penumbra 4. The light expands to form a larger box compared to the size of the pinhole. 5. The illustration is correct; the shape of the filament is rotated 180 degrees 6. The ball is black because no light is able to travel through the filter 7. The ball is black because no light is able to travel through the filter 8. The ball is blue because the blue light is able to pass through the filter 9. There are different areas of shadow because the light source is an extended source so some areas receive all light; others only receive some light, while others receive no light. 10. The Electromagnetic Spectrum is being illustrated and this is occurring because different colors of light have different frequencies, so white light splits up when it refracts through a prism. 11. Even though the sun seems like a point to us on Earth, it’s still an extended light source. 12. Jason can see A and C, Joy can see A, B, and D, Bob can see A and C, and Xavier can see A and C. 13. The moon is not a luminous object. It may seem like one at night, but it’s only reflecting the sun’s light. 14. You could see a dot from the laser all around the walls, or you could step into the laser beam. You wouldn’t do this because you could severely damage your eyes. 15. Persons 2, 4, and 5 would be able to see it. 16. No matter what, whenever you’re in a pitch-black room, you will never see anything