Doublet Separation, ExperimentThe experimental procedure involves examining the interference fringes of the light at normal incidence ( angle = 0°) such that constructive interference is produced by one wavelength of the doublet while the other wavelength is adjusted to a halfinteger condition. This can be seen in the interference pattern as a weaker ring halfway between the stronger interference maxima. This corresponds to Advancing the micrometer until this condition occurs again provides data from which the doublet separation can be calculated. Subraction of the two distances obtained in the doublet experiment gives the following expression: This expresses the fact that the shorter wavelength has gained an order on the longer wavelength. Combining terms gives Substituting this back into the top equation gives The relationship for the doublet separation is then

Index Interference concepts Example of sodium doublet  

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Doublet Separation, FabryPerotThe resolvance of the FabryPerot interferometer is sufficient to resolve closely spaced spectral doublets like the sodium doublet and the hydrogendeuterium doublet. The experimental procedure yields a distance of movement of the interferometer mirror which allows the doublet separation to be calculated from the relationship
The average wavelength of the doublet must be known from other measurements to make use of this relationship. The default values in the calculation are for the sodium doublet. You may change the numbers to explore variations. 
Index Interference concepts Example of sodium doublet  

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