318CHAPTER 24 24.28 As light emerging from the glass reflects from the top of the air layer, there is no phase reversal produced. However, the light reflecting from the end of the metal rod at the bottom of the air layer does experience phase reversal. Thus, the condition for constructive interference in the reflected light is ( )122airtmλ=+. As the metal rod expands, the thickness of the air layer decreases. The increase in the length of the rod is given by ()1122airairairifLtmmm2λλ∆=∆=+−+=∆The order number changes by one each time the film changes from bright to dark and back to bright. Thus, during the expansion, the measured change in the length of the rod
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