Experiment 9. Law of reflection and refraction of light 1. Purpose Invest light passing through two mediums boundary surface in order to understand reflection and refraction of light 2. Principle As shown in Fig. 1, light, which reaches boundary surface, is separated by reflection (1) and refraction (2). Each light is named as incident rays, reflected rays and refracted rays. Law of reflection and refraction is defined 1) Reflected rays and refracted rays are in surface which is made by incident rays and normal line. 2) Law of reflection : θ 1 = θ 1 3) Law of refraction : sinθ 1 sinθ 2 = n 2 n 1 = n 21 Fig. 1. Incident rays, reflected rays and refracted rays. Let s consider that incident rays inject from dense medium to light medium. As shown in Fig. 2, while incident angle is increasing, refracted rays will reach a point following along the surface. Refracted angle is 90 degree. This is called as total reflection. sinθ C = n 21-1 -
Fig. 2. Total reflection point e. θ C is critical angle. Fig. 3 shows reflection when reflected surface is plane. Rays come from light source O can flow A when it reaches A and can flow b when it reaches B. These reflected rays look like coming from light source O. O is in symmetrical position of O. Fig. 3. Reflection of light by plane mirror Fig. 4 illustrates that rays can flow medium with displacement as expressed by following equation d = a sin (θ 1 θ 2 ) cosθ 2 Fig. 4. Refraction of light - 2 -
3. Equipment Equipment # Equipment # Laser Diode 1 Rotational plate 1 Light Sensor 1 Acrylic 1 Rotary Motion Sensor 1 Linear converter 1 1m Optical stand 1 Lense stage 1 1) Data-Studio program Setting (1) Connect ScienceWorkshop interface to computer (2) Connect yellow and black line of rotary motion sensor into ScienceWorkshop. Connect Light Sensor into Channel A. Turn on interface and computer. Fig. 5. Connection interface with Sensor 2) Equipment Setting As shown in Fig. 6, Put laser diode at the end of Optical stand and fix light sensor on Rotational plate. Connect rotary motion sensor at the center of rotational plate in order to measure angle. Set maximum measurement range at light sensor for measuring light amplitudes. (1) Measurement of ray route along plane plate Fig. 6 Measurement of ray route along plane plate - 3 -
(2) Displacement measurement Fig. 7 Displacement measurement Linear converter assembly is shown in Fig. 8. Saw-tooth rod enters in rotary motion sensor and fix by screws. And put light sensor on Aperture Bracket and fix by crews and connect din connector. Fig. 8. Installation of rotary motion sensor Fig. 9. Installation of light Sensor - 4 -
4. Experiment Caution - use carefully acrylic for finger print - Keep away laser from eyes - Use carefully Aperture for adjusting range Before Experiment - It is important whether rotation angle of rotational plate is real value. Rotate rotational plate as 180 degree and compare with measured angle in order to know real angle value. 1. Measurement of ray route along plane plate (1) Install the equipment as shown in Fig. 6. Put plane of acrylic toward light source. (2) Put plane of acrylic in order to make 0 degree. (3) Connect Light sensor and motion sensor to Data Studio (4) Adjust acrylic for 30 degree between plane of acrylic and light source. Confirm reflected rays and refracted rays. (5) Choose the Light sensor s graph button of Data Studio which is on right bottom. Click x-axis of graph and change graph to Light intensity Vs Angle. (6) Put START button and rotate rotational plane. Measure Light intensity of reflected rays and refracted rays coming from light sensor. Measure rotation angle of rotary motion sensor. (7) Find out reflected angle and refracted angle by reading maximum light intensity. (8) Repeat (4)~(7) by changing incident angles recorded in Table 1. (9) Confirm the law of reflection and calculate a refractive index by using snell s law and refracted angle. 2. Critical angle measurement (1) Install the equipment by following Experiment 1. Put arc of acrylic toward light source. (2) Put arc of acrylic in order to make 0 degree (3) Confirm reflected rays and refracted rays by changing angle of acrylic. (4) Record incident angle when refracted angle is 90 degree by changing angle of acrylic. (5) Compare refractive index. 3. Displacement measurement (1) Install the equipment as shown in Fig. 7. (2) Connect light sensor and rotary motion sensor to Data Studio. - 5 -
(3) Click rotary motion sensor on Data Studio and Set Linear Calibration as Rack on 3 rd item Rotary Motion Sensor. Check Position on 2 nd item Measurement. (4) Confirm whether penetrated rays are parallel to incident rays by changing angle of acrylic. (5) Choose the Light sensor s graph button of Data Studio which is on right bottom. Click x-axis of graph and change graph to Light intensity Vs Position. (6) Run Data Studio and put acrylic in order to make 0 degree incident rays. Record d 1. (7) Put light sensor at the end of linear converter and rotate acrylic. Record θ 1 and d 2 on Table 3. (8) Confirm maximum light intensity point. Find out displacement by subtracting d 1 and d 2. (9) Repeat (6)~(8) with different angle. (10) Find out refractive angle θ 2 by using measured displacement. Compare refractive index. Reference : equation for θ 2 at displacement experiment. - 6 -
5. Results Table 1. : Measurement of ray route along plane plate Incident angle Reflective angle Refractive angle Refractive index 30 degree 40 degree 50 degree 60 degree Average refractive index Table 2. : Critical angle measurement # Critical angle Refractive index 1 2 3 Average Table 3. : Displacement measurement Thickness of medium : Incident angle d1 d2 Displacement (d2-d1) 20 degree 30 degree 40 degree 50 degree Refractive angle Refractive index - 7 -