CAUTION: Direct eye exposure to lasers can damage your sight. Do not shine laser pointers near anyone s face, or look directly into the beam.

Transcription

1 Name: Date: Partners: Purpose: To understand the basic properties of light and how it interacts with matter to reflect, refract, disperse or diffract. Part 1A: Reflection Materials: 1. mirror 2. ruler 3. laser pointer 4. loose-leaf paper 5. protractor 6. spoon CAUTION: Direct eye exposure to lasers can damage your sight. Do not shine laser pointers near anyone s face, or look directly into the beam. Diagram: Mirror Reflected Beam Reflection Incidence Incident Beam Laser Paper Normal to the mirror Rev /13

2 Procedure: 1. Place a sheet of paper down on the table. 2. Hold the mirror so its edge rests on the sheet of paper, holding the mirror perpendicular to the table. 3. Draw a line on the paper indicating the location of the front edge of the mirror. 4. If necessary, dim the lights. 5. Shine the beam onto the center of the mirror at an angle. 6. Trace the path the beam takes on the paper, and label the trace as Trial #1. 7. Repeat steps 5&6 three more times to obtain Trials 2, 3, and 4, but change the angle of incidence for each trial. Make sure to label each trial clearly. Data Analysis: 1. For each trial, locate on your sheet of paper where the beam of light hit the mirror, and draw a line perpendicular to the mirror edge at this point. 2. The angle of incidence is the angle between the beam coming from the laser pointer and the line perpendicular to the mirror. Measure the angle of incidence for each trial. 3. The angle of reflection is the angle between the beam reflected off the mirror and the line perpendicular to the mirror. Measure the angle of reflection for each trial. Trial 1 Trial 2 Trial 3 Trial 4 Incidence Reflection Questions: 1. What mathematical truth can you conclude from the data? Rev /13

3 2. This fundamental rule about the angle of incidence and reflection doesn t only apply to the behaviour of light. Explain how playing a game of billiards or pool relates to the behaviour of light. Part 1B: Images Formed by Flat (Plane) Mirrors Procedure and Observations: 1. Place an object in front of a mirror, and note the image of this object formed by the mirror. 2. Sketch a diagram below of how the rays of light get from the object to your eye. Draw a ray from the top of the object and a ray from the bottom of the object. Use the law of reflection when you draw rays being reflected from the mirror. 3. Using this sketch and your observations of the actual image formed by the mirror, answer the following questions: Where does the image appear to be (ie. where do the rays of light from the object appear to come from)? Is the image real or virtual? A real image is where light rays actually converge-- if you placed a camera at the location of a real image, you would see a picture of the image. A virtual image is where rays of light appear to converge-- if you place a camera at the location of a virtual image, no picture would be recorded. Is the image upright or upside-down? How does the distance of the image behind the mirror (the image distance, d i ) compare with the distance of the object in front of the mirror (the object distance, d o )? Rev /13

4 How does the height of the image compare with the height of the object? In other words, does a flat mirror magnify the image (make it larger or smaller)? Do you think you would have found the same image properties if you had used a curved mirror instead of a flat mirror? Try investigating with a spoon or curved mirror, and describe the differences. Part 2A: Refraction in liquids Additional Materials: 1. Rectangular prism, equilateral triangle prism 2. Straw or Ruler mL Pyrex beaker (or larger) 4. water and glycerine 5. Pyrex Test tube Activity 1 The bent straw Most of you by now have noticed if you place a stick in water the stick appears to bend under the surface. Consider the following: PREDICT! If you place a straw in a beaker of water as shown below, and look at the straw from above, which way will the straw appear to bend, away from you or towards you? Write down your prediction below. Rev /13

5 Straw Water Bends away from observer behind straw Bends towards observer behind straw OBSERVE! 1. Fill up a 600mL beaker full of water, and place a straw in it as shown. Look down on the beaker from above the straw. What do you notice? Did this match your prediction? 2. Why does this occur? What would happen if you immersed the straw in another transparent fluid like dish soap or glycerin, or mineral oil? Would the straw appear to bend in exactly the same way each time? 3. Now fill up a 600mL beaker full of glycerin (also known as glycerol, used in the food industry as a sweetener). This time place an empty test tube in the solution, and observer the degree of refraction. Write your observations below. Did this match your prediction? 4. While watching the test tube carefully, slowly fill up the test tube with glycerin. What do you observe? Rev /13

6 5. Can you explain this observation in terms of refraction? Part 2B: Reflection and Refraction in rectangular and triangular prisms Activity - Rectangular prism normal incident light 1. PREDICT: You ve placed a rectangular glass or plastic prism so one of the wide faces is lying flat on the table. If you shine a laser light with an angle of incidence of 0 (i.e. normal to the surface of the prism), how will the light pass through the prism? Will it bend like the straw did in water, or go straight through? Sketch your prediction below: 2. OBSERVE: Give it a try! Sketch how the light passes through the prism. Is this what you predicted? 3. PREDICT: Consider shining a light on the same surface of the rectangular prism, but at an angle of incidence between 0 and 90 degrees. How will the beam of light pass through the prism? Sketch your predictions below: Rev /13

7 4. OBSERVE: Try it! Have a person in your group shine the laser at the surface of the prism, and have someone else measure the angle of incidence (I1), the angle of refraction when the light enters the prism (R1), the angle of incidence as the light hits the opposite face of the prism, (I2), and the angle of refraction (R2) when the light hits the air again on the other side of the prism (see diagram). Record your results in the table. Try this for 3 different angles of incidence I1. R 2 R 1 I 2 I 1 Rev /13

8 Data: Refraction in a rectangular prism Trial 1 Incidence I 1 Refraction R 1 Incidence I 2 Refraction R 2 Trial 2 Trial 3 5. QUESTIONS: a) For each trial, how did I1 and R2 compare? How did I2 and R1 compare? Does this make sense? How do R1 and I1 compare? b) What did you notice about the reflected rays of light? Where did reflection occur? Why didn t you see this when you shone the light perpendicular to the prism s surface? Activity - Triangular prism 1. Take a look at the triangular prism, hold it up so you are looking through one of the narrow faces. If you look through the prism this way, what do you see? Do you see what is directly in front of you? 2. Place the prism flat on the table. Shine the light into one of the narrow faces, and sketch what you see. Try this for a few different incident angles. Instead of measuring angles of incidence and refraction, focus more on sketching the rays, including the refracted and reflected rays. Rev /13

9 Part 3: Dispersion: Now that you have understood the basics of refraction, you can look at dispersion. It turns out that the angle of refraction depends on the colour (or wavelength) of the light. Typical white light from the sun is actually a blend of all colours in the visible spectrum. PREDICT: If it is a bright sunny day, what will you observe when sunlight passes through the triangular prism? Observe: 1. Find a sunny spot and sketch the spectrum you are able to observe. You will need to rotate and fiddle a bit with the prism in order to get the alignment right. Sketch the resultant colour pattern on the diagram below Rev /13

10 2. Why can t you observe this pattern when using the rectangular block? The most common example of dispersion is probably the rainbow. In this case, refractions and reflections in raindrops or mist disperse the white light from the sun into the rainbow spectrum. This can only be seen well if you are in the right location with respect to the sun and the raindrops. Rev /13

11 Part 4: Diffraction: Additional Materials: 1. Spectrograph When light passes by the edge of a material, it is observed to bend. A common way of describing this is in analogy with sound. If you hide behind a tree, and somebody yells from the other side, you can still hear it. The sound bends around the edge of the tree. You are not in a perfect shadow. Light passing by a sharp edge can also bend, so you do not get a perfect shadow. The effects can be observed easily. For example: Observe: Hold your thumb and forefinger about 20 cm away from your eyes. What happens when you look through the narrow gap between them, just before they physically touch? Can you see the black lines that are being formed? Sketch what you observe. Explain: Explain why these alternating dark and light patterns appear. Note this is beyond what would be expected of grade 7 and 8 children and you probably need help to understand it properly! Observe: Another common device to bend light in this way is known as a diffraction grating. This is basically a series of very fine slits that deflect the light. As it turns out, this is also wavelength dependent, so this is another way of dispersing light. Use the small spectrograph provided to view a number of objects. Say an incandescent light bulb, a fluorescent light bulb and the sun. Describe what you observe: Caution. Don t look directly at the sun, but only at the reflected light from the sky! Rev /13

12 Incandescent Bulb: Fluorescent Bulb: Sun: Questions: 1. Using the chart provided on the spectrograph, can you identify any chemical elements in the fluorescent bulb? 2. Can you explain why the incandescent bulb has all colours, but the fluorescent light has only a few? 3. Why are there some colours missing (black lines in the spectrum from the sun)? Rev /13

13 Summary and Suggestions for the Future: a. What were the important concepts of physics/science that you learned from this activity? What else did you learn? b. Can you think of alternative hands-on ways in which these concepts could be demonstrated? c. What changes (if any) would you make to teach these activities in a Grade 7-8 classroom Rev /13