Lesson Guide Intermediate Reflection and Refraction

Transcription

1 Lesson Guide Intermediate Reflection and Refraction

2 What s in the kit One set of (3) Light Blox - red, green and blue One set of three lenses 2 Mirrors Semi-circular Refraction Cell One protractor 6 AA Batteries What s else you ll need Ruler Plain white paper Aluminum Foil Blank wall or screen

3 Light Blox Instructions Light Blox are safe, durable, and long lasting LED s designed specifically for teaching and learning about light. Batteries Light Blox comes with 6 AA batteries. Install 2 AA batteries in each unit by sliding open the case and inserting batteries. Replace cover. Operation Turn Light Blox on with the switch on the side of the unit. Slide the switch to turn the unit on. The color of the switch matches the color of the light. Removable Caps Each Light Blox comes with a removable cap. When the cap is on, the units produce a ray of light for studying simple geometric optics such as reflection and refraction. When the cap is removed, the units produce a block of light for studying color and shadows. MORE Kits, Lessons and Activities! For more ideas, lessons and activities for using Light Blox in the classroom, visit

4 Law of Reflection Light behaves in a very predictable way. Students discover that a ray of light approaching and reflecting off of a flat mirror, follows a predictable law known as the law of reflection. NGSS Connection Disciplinary Core Idea: Electromagnetic Radiation When light shines on an object, it is absorbed, reflected or transmitted through the object. What You ll Need * Set of Light Blox - caps on * Mirror with Stand * Protractor Background and Introduction The ray of light approaching the mirror is the Incident Ray (labeled I in the diagram). The ray of light that bounces from the mirror is the Reflected Ray (labeled R in the diagram). The Normal Line (labeled N in the diagram) is drawn beginning at the point where the ray strikes the mirror. The Normal Line divides the angle between the incident ray and the reflected ray into two equal angles. The angle between the Incident Ray and the Normal is the Angle of Incidence. The angle between the Normal and the Reflected Ray is the Angle of Reflection. The Law of Reflection says: when a ray of light reflects off of a surface, the Angle of Incidence is equal to the Angle of Reflection.

5 Setting Up Have students set up their Light Blox, mirror and protractor, as shown to the left. Place the mirror directly in the middle of the the protractor so that the mirror lines up with the 90 degree line and the center of the mirror is aligned with the 0 degree mark. Observing and Investigating Have students shine the light blox at the mirror to explore how the light bounces. Ask them to adjust the angle from which the light hits the mirror and ask: What do you notice about how the light bounces? - a predictable, straight line that moves when the light source moves - be sure they notice that it is the change in the angle, not distance, that changes how the light bounces. Students can quantify their observations - have students place the mirror with the stand in the center of the protractor so that the mirror is parallel to the 90 degree axis of the protractor. Have students aim the light ray from the 30 degree angle (with respect to the vertical) directly at the center of the mirror and measure the angle to the reflected beam with respect to the vertical. Ask students to record their results and repeat with two more angles of incidence of their choice. Next, have students neatly fold the aluminum foil to make a flat, shiny surface. Repeat the above, as with the mirror. Students will observe that the foil interacts with light in much the same way as the mirror. Have students crumple the foil into a ball and repeat the exploration. Students will quickly notice that the light scatters and bounces in all directions. Explain that when a surface is rough, like with the crumpled foil, light still bounces according to the same rules they just observed: in a straight line with incident angles = to reflected angles - however, many rays hit many surfaces, scattering the rays as they hit different surfaces of the crumpled foil. Now you can explain that light bounces off everything that we see. It all bounces in straight lines. All we ever see is light - bouncing off surfaces into our eyes. Hold a group discussion of students findings. Students should notice that the angle of incidence = the angle of refection. Specular Diffuse

6 Refraction Light travels at the speed of light in a vacuum. Once light his our atmosphere, it slows down just a bit. This is because when light interacts with matter, atoms and molecules absorb and re-radiate the light (start thinking energy). This slowing down of the light causes it to bend as it travels from one medium (such as air) into another, more dense medium (such as water, glass or plastic). NGSS Connection MS-PS4-B Electromagnetic Radiation A wave model of light is useful for explaining brightness, color and the frequency dependent bending of light at As we explore refraction and the ways in which light interacts with matter, it is helpful to think of light as a wave rather than a ray. Specifically lots of waves of various frequencies or wavelengths. When waves of light hit a boundary - such as when they move from air into glass or water - they interact with the material and slow down. As light leaves that medium and travels back into air, they speed up. Different frequencies (wavelengths) of light correspond with different energies and different colors. Blue light has the greatest frequency, the shortest wavelength and the most energy of the visible spectrum. Red light has the smaller frequency, longest wavelength and least amount of energy of the visible spectrum. Because different frequencies of light have more or less energy, the speed with which they move through a medium is different. Blue light, with more energy will slow down less as it passes thru a prism than will red light with it s lower energy. The angle at which a specific frequency of light bends is determined by the change in speed of the light as it travels from one medium into (or out of) another. So, blue light will bend at a different angle from red light. Now recall that visible white light is actually a range of specific frequencies (or wavelengths) of light that the eye is able to sense, or see. So, when white light hits a medium, each of the frequencies (wavelengths) of light in that white light bend at slightly different angels from one another. This is how a prism creates a spectrum or rainbow of color. In effect, refraction splits or un-mixes the many frequencies of light by bending each at a slightly different angle.

7 A Kinesthetic Understanding of Refraction This kinesthetic activity helps young scientists grasp how light bends at a boundary and why. NGSS Connection MS-PS4-B Electromagnetic Radiation The path that light travels can be traced in straight lines, except at surfaces between certain transparent materials (water, glass) where the light path bends. What You ll Need * A large flat area, indoors or out doors * Either a natural boundary or masking tape To begin, think about light as a series of wave fronts approaching a boundary. As the waves cross the boundary and enter the second medium they slow down and bend. This activity gives students an intuitive grasp of what is going on. Bring your whole classroom to an outdoor area with a natural boundary such as a side walk or pavement. If an outdoor area is not available for you, clear a large area in the room and create a boundary with a long piece of masking tape. Tell students that on one side of the boundary, they will be marching (in parallel lines) on pavement. But when they cross the boundary, they will be marching in knee deep jello! Line students up in lines of 4-5 students with linked elbows as in the diagram and have them march in step at a normal stride. Point them towards the boundary at an angle. Ask students: what happens when you get to the jello? The SLOW DOWN! Instruct students to march at 1/2 stride once they reach the boundary, but not before. See how the line changes direction?! Once they have all crossed, have them travel back in the other direction, taking 1/2 steps until they reach the boundary where they again resume full steps, and bend back to their original angle. Notice what happens to the wave front if they approach the boundary head on or not at an angle. The still slow down, but they do not bend. This is important.

8 Refraction and Lenses The shape of a lens impacts HOW light bends. A concave lens will cause rays to bend towards Normal Line; a convex lens will cause rays to bend away from the Normal Line. NGSS Connection MS-PS4-B Electromagnetic Radiation The path that light travels can be traced in straight lines, except at surfaces between certain transparent materials (water, glass) where the light path bends. What You ll Need * A set of Light Blox with the line caps on. * A set of lenses - concave, convex and trapezoidal. * The refraction cup * 1/2 cup of water Now that students have a feel for what is happening with refraction and why, they can observe light as it refracts or bends as it travels from one medium to another. Start with a single Light Blox with the line cap on, and the trapezoid lens. Place the trapezoid lens, frosted side down, on a flat, preferably white, surface and dim the lights as much as you are able to. Turn on the Light Blox and orient it so that the ray of light hits the edge of the trapezoid lens straight on. Does it bend? Why? Why not? Now orient the ray of light at an angle, as shown in the photo above. Notice how the light (observed as a ray, but thought of like a wave) enters as a straight line that abruptly changes angle as it travels through the more dense medium - in this case, acrylic - just like the line of kids bent when it crossed the boundary at an angle. Have students spend some time changing and observing and discussing the relationship between the angle of the light ray and the boundary.

9 Predicting the path of a light ray Developing a conceptual understanding of how lenses bend light in predictable paths, lays the foundation for later mathematical interpretations of lens/light relationships which is the basis for a lot of familiar technology such as eyeglasses, compound microscopes and refraction telescope. With some careful thinking about what we understand about the refraction of light, we can begin to predict how light might bend when passing through lenses of various shapes. Give each group of students a set of Light Blox and an empty refraction cup and a plain piece of white paper to draw on. Have students set up the cup and lights as in Figure 1. Now, drawing on their kinesthetic experience with refraction invite them to work together to a. write down what they understand about how light refracts and why b. predict how (specifically in what direction) each ray of light will bend and WHY. Invite students to use both words and pictures to make their predictions and justifications. Figure 1 This is not easy. It requires a real integration of what is happening with the light as it moves into and out of the denser than air medium. Once students have wrestled with it for an appropriate length of time and come up with some possibilities, invite them to carefully pour water into the refraction cup and observe what happens. Now that they see HOW the light bends, towards the center, invite them to revisit the question of WHY? Why does the light bend towards the center and not away? What else do they notice? Figure 2 See Figure 2 for the answer to how, or in what direction will the light bend: the middle (blue) ray passes straight through, while both the ray on the left (green) and on the right (red) bend towards the center.

10 Here s why: Recall from the kinesthetic model that it is a difference in speed between the first part of the wave front entering the denser medium and the latter part of the wave front entering the denser medium that causes the bending or refracting. If the wave front hits the surface head on (not at an angle) the wave front s speed remains consistent. CONVEX LENS For example, if the kids march into the jello head on, they slow down, but their line does not bend. So, the center, blue ray does not refract (bend) at all, it passes straight through the lens - just like the single ray when oriented perpendicular to the straight edge of the trapezoid did not bend, or refract. The beam (wave fronts) must encounter the boundary at an angle in order to create a change in speed between the first part of the wave front and the rest of the wave front to cause refract. With a curved lens, the angle changes because of the curvature of the lens, not because of the angle of the incident ray. So, the green and the red rays hit a curved surface and bend or refract accordingly. The amount of distance the ray travels inside the denser medium and the curvature of the lens determine how steeply the light bends or refracts. Knowing that both the surface curvature and the thickness of the lens impact the amount of bending, notice the two additional black rays. Because of where they hit the lens, they travel a longer distance inside the lens than either the red or the blue ray. This causes them to bend, or refract, more. The end result is that a CONVEX lens (one that is curved outwards) causes the light to travel to a single point called the focal point or CONVERGE. The distance from the center of the lens to the focal point is called the focal length of the lens.

11 Now take a look at the double CONCAVE lens. Notice the curvature of this lens is opposite of the convex lens. This lens, for the same reasons, causes light to DIVERGE or bend outwards, away from the center. This basic understanding allows students to move onto describing lenses and the way they bend light in mathematical terms. It also allows them to begin to understand how a lot of optical technology works such as microscopes, corrective lenses for vision problems and telescopes.

12 Template Page Place the CONCAVE LENS Here Place the CONVEX LENS Here

13 LASER Classroom is the home of LASER Blox and of all the tools and resources you need to teach and learn about Light, Lasers and Optics. LASER Classroom mission is to create resources and partnerships that make teaching and learning about light, lasers and optics accessible, engaging and fun so that teachers and students are empowered and prepared to enter the 21st century workforce LASER Classroom is a registered DBA of OnPoint Lasers, Ltd. D&B: EIN: MN ID: CCR#: 1B2H1 NAICS (Laser systems) (Science Kits) (School Supply Stores) LASER Classroom Corporate Office 1419 Main St. NE Minneapolis, MN order@laserclassroom.com LASER Classroom Warehouse 507 Wilkie Drive Winona, MN 55987