Experiment 11. Grating constant of diffraction grating

Transcription

1 Experiment 11. Grating constant of diffraction grating 1. Purpose Measure light source s wavelength by using diffraction grating, those grating constant is already known, in order to understand diffraction phenomenon. With this light source, measure unknown diffraction grating s grating constant. 2. Principle Figure 1 illustrates light, which has λ wavelength, passes through glass plate of diffraction grating (G) and m th diffraction light tilted to ф angle. Optical pass difference between nearest linear light is Δ=dsinф Figure 1. where all diffracted light show constructive interference. It makes light intensity maximum. The d is grating constant and m is number of diffracted light. Therefore, if we know grating constant d while measuring diffraction angle ф, we could know wavelength of light source and vise versa

2 3. Equipment and set up Equipment Quantity Laser Diode 1 Light Sensor 1 Rotary Motion Sensor 1 Optics Bench 1 1M Rotation plate 2 Diffraction grating 1 Known grating constant Diffraction grating 1 Unknown grating constant 4. Experiment procedure Caution - Use diffraction grating carefully. - Use Light sensor under limit sensitivity of light intensity. 1-1) Install Data studio program (1) Connect Science Workshop interface into computer and turn on. (2) Connect light sensor and rotary motion sensor to Analog Channel A and Digital Channel, respectively

3 1-2) Equipment and Setup As shown below Figure, Put light sensor at the end of optical bench and laser sensor onto rotation plate. Put rotary motion sensor at the center of rotation plate in order to measure rotation angle. Since ratio of motion sensor to rotation plate is not 1:1, we must rotate rotation plate to 180º for getting correction ratio. Meanwhile, Put sensitivity of light sensor to 1 to control intensity of rotational light. 2) Wavelength measurement of laser diode (1) Put laser diode on optical bench and diffraction grating onto rotation plate as shown above Figure. Attach rotation plate on rotary motion sensor. (2) Put slit in front of light sensor for measuring intensity of passed light precisely. (3) Set up beginning stage by watching quantity of light passing through light sensor to be 0 th diffraction angle. (4) Run Data Studio for connecting light sensor and rotary motion sensor (5) Choose Light Sensor graph located on the lower right side menu. Click x-axis on graph and invert graph as intensity of light vs angle. (6) Rotate rotation plate and observe intensity of light dependence on angle. (7) Read angle when intensity of light show large value after 0 degree and get constant each diffraction angle. (8) Read until 3 rd diffraction angle appears. ( Measure light intensity with increasing sensitivity up to

4 when you have difficulties reading 2 nd diffraction angle. ) (9) Move initial position of diffraction plate and rotate rotation plate opposite direction. Repeat (6)-(8) process and measure angle until 3 rd diffraction angle appears. (10) Record results on Table 1. Calculate average values of each diffraction angle. Calculate wavelength of laser diode by using already known grating constant. 3) Measurement of unknown grating constant - 4 -

5 5. Results Diffraction grating experiment DARA SHEET Table 1. laser diode wavelength measurement (θ m and θ m represents different direction angle) Grating number per mm (n) Grating constant (1mm/n) Diffraction number 1 st (m=1) 2 nd (m=2) 3 rd (m=3) θ m θ m 2Φ m = θ m - θ m Φ m Λ m [mm] Wavelength of laser diode : Λ=(Λ 1 +Λ 2 +Λ 3 )/3 [μm] Λ m =(sin Φ m )/mm [μm] Table 2. Measurement of unknown grating constant Wavelength of laser diode [mm] Diffraction number 1 st (m=1) 2 nd (m=2) 3 rd (m=3) θ m θ m 2Φ m Φ m N Grating number : n=(n 1 +n 2 +n 3 )/3 n=(sin Φ m )/ Λ m m - 5 -

6 6. Questions 1) Why do we do experiments with putting slit in front of laser diode? 2) We user monochromatic light which is emitted from laser diode. But, the light what we see usually is white light which is composed of several wavelengths. What if we use this light as laser diode? Which do we add equipment for doing experiment? And how do we install? 3) Which circumstances are happened when grating constant is smaller than wavelength of laser diode? 4) What kind of laser diode do we need in order to investigate diffraction by passing through grating structure having atom-size grating constant? - 6 -