Investigation of interference effects of Young s double slits and diffraction gratings

Transcription

1 Name: Teacher: Class: Investigation of interference effects of Young s double slits and diffraction gratings Practical skills covered A use of analogue equipment to measure lengths J use of laser or light source to investigate characteristics of light, including interference and diffraction Endorsement skills being assessed 1 Following written procedures 3 safely complete practical work 4 Making and recording observations You are now going to complete a pair of experiments to determine the wavelength of light using; Experiment A: Experiment B: Equipment Young s double slits diffraction grating For both experiments you will need the following equipment: Double slit 0.1mm separation Diffraction grating (300 lines/mm) Three short paper strips (0.3m approx.) One long paper strip (0.8m approx.) Pencil Laser Laser holding case Laser safety glasses Metre rule Blutak Experiment A: Determining the wavelength of a laser using Young s double slits

2 AS Physics Practical : Measuring the wavelength of laser light Page 2 Safety laser light Wear laser safety glasses at all times when any lasers are in operation in the laboratory. You may need to remove safety glasses briefly to mark the positions of maxima and fringes. Be careful not to turn towards the laser. Method 1. Fix a short strip of paper onto the screen. Set D, the distance between the double slits and screen, to be about 1m. Measure and record D on the sheet. 2. Position the double slit (0.1mm separation) in the holder. 3. Turn the laser on and carefully mark the centre of each bright fringe on the sheet to allow you to take suitable measurements of the fringe width, w. 4. Turn the laser off. 5. Repeat the above steps for five further values of D. Mark the maxima on separate sides of the short strips, using both sides of each strip. 6. Once you have gathered the six sets of data on separate sides, for each dataset record the value of D in a suitable table. 7. For each dataset measure the distance across five fringes and calculate the fringe spacing, w. Record w in a table against the corresponding value of D. Repeat the measurement across five different fringes and record this value of w in the table. 8. Calculate the average fringe separation, wave. Record wave in the table. Record all of your results in a suitable table in the space below.

3 AS Physics Practical : Measuring the wavelength of laser light Page 3 9. Plot a graph of w (y-axis) against D (x-axis). 10. Plot a line of best fit and determine the gradient Gradient =. 11. Rearrange the fringe spacing equation, w = #$ % into the form y = mx + c Use your value of the gradient and the rearranged equation above to determine the wavelength, λ, of the laser light. Wavelength =. The manufacturer s value for the wavelength is.. Analysis 1. What precautions did you take when using the laser, why are they necessary? 2. How did you ensure an accurate measurement of w?

4 AS Physics Practical : Measuring the wavelength of laser light Page 4 3. How does the method reduce random error? 4. a. What is the percentage error on the measurement of w? b. What is the percentage error on the measurement of D? c. The percentage error of s is..%. What is the percentage error on your calculated value of λ? 5. a. Describe what would happen to the pattern if the laser was changed from green to red? b. Would your measurement of green light be more or less accurate?

5 AS Physics Practical : Measuring the wavelength of laser light Page 5 Experiment B: Determining the wavelength of a laser using a diffraction grating Safety laser Wear laser safety glasses at all times when any lasers are in operation in the laboratory. You may need to remove safety glasses briefly to mark the positions of maxima and fringes. Be careful not to turn towards the laser. Method 1. Fix a long strip of paper onto the screen/wall. 2. Position the diffraction grating in the holder. Note N, the number of lines per metre: N =... m Switch the laser on and set D, the distance between the diffraction grating and screen, so that at least five orders of maxima are visible on the strip on both sides of the central maximum. Record D: D = m 4. Carefully mark the central maximum and label it. Then carefully mark the higher order maxima on both sides of the central maximum and label them with their order numbers. 5. Turn off the laser and remove the sheet from the wall. 6. Measure X, the distance between central maximum and the n-th order maximum to the left of the central maximum. Repeat the measurement for the n-th order maximum to the right and calculate the mean.

6 AS Physics Practical : Measuring the wavelength of laser light Page 6 7. Repeat the measurements for each visible order of maxima. 8. Using your measurements of Xave and D calculate θ, the angle between the central maximum and the n-th order maxima, and sin(θ). X q D Record these calculations in the table below. n X L / m X R / m X ave / m θ / sin(θ)

7 AS Physics Practical : Measuring the wavelength of laser light Page 7 6. Plot a graph of sin(θ) (y-axis) against n (x-axis). 7. Plot a line of best fit and determine the gradient Gradient =. 9. Rearrange the diffraction equation, nλ = dsinθ into the form y = mx + c 9. Determine d, the grating spacing using the formula d = 3 where N is the number of lines 4 per metre. Gradient =. 10. Use your value of the gradient and the rearranged equation above to determine the wavelength, λ, of the laser light. Wavelength =. The manufacturer s value for wavelength is..

8 AS Physics Practical : Measuring the wavelength of laser light Page 8 Analysis 1. How did you ensure an accurate measurement of θ? 2. Why should a diffraction grating give a more accurate measurement of wavelength, λ, than using a double slit?