PROCEDURE Clamp a metre stick horizontally in a stand. Allow the laser beam to hit the metre stick normally (at 90°). Move the metre stick sideways until the spot is on the 50 cm mark. Place the grating between the laser and the metre stick, at right angles to the beam. Observe the interference pattern on the metre stick a series of bright spots. – Calculate the mean distance x between the centre (n=0) bright spot and the first (n =1) bright spot on both sides of centre. Measure the distance D from the grating to the metre stick. Calculate using tan = x/D. θ θ Calculate the distance d between the slits, using d =1/N, where N is the number of lines per metre on the grating. Calculate the wavelength using = λ nλ dsinθ. Repeat this procedure for different values of n and get the average value for λ. RESULTS n x (m) D (m) θ λ CONCLUSION We got a value of 7.10 × 10-7 m, which sits nicely in the accepted range of 620 750 nm for red light. Job is oxo – SOURCES OF ERROR / PRECAUTIONS The diffraction grating may not have been exactly at right angles to the beam of light; x should be the same on both sides. Measure x on both sides of the n = 0 position and take the average of the two readings. To reduce the percentage error further just measure the total distance between both points and divide by 2 Determining the exact middle of the dot on the screen was difficult (repeat and get an average) NOTE A nice variation on this is to take a school laser into a large darkened room (e.g. a gym) and get very large distances for D and x on either side. Leaving Cert Physics Syllabus Content Depth of Treatment Activities STS 1. Diffraction and Use of diffraction grating Suitable method of Interference colours interference formula: n = d Sin demonstrating the wave Petrol film, soap nature of light. bubbles. Derivation of formula Appropriate calculations. 2. Light as a transverse Polarisation Demonstration of Stress polarisation. wave motion polarisation using polaroids Polaroid sungalsses. other method. or 3. Dispersion Dispersion by a prism and Demonstration. Rainbows, polished
PROCEDURE Clamp a metre stick horizontally in a stand. Allow the laser beam to hit the metre stick normally (at 90°). Move the metre stick sideways until the spot is on the 50 cm mark. Place the grating between the laser and the metre stick, at right angles to the beam. Observe the interference pattern on the metre stick – a series of bright spots. Calculate the mean distance x between the centre (n=0) bright spot and the first (n =1) bright spot on both sides of centre. Measure the distance D from the grating to the metre stick. Calculate θ using tan θ = x/D. Calculate the distance d between the slits, using d =1/N, where N is the number of lines per metre on the grating. Calculate the wavelength λ using nλ = dsinθ. Repeat this procedure for different values of n and get the average value for λ. RESULTS n x (m) D (m) θ λ CONCLUSION We got a value of 7.10 × 10-7 m, which sits nicely in the accepted range of 620 – 750 nm for red light. Job is oxo SOURCES OF ERROR / PRECAUTIONS The diffraction grating may not have been exactly at right angles to the beam of light; x should be the same on both sides. Measure x on both sides of the n = 0 position and take the average of the two readings. To reduce the percentage error further just measure the total distance between both points and divide by 2 Determining the exact middle of the dot on the screen was difficult (repeat and get an average) NOTE A nice variation on this is to take a school laser into a large darkened room (e.g. a gym) and get very large distances for D and x on either side. Leaving Cert Physics Syllabus Content Depth of Treatment Activities STS 1. Diffraction and interference Use of diffraction grating formula: n = d Sin Derivation of formula Suitable method of demonstrating the wave nature of light. Appropriate calculations. Interference colours Petrol film, soap bubbles. 2. Light as a transverse wave motion Polarisation Demonstration of polarisation using polaroids or other method. Stress polarisation. Polaroid sungalsses. 3. Dispersion Dispersion by a prism and Demonstration. Rainbows, polished
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