Science Summary on Light

Transcription

1 Science Summary on Light Lauren Murray SME 301, Section 3 Misconception Incorrect: We see objects only if they reflect light. False. Correct: We can see objects that do not reflect light, but only if they appear in contrast to another object that does reflect light. True. Content Expectation that relates to the misconception Discipline Physical Science Standard Properties of Matter Content Expectation Explain how we need light to see objects: light from a source reflects off objects and enters our eyes. Activity Materials: Directions: Several sheets of red cellophane Flashlight Rubber band Printed sheet with red, green, and blue dots 1) Stack the sheets of cellophane together and use them to cover the light end of the flashlight. Hold in place with the rubber band 2) Go into a room that is in complete darkness so that the only light that appears in the room will be the light of the flashlight 3) Turn on the flashlight and shine it on the printed paper with the dots. Observe what you see on the paper 4) Compare what you saw on the paper in the dark room to what is seen in a normally lighted room Results o The white parts of the paper appear completely red o The red dots on the paper cannot be seen at all o The green and blue dots all appear black. Credit: Tim Holcombe in class for SME 301 Explanation When the red cellophane, a transparent material, covers the flashlight the light from the flashlight appears red because the cellophane absorbs all wavelengths of white light that comes from the flashlight except for the red wavelength. This wavelength is transmitted through the cellophane making the light appear red. The white paper also appears red because the color white is a reflection of all wavelengths of light together, and therefore is able to reflect the red light

2 transmitted onto it. The red light is able to completely cover the white paper because no other wavelengths of light are visible in the darkened room. The red dots cannot be seen because they are reflecting the same wavelength of light that is being transmitted onto them. Since the red light is the only light in the room, when it hits the red dots on the white paper the only wavelength of light that is reflected back is red. There is no contrast between two different colors caused by multiple wavelengths of light so the colors cannot be distinguished from one another. The green and blue dots both appear black in the red light because they absorb the red light and have no other wavelength of light to reflect. A green dot appears green in white light because when the light hits it it absorbs all wavelengths except for green, which is reflected. The same occurs for blue, but the blue wavelenght is reflected. The color black occurs when all wavelengths of light that hit an object are absorbed. We are used to seeing this occur in white light when a black object appears that color because it absorbs all wavelengths of white light. In the case of this activity, the green and blue dots can appear black because red is the only wavelength of light reaching them. They absorb it, and because there is no other light in the room all light is absorbed and none is reflected. Yet even though the color black is completely absorption of light we are still able to perceive it because of the contrast to other colors around it, which are reflecting wavelengths of light. The dots on the paper can be seen as black because the paper surrounding them is reflecting red light and providing contrast. This explains my misconception because while I knew that the reflection of light off objects allows us to see them, I forgot about the fact that objects that are black reflect no light yet they can still be seen when placed in contrast to another object that is reflecting light. By isolating the light in this activity to one color (or one wavelength), I was able to see the difference between reflection of some wavelengths of light (viewing the paper in normal white light) and when all available wavelengths of light are absorbed (in the dark room with only red light) and how that relates to contrast. Big Idea #1- Light is an electromagnetic wave caused by the vibration of high energy electrons that then travels in a straight line until it hits an object of matter. Electromagnetic waves are a combination of electric and magnetic fields, running perpendicular to one another. This is a type of transverse wave, in which the motion and direction of the wave are at right angles to each other. These waves are caused by the vibration of electrons in both electric and magnetic fields (hence the name electromagnetic). These electrons will move to higher energy levels around the nucleus of the atom, releasing a particle of energy called a photon. These electromagnetic waves occur at a variety of frequencies and are often organized into a spectrum to illustrate the difference in their uses at different frequencies. Visible light occurs at a specific range of frequencies in the electromagnetic spectrum. In the electromagnetic spectrum the lowest frequency waves are radio waves, followed by microwaves, infrared radiation, visible light, ultraviolet radiation, x-rays, and gamma rays at the highest frequencies. All electromagnetic waves, including light, travel in straight lines until they hit an object of matter. Light cannot bend around an object. A shadow is a dark outline of an object that blocks some rays of light from reaching the surface beyond it. All electromagnetic waves,

3 including light can travel without the prescence of matter. The vibration of electrons and release of photons can occur even in a vacuum such as outer space. Related GLCEs P.EN Identify light as a form of energy P.EN P.EN Demonstrate that light travels in a straight line and that shadows are made by placing an object in a path of light. Describe how waves are produced by vibrations in matter. Big Idea #2- The amplitude of a light wave, which is effected by the amount of energy carried by the wave, effects the perceived brightness of the light wave, and the wavelength of a light wave has an effect on the perception of color. Every wave has certain properties, including wavelength, amplitude, and frequency. The wavelength is the distance from one wave to the next, usually measured from the crest of one wave to the crest of the next wave. The frequency of a wave is measured as the number of wavelengths to pass a certain point in a given period of time. If the frequency of a wave is high, many wavelengths are passing in a given period of time, therefore the wavelengths are very short (there is little distance between each wave). If the frequency of a wave is low, fewer wavelengths are passing in a that same given period of time, therefore the wavelengths are longer (there is a longer distance between each wave). As mentioned in Big Idea #1, the size of the frequency of an electromagnetic wave effects its type and its use. Low frequency electromagnetic waves such as radio waves have very long wavelengths. High frequency electromagnetic waves such as x- rays and gamma rays have very short wavelengths. Visible light occurs over a range of wavelengths in the electromagnetc spectrum and the color of light is determined by these different wavelengths. The spectrum of colors of visible light is in the order red, orange, yellow, green, blue, indigo, and violet. Colors of light toward the red end of the spectrum have longer wavelengths (lower frequencies) and colors toward the violet end have shorter wavelengths (higher frequencies). Another property of waves if their amplitude, which is the distance measured from the midpoint of the wave to the top of its crest. In other words, amplitude is the height of the wave. Amplitude is determined by how much energy a wave is carrying. Waves with a large amount of energy will have large amplitudes and waves with a smaller amount of energy with have a smaller amplitude. The amount of energy carried by a light wave will determine its brightness. A very bright light will have more energy and its waves will have large amplitudes. A dimmer light will have less energy and its waves will have smaller amplitudes. Related GLCEs P.EN Describe how waves are produced by vibrations in matter. P.EN Identify light as a form of energy

4 Big Idea #3- When interacting with matter light can either pass through matter (transmission), be reflected by it, or be absorbed by it. When light is transmitted by an object of matter that object allows the light waves to pass through it, transferring energy from one molecule to the next. This can occur in solids, liquids, and gases. If an object is transparent, the light wave will travel through the object and beyond it in the same straight line that it entered the object in. Window glass is transparent because it allows us to see the objects on the other side of the window as clearly as we would see them without the window there. A translucent object diffuses that pass through it into many different directions. Looking through a translucent substance such as wax paper makes the image on the other side appear blurry because the light is split into all different directions as it passes through. An opaque object does not transmit the light that hits it, and reflects or absorbs the light instead. The color that we see is a product of these differences between transmission, reflection, and absorption. Most of the light that we see including light from the sun and light from a typical light bulb is in a form called white light which is a combination of all wavelengths of the visible light spectrum. We see an object s color because when white light strikes that object it absorbs all wavelengths of light except for the specific wavelength of its color. If the colored object is opaque that wavelength is reflected and if the object is transparent that wavelength is transmitted. Only that wavelength enters our eyes and we see that color. For example, a book with a blue cover absorbs all wavelengths of light except for the wavelengths that corresponds to the color blue, that wavelength is reflected back to our eyes and we see the book cover as blue. The speed of light is slowed down when it interacts with matter so the more separated the molecules of matter are the faster the light will travel through it. Therefore, light travels fastest through a gas, then through a liquid, and slowest through a solid. This can be seen when a straw is placed in a glass of water. The straw appears to be bent when it enters the water because the light waves are slowed down and bent in a different direction when they pass from the air (a gas) to the water (a liquid). This is known as refraction. Interactions with matter can also cause light energy to be transformed to other forms of energy, most commonly heat energy. Due to the law of conservation of energy in which no energy can be created or destroyed only changed in form, light energy that is absorbed by matter is not lost, simply transformed into another form, usually heat. This occurs on a large scale in the process of heating the Earth when the Earth s solid surface absorbs light energy from the sun and radiates it out to the atmosphere as heat energy. The use of solar panels can also transform light energy into other forms of energy other than heat such as electrical or mechanical energy. Light energy itself can also be a product of energy transformations from other forms of energy, such as the electrical energy needed to light a lightbulb or the fusion reactions in the sun that transform nuclear energy into light energy. Related GLCEs P.EN Demonstrate what happens to light when it travels from water to air (straw half in water looks bent). P.PM Explain how we need light to see objects: light from a source reflects off objects and enters our eyes.

5 P.EN P.EN Demonstrate how waves transfer energy when they interact with matter. Illustrate how energy can be transferred while no energy is lost or gained in the transfer. Real World Examples Big Idea #1 Lampshades (Light travels in a straight line) Light from a source will travel in straight lines in all directions away from that source. However, objects such as lampshades can be used to aim the rays of light in a desired direction. The light will still travel in all directions from the source even with an object to guide its direction, but the sides of the lampshade will block the light from traveling further. Rays of light will be able to travel out the bottom part of the lampshade, aimed in a straight line in the direction it has been aimed in. Big Idea #2 A three-way light bulb (Brightness, energy, amplitude) Brightness of light is related to the size of the amplitude of the light wave, which is related to the amount of energy in the light wave. A three-way light bulb, which has three levels of brightness increasing from lowest to highest as the knob on a lamp is turned, shows this relationship between energy and brightness. Each time the knob is turned on the lamp more electrical is allowed to flow to the bulb. This electrical energy is transformed into light energy inside the bulb and the greater amount of energy that flows into the bulb the brighter the light. If the light waves could be slowed down so that the wave vibrations could be seen, the amplitude of the wave would be increasing each time the knob was turned and more energy was allowed into the bulb. Big Idea #3 Lights on a stage (Energy transformations when light interacts with matter) When performing on a stage under a set of many lights, it can usually be noted that this space is very warm. The heating that occurs on a stage is very similar to the way the Earth s surface is heated, but on a smaller scale. The bright, high intensity stage lights emit their energy down onto the stage to light the performance. This light hits the stage floor, which is usually black in color, and is almost all absorbed. (Black colors absorb all wavelengths of light.) This absorbed light is an interaction with the matter of the stage floor and is transformed into heat energy which is then is radiated up to the air above the floor, heating the air above the stage floor. For this reason, the floor of a stage and the air above it are usually very warm in comparison to the floor and air in the backstage area, which is usually kept unlit. Classroom Resource This activity is a variation that I developed based on the Big Idea Activity we did in class titled Earth Heats Unevenly, presented by Tim Holcombe. That activity showed how two cans of water will be heated at different rates based on the color of the can. This activity uses the same materials and the same principle of an energy transformation from light to heat when interacting with matter, however the different rates of heating here are varied using two lamps with light bulbs of different wattages.

6 Materials: Directions: Two cans, both painted the same color Two thermometers with foam supports to hold one in each can Two lamps, one with a 100 watt light bulb, the other with a 60 watt light bulb 1) Set up the two cans like in the activity illustrated on page 191 in the course pack, however make sure they are far enough apart so that each lamp only shines on its individual can 2) Take the temperature of the water before turning on the lamps. Record these temperatures. 3) Turn on the two lamps at the same time and let them shine on the cans for minutes 4) Observe which light bulb is brighter than the other and take note of its wattage 5) Take the temperature of the water after minutes and record these temperatures. The initial observation to be made in this activity should be that the 100 watt light bulb is brighter than the 60 watt light bulb. Big Idea #2 for light gives that the brightness of light is related to the amplitude of its light wave, which is related to the amount of energy carried by the wave. A 100 watt light bulb is brighter than a 60 watt light bulb, therefore its light wave has a greater amplitude, and this wave is carrying more energy. This will result in the can lighted by the 100 watt bulb being heated more in the same period of time than the can lighted by the 60 watt bulb because more energy is emitted in the form of light from this bulb which means that there is more energy to transform to heat when it interacts with the metal of the can. This heat will be transferred to the water inside the can and the thermometer should show a greater increase in temperature for the can lighted by the 100 watt bulb than the 60 watt bulb. This activity does illustrate Big Idea #3 in the way that it shows light energy transformed into heat energy when interacting with matter, but the emphasis should be placed on the fact that the brighter light bulb caused the water to heat more, leading to an explanation of a greater amount of energy coming from the brighter bulb. Some guiding questions to follow this activity could include: o Which bulb has a brighter light? o What happened to the temperature of water in the can lit by the 100 watt bulb? the 60 watt bulb? Why did this occur? o What was the difference between the temperature in the can lit by the 100 watt bulb than the 60 watt bulb? Why was there a difference? This explanation should discuss the relationship between brightness, amplitude, and energy and how the greater amount of light energy resulted in the greater amount of heat energy. o Draw a diagram of the light waves from the 100 watt bulb and the 60 watt bulb. Show the difference in their amplitudes.