Refraction Investigations You will find the Refraction slideshow notes helpful as you complete these investigations. How Refraction Affects What We See: Part 1 You probably won t find anyone who is not familiar with the optical effects one observes when placing a straw into a glass of water. But how familiar are we really? Place a straw in the glass of water and record your observations in each case. I. Describe what you see when viewing the straw from the side of the glass with your eye above water level looking down at the surface of the water. Include a sketch. II. Describe what you see when viewing the straw from the side of the glass with your eye at water level. Include a sketch. III. Describe what you see when looking up at the straw from the side of the glass with your eye below water level looking upward at the surface of the water. Include a sketch. List factors that might influence your observations. How could you test these factors? How Refraction Affects What We See: Part 2 Examine the location and appearance of your finger as it is viewed through these two shapes. Describe the location of the finger portion viewed through this double concave lens. Show the finger above, below, and inside the shape. top view side view 1: finger on left side view 2: finger on right side view 3: finger in middle 1
Describe the location of the finger portion viewed through this double convex lens. Show the finger above, below, and inside the shape. top view side view 1: finger on left side view 2: finger on right side view 3: finger in middle Name two ways in which the image of the finger portion observed when looking through the double concave shape is different from the image observed when looking through the double convex shape. (HINT: size and location) Refraction through Lenses Draw a sketch of each lens type. Converging Lenses double-convex plano-convex concavo-convex Diverging Lenses double-concave plano-concave convexo-concave What is common about the shapes of the convex (converging) lenses? What is common about the shapes of the concave (diverging) lenses? Which, if any, of these lenses can be used to magnify objects when placed close to the object? Hold each lens at arms length and look at a distant object. Which, if any, of these lenses will make distant objects appear inverted? 2
Each of the previous optical effects occurred due to the property of light known as refraction. You will now take a closer look at this behavior of light. Refraction of Light at the Boundary between Two Media: Theory Refraction is defined as the of a wave as it passes from one medium into another medium in which it has a different transmission. When the continuous wave fronts obliquely reach a new medium, part of the wave front changes speed before the rest of the wave front. This causes one portion of the wave front to either move ahead of, or lag behind, the rest. This difference in speed causes the direction of the wave to change. This change in direction is known as. If the wave front enters the new medium head on, there is no refraction because. Make sketches showing continuous wave fronts passing into new mediums in which they A) speed up, and B) slow down. A) wave speeds up B) wave slows down slow fast fast slow Suppose light speeds up as it passes from Medium A into Medium B. When light speeds up, it always refracts the normal. Make a sketch illustrating the path of light through these materials. Draw a dotted normal at the location in which the ray reaches the boundary between the two media. Label the angle of incidence Ө A and the angle of refraction Ө B. In this case, Ө A Ө B. A B Suppose light slows down as it passes from Medium C Into Medium D. When light slows down, it always refracts the normal. Make a sketch illustrating the path of light through these materials. Draw a dotted normal at the location in which the ray Reaches the boundary between the two media. Label the angle of incidence Ө C and the angle of refraction Ө D. In this case, Ө C Ө D. C D 3
Refraction of Light at the Boundary between Two Media: Experiment 1 Place the jello square on the square below. Shine the laser beam into the jello square along the ray shown on the left side. Notice the beam as it passes through the jello. Mark the location in which the beam exits the jello square on the right side. You might also notice some reflection both inside and outside the jello. Mark another spot on your paper that can be used to sketch the path of the beam as it travels through the air on the right side of the jello square. Remove the jello square and use a ruler to sketch the path of the beam as it travels through the jello and back into the air. Be sure to place arrowheads on your light rays to indicate the direction the light traveled. Use your ruler and protractor to draw a dotted normal line at the location in which the beam entered the jello. Label the angle in the air outside the jello between the beam and the normal Ө A. Label the angle inside the jello between the beam and the normal Ө B. When light (the laser beam) passed from the air into the jello, its speed. This made the laser beam refract the normal such that Ө A Ө B. Now use your ruler and protractor to draw a dotted normal line at the location in which the beam exited the jello. Label the angle inside the jello between the beam and the normal Ө C. Label the angle in the air outside the jello between the beam and the normal Ө D. When light (the laser beam) passed from the jello into the air, its speed. This made the laser beam refract the normal such that Ө C Ө D. 4
Refraction of Light at the Boundary between Two Media: Experiment 2 5
Place the jello semicircle on the semicircle in the preceding page. Shine the laser beam into the jello semicircle along the ray shown on the left side. Notice the beam as it passes through the jello. Mark the location in which the beam exits the semicircle on the curved side. Mark another spot on your paper that can be used to sketch the path of the beam as it travels through the air on the far side of the semicircle. Remove the jello semicircle and use a ruler to sketch the path of the beam as it travels through the jello and back into the air. Be sure to place arrowheads on your light rays to indicate the direction the light traveled. Repeat these steps to find the path of the laser beam as it passes through the semicircle along the ray shown on the right side. Use your ruler and protractor to draw two dotted normal lines at the positions on the far side of the semicircle where the beams exited the jello. The normal lines will originate at the center of curvature of the circular surface and pass through the locations in which the beam passed from the jello back into the air. Label the angles inside the jello between the beams and the normals Ө A and Ө C. Label the angles in the air outside the jello between the beams and the normals Ө B and Ө D. Use your diagram to complete the following statements. When light (the laser beam) passed from the air into the jello, its speed. However, the beam did not refract at this location because. When light (the laser beam) passed from the jello back into the air along the path on the left side, its speed. This made the laser beam refract the normal such that Ө A Ө B. When light (the laser beam) passed from the jello back into the air along the path on the right side, its speed. This made the laser beam refract the normal such that Ө C Ө D. The location in which the two beams would eventually cross in the air on the far side of the semicircle is known as the. Because light passing through this semicircle shape converges to a point, this shape acts as a. Use your ruler to complete the paths the two rays shown below would take if they passed through a jello lens having this shape. 6
Total Internal Reflection: One Interesting Result of Refraction When light reaches the boundary between two material media, some of the light energy is reflected and some is transmitted. The relative amounts of each depend on the nature of the materials and the incident angle of the light. Under the right conditions, it is possible for light to have 100% reflection and no transmission at all. This is know as TOTAL INTERNAL REFLECTION and is only possible when light speeds up as it enters the new medium. Therefore, we will never notice this phenomenon when light travels from into, but may notice it when light travels from into. Shine your laser beam into the jello square and jello semicircle at different angles until you notice total internal reflection. Sketch the path of the laser beam in each diagram below to show how you obtained total internal reflection. References: This lab is from http://jabryan.iweb.bsu.edu/lowcostphysics/ 7