REFLECTION AND REFRACTION OF LIGHT INTRODUCTION: A light ray travels in a straight line from a source until it encounters some object. What happens next depends on several factors including the nature of the material making up the object, the smoothness of the surface, and the angle at which it strikes the surface. If the material is transparent, the light ray may be transmitted through the material. In this case the light ray appears to become bent, undergoing a change in the direction of travel at the boundary between two transparent materials (such as air and water). The change of direction of a light ray at the boundary is called refraction. Light rays traveling from a source, before they are reflected or refracted, are called incident rays. If an incident ray undergoes reflection, it is called a reflected ray. A line perpendicular to the surface, at the point where the incident rays strikes is called the normal. The angle between an incident ray and the normal to the surface is called the angle of incidence. The angle between a reflected ray and the normal to the surface is called the angle of reflection. The angle between a refracted ray and the normal to the surface is called the angle of refraction. These terms are descriptive of their meaning, but in each case you will need to remember that the angle is measured from a line perpendicular to the surface called the normal. EQUIPMENT: Pin board, pins, ruler, protractor, plane mirror, block to hold a mirror, plate glass block, white paper, and a compass. PROCEDURE: Part A. Reflection of Light 1. Using a ruler, draw a straight line across a sheet of plain (unlined) white paper near and parallel to the long edge. Place the paper on the pin board. Label the line with a B at one end and B' at the other end (B is for boundary). 2. Attach a small, flat mirror to a block of wood similar to that shown in Figure 1. Place the mirror and block combination on the paper with the back of the mirror (the reflecting surface) on line BB'. 3. Stick a pin straight up and down into the paper about 10 cm from the mirror and slightly to the right side as shown in Figure 1. This is point I. On the left side, carefully align the edge of a ruler with the reflected image as shown in the illustration. Then firmly hold the ruler and draw a pencil line along this edge. Move the mirror and extend this line to the mirror boundary line BB'. Label the point of reflection with the letter P. 4. Place a protractor on line BB' and mark a point 90 from the line at point P. From this point, use the ruler to draw a dashed normal, NP, as shown in Figure 1. Complete your ray diagram by using the ruler to draw a line from the point of reflection, P, to the source of the light ray at the pin, I. Place arrows on line IP and line PR to show which way the light travels.
Figure 1 5. Use the protractor to measure the angle of incidence and the angle of reflection. Record these angles in Data Table 1 under Trial 1. 6. Place the mirror with its back edge again on line BB' and conduct a second and third trial at different sighting angles. The pin should not be moved, simply look at the image of the pin from a different angle. The point of intersection of this line with BB will be different for each trial. Record these measurements in Data Table 1. Figure 2
Part B: Refraction of Light 1. Place a clean sheet of white (unlined) paper on the pin board. Place an approximately 5 cm square glass plate flat on the center of the paper. Use a pencil to outline the glass plate, then move the plate aside. 2. Use a ruler to draw a straight line from near the upper right corner of the plate outline, making an angle of about 45 to the edge. Label this line IA as shown in Figure 2. Place one upright pin at point A immediately outside the plate outline and a second upright pin at point I. Return the glass plate to the outline. 3. Bring the pin board, paper, and glass plate near the edge of the table so you can sight through the glass plate toward the two pins. Position a ruler so that one edge aligns with the two pins as shown in Figure 2. Draw a line along the ruler and label the line BC. Move the glass plate aside for a second time. 4. Draw a line from A to B, showing the path of the light ray through the glass. Overall, the path of the light ray is from I to A to B to C, showing that the light ray was bent twice. 5. Draw normals to the surface of the glass at A and B. Show the angle of incidence and the angle of refraction with curved arrows at both boundaries. 6. Using Figure 3 as an example, draw a circle of 6.5 to 7.0 cm radius on your drawing with point A as the center. Record the radius used on your drawing. 7. Where the circle cuts the incident ray, label point R. Point S is where the circle cuts the refracted ray. Label point S. 8. Using a protractor, carefully draw a line from R perpendicular to the normal. Label this intersection M. Also draw a line from S perpendicular to the same normal. Label this intersection N. 9. Measure the length of lines MR and SN in millimeters. Record your answers on your drawing and in Data Table 2. Index of MR refraction= SN Calculate the index of refraction of the glass, and record your answer on Data Table 2. 10. The index of refraction, n, is the ratio of the speed of light in air to the speed of light in some substance such as glass. Mathematically this can be written as n = SPEED OF LIGHT IN AIR SPEED OF LIGHT IN GLASS Calculate the speed of light in glass using your experimental value for n and assuming the speed of light in air is 3.0 x 10 8 m/sec. Show your work at the bottom of the page after Data Table 2. Express your answer using scientific notation.
Figure 3
Data Table 1: Reflection of Light Trial Angle of Incidence (degrees) Angle of Reflection (degrees) 1 2 3 Data Table 2: Refraction of Light Length MR (mm) Length SN (mm) Index of Refraction Speed of Light in Glass (m/s)
RESULTS: 1. Describe any pattern you found in the data between the angle of incidence and the angle of reflection. 2. Describe any generalizations or rules you found concerning reflection. 3. Describe what happens to a light ray as it travels (a) from air into glass and (b) from glass into air. 4. Glass has a greater index of refraction than air. Make a generalized statement about what happens to a light ray with respect to the normal as it travels from a medium with a low index of refraction to one with a higher index of refraction. 5. Make a generalized statement about what happens to a light ray with respect to the normal as it travels from a medium with a high index of refraction to one with a lower index of refraction.