Level+3+Physics

On Interference slits.
Hi Johanan You are right. If all else is equal two slits will let through twice as much light as one slit, resulting in the addition of in-phase waves causing a much brighter central maximum than one slit. I have not idea how the error slipped through – we will get it correctly, hopefully for next year. Yes brightness means pretty much the same as intensity: intensity is a bit more scientific and whereas brightness refers to the appearance of something, and colour, for example, affects how bright things appear. (Our eyes are more sensitive to some wavelengths than others, so a spectrum of equal intensity will look dimmer in places… Have you noticed how rainbows fade out at the edges as your eyes are less sensitive to the radiation?)

[] has some good images which allow you compare intensity with slit number Kerry Parker, May 2010

On significant Figures
Significant figures – most school equipment measures to an absolute maximum of two significant figures, most often it measures only to one significant figure. Your final value of a calculation should reflect this. For example, measuring mass or 4.7g, any force calculated using F = ma where a = 9.8 Nkg-1 would have an answer of 46 N. However don’t round too soon. If you need to carry this value of the force into another calculation, use the full 46.07 Also remember when quoting absolute uncertainties, you can only justify stating them to one significant figure. For example an ordinary 30 cm ruler measures to ± 1 mm. So lengths can be written as 12.5 ± 0.1 cm or 125 ± 1 mm Incidentally, even with a digital watch, stopwatch or phone, you can only time accurately to ± 0.1 s. Also most electronic time (which is automatic) will only time accurately to ±0.01 s. Swimming is a bit different because swimmers touch a pad. We teach a particular method of handling uncertainties in PH304, ‘Graphical Analysis’ for handling values in measurements for experiments. Tish glasson May 2011