How Do You Measure the Index of Refraction in Water?

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How Do You Measure the Index of Refraction in Water? Description: Students will use glass and water to measure the indices of. Student s (per group): Semicircular Dish Water Glass plate Graph Paper Ruler
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How Do You Measure the Index of Refraction in Water? Description: Students will use glass and water to measure the indices of. Student s (per group): Semicircular Dish Water Glass plate Graph Paper Ruler Cardboard Pin Additional Teacher s: None Background and Misconceptions: When light goes from one medium to another, it can bend or refract. Refraction is evidence that light can behave with wave-like properties. Pictured below, the wave represented by the red line will bend when the ray goes from air to water. Since water is more dense than the air, the ray will bend toward the dashed line, called the Normal. The normal is a line that is perpendicular at the point where the ray is striking the surface. If you were to follow the ray in the opposite direction, going from the water into the air, the ray will bend away from the normal. Light will bend away from the normal when light goes from a more dense material into a less dense material. If we measure the angle between the normal and the direction of the rays, we can use a formula for the index of to find a value that represents the amount of bending. The value for the index of is a dimensionless unit. Larger values indicate greater amounts of bending. The table shows some values of common substances. Sin Index of Refraction = i Sin r incidence. i r. Index of Refraction (n) Air 1.00 Water 1.33 Diamond 2.42 Human Lens 1.4 Crown Glass Teacher Guided Questions to Inquiry: Use these questions to get the students started on their own inquiry! 1. How does light behave when it travels through glass? 2. Why do your legs look shorter when you are sitting on the edge of the pool dangling them in the water? 3. How can you use water or glass to bend light? 4. How can you measure the index of of light? Additional Hints: The curved dishes can be purchased at some science supply companies. Alternatively it is possible to purchase round clear dishes, cut them in half, and then glue a piece of Plexiglas on the open half to create a semicircular dish. This lab has been broken into two parts and will likely take at least 2 days to complete both parts. This lab requires careful measurement to obtain good results. How Do You Measure the Index of Refraction in Water? TEACHER ANSWER SHEET Water incidence Sin I Sin r Index of How Do You Measure the Index of Refraction in Glass? TEACHER ANSWER SHEET Glass incidence Sin I Sin r Index of (glass type will affect this number) Questions: 1. What happened to the light as it entered the water and glass? (Describe how it behaved with respect to the normal line.) The light bends toward the normal as it enters the water or glass and bends away as it leaves the glass. 2. Determine the percentage of error for your answer for the index of for water to that accepted value which is Explain what might have contributed to your error if you were off. Percentage Error (Your Value - Accepted Value) x100% Accepted Value Students answers should be less than 10% error. 3. You hold a flash light and shine it on the flat side of the curved dish. What happens to the light after it passes through the dish. (Does it spread out, come to a point, etc.?) It will spread out. 4. Why is it important that we know how much light bends as it goes through different materials? What are some applications of in everyday life? One important application is for making lenses for people. By knowing the index of for various substances the design of eyeglasses can be changed or lightened. Glass and plastics bend light differently and new materials can be created to cause light to bend more or less while making the eyeglasses lighter. How Do You Measure the Index of Refraction in Water? Name Date Description: You may often sit on the edge of a pool and dangle your feet in the water. When you look at your feet and legs in the water, they look distorted. This effect is caused by REFRACTION. Refraction of a wave occurs when a light ray enters a material of different density. As it passes from one medium to the other, the light ray bends, or refracts. While it is interesting to see your legs shorter and fatter than they actually are, this effect is also important for making corrective glasses, contact lenses, and telescopes. Knowing how much light bends is important and scientists have assigned numbers to the amount that light bends when it enters different media. These numbers start at 1.00 and increase. A number of 1.00 indicates air or a vacuum. Some materials are many times more dense than air and will bend the light more. Diamond, for example, bends light dramatically. These numbers are called the INDEX OF REFRACTION. In this lab, you are going to determine the index of for water and glass. s: Semicircular Dish. Water Glass plate Graph Paper Ruler Cardboard Pin Procedures, Lab 1: 1. Draw a line on the graph paper dividing the paper in half. 2. Make sure there is a line drawn on the curved edge of the dish. This line will be your object. Top Half of Paper 3. Place the edge of the dish along the straight line so that the dish is on the bottom half of the paper and trace the outline of the dish on the paper. 4. Mark the position of the object (the line) on your paper. 5. Lay a ruler on the bottom half of the paper. Adjust the position until the edge of the ruler seems to point at the object when you look through the water. 6. Draw a line along the ruler edge to the edge of the dish. Line Bottom Half of Paper 7. Repeat steps above but line up the ruler at a different position. 8. THIS PICTURE SHOWS WHAT YOUR PAPER SHOULD LOOK LIKE AFTER YOU ARE FINISHED DRAWING THE LINES AND TRACING THE DISH. 9. Remove the dish and connect the lines on the sheet with the object position. View from the Side Top Half of Paper 10. At the point where the line is drawn on the flat surface, draw a line perpendicular to the surface. This is called the normal. 11. Measure the angle of the light to the normal in both the water (which is angle r in the table) and air (which is angle I in the table). Trace of dish Line Line connecting dot on dish and ruler line Line Drawn on Paper 12. Calculate the index of for water using the formula below. Sin Index of Refraction = i Sin r Bottom Half of Paper incidence. i r. The index of tells us how much light will bend when it goes from one medium to another. The index of for air is incidence Sin I Sin r Index of Water How Do You Measure the Index of Refraction in Glass? 1. Repeat the above steps but this time use a glass plate. You will use the pin as a way of looking through the glass for a reference. See the diagram. Glass plate Pin 2. Place the glass plate in the center of the paper. Trace an outline of the glass. A 3. Remove the glass plate. Mark a point called B 4. From Point B, draw a line that is perpendicular to the glass plate and call this Normal Draw a line that is at a 30 o angle from the normal and label the end of that line as A. The angle formed at is 30 o is I, the angle of incidence. 6. Replace the glass plate over the outline on the paper. With your eyes on a level with the table and end of the glass plate, look through the plate until you see the line AB. Use your ruler to line up the line AB so that it seems to continue the line through the glass. Draw this line and label it as CD. i Normal 1 r B Normal 2 C 7. Remove the glass plate and draw a line between the two lines that were drawn. D 8. At point C, on the edge of where the paper draw another normal line, Normal Measure the angle formed between BC and Normal 1. This is the angle of, r. Record the value of this angle in the table below. Using the formulas, solve for the index of. Sin Index of Refraction = Sin i r incidence. i r. The index of tells us how much light will bend when it goes from one medium to another. The index of for air is incidence Sin I Sin r Index of Glass Questions: 1. What happened to the light as it entered the water and glass? (Describe how it behaved with respect to the normal line.) 2. Determine the percentage of error for your answer for the index of for water to that accepted value which is Explain what might have contributed to your error if you were off. (Your Value Accepted Value) PercentageError x100% Accepted Value 3. You hold a flash light and shine it on the flat side of the curved dish. What happens to the light after it passes through the dish. (Does it spread out, come to a point, etc.?) 4. Why is it important that we know how much light bends as it goes through different materials? What are some applications of in everyday life?
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