Core Practical 8: Diffraction and Wavelength

In Huygens’ construction, points on a wavefront act as sources of secondary wavelets; their forward envelope gives a subsequent wavefront. At an opening or obstacle edge, spreading into the geometrical shadow can be understood through these wavelets.

Diffraction is appreciable when the opening is comparable with wavelength. A wider slit gives less angular spreading for the same wavelength. This description concerns the wave pattern, and should not be confused with a lens refracting rays to an image.

Adjacent grating slits have path difference d sinθ in a far-field direction θ. Bright orders occur when this equals an integer number n of wavelengths. The central maximum is n=0; first order means n=1, not the first bright spot counted from the left of a photograph.

Convert line density to lines per metre before taking its reciprocal for d. The condition |sinθ|≤1 limits the highest possible order; an algebraic value above one means that order cannot occur, not that the calculator must be forced to return an angle.

Use an approved low-power laser or suitable monochromatic source, a grating and screen or angular measurement apparatus. Align the beam normal to the grating and identify the central maximum. Measure matching orders on each side to reduce a zero-offset or alignment bias.

If the screen is distance D away and an order lies transverse distance x from the centre, calculate θ=arctan(x/D). Using x/D directly as an angle is a small-angle approximation and may be poor for a grating. Avoid direct or reflected beams entering eyes and keep the beam safely controlled. Repeat alignment and compare wavelength estimates from different usable orders.

Try each problem before opening the worked solution. State the physical model, show substitutions with consistent units, and explain what your answer means.