Core Practical 6: Investigating Diffraction Gratings PDF

Head to www.savemyexams.com for more awesome resources 2.20 Core Practical 6: Investigating Diffraction Gratings Your notes Core Practical 6: Investigating Diffraction Gratings Aim of...

Head to www.savemyexams.com for more awesome resources 2.20 Core Practical 6: Investigating Diffraction Gratings Your notes Core Practical 6: Investigating Diffraction Gratings Aim of the Experiment To find the wavelength of light using a diffraction grating Variables Independent variable = Distance between maxima, h Dependent variable = The angle between the normal and each order, θn (where n = 1, 2, 3 etc) Control variables Distance between the slits and the screen, D Laser wavelength, λ Slit separation, d Equipment List Resolution of measuring equipment: Metre ruler = 1 mm Page 12 of 38 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Vernier Callipers = 0.01 mm Method Your notes The setup of apparatus required to measure the distance between maxima h at different angles θ 1. Place the laser on a retort stand with the diffraction grating in front of it 2. Use a set square to ensure the beam passes through the grating at normal incidence and meets the screen perpendicularly 3. Set the distance D between the grating and the screen to be 1.0 m using a metre ruler 4. Darken the room and turn on the laser 5. Identify the zero-order maximum (the central beam) 6. Measure the distance h to the nearest two first-order maxima (i.e. n = 1, n = 2) using a vernier calliper 7. Calculate the mean of these two values 8. Measure distance h for increasing orders 9. Repeat with a diffraction grating with a different number of slits per mm An example table might look like this: Page 13 of 38 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Your notes Analysing the Results The diffraction grating equation is given by: nλ = d sin θ Where: n = the order of the diffraction pattern λ = the wavelength of the laser light (m) d = the distance between the slits (m) θ = the angle between the normal and the maxima (°) The distance between the slits is equal to: Where N = the number of slits per metre (m–1) Since the angle is not small, it must be calculated using trigonometry with the measurements for the distance between maxima, h, and the distance between the slits and the screen, D Calculate a mean θ value for each order Calculate a mean value for the wavelength of the laser light and compare the value with the accepted wavelength Page 14 of 38 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources This is usually 635 nm for a standard school red laser Evaluating the Experiments Your notes Systematic errors: Ensure the use of the set square to avoid parallax error in the measurement of the fringe width Using a grating with more lines per mm will result in greater values of h. This lowers its percentage uncertainty Random errors: The fringe spacing can be subjective depending on its intensity on the screen, therefore, take multiple measurements of w and h (between 3-8) and find the average Use a Vernier scale to record distances w and h to reduce percentage uncertainty Reduce the uncertainty in w and h by measuring across all visible fringes and dividing by the number of fringes Increase the grating to screen distance D to increase the fringe separation (although this may decrease the intensity of light reaching the screen) Conduct the experiment in a darkened room, so the fringes are clear Safety Considerations Lasers should be Class 2 and have a maximum output of no more than 1 mW Do not allow laser beams to shine into anyone’s eyes Remove reflective surfaces from the room to ensure no laser light is reflected into anyone’s eyes Page 15 of 38 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Worked example Your notes A student investigates the interference patterns produced by two different diffraction gratings. One grating used was marked 100 slits / mm, and the other was marked 300 slits / mm. The distance between the grating and the screen is measured to be 3.75 m. The student recorded the distance between adjacent maxima after passing a monochromatic laser source through each grating. These results are shown in the tables below. Calculate the mean wavelength of the laser light and compare it with the accepted value of 635 nm. Assess the percentage uncertainty in this result. Page 16 of 38 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Your notes Page 17 of 38 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Your notes Examiner Tip Remember to read the question carefully and make sure dimensions such as the fringe separation are put into meters. 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