22P3Q07

7a

Light waves which have a constant phase difference among them.

COMMENT: Careful. Coherence does not mean zero phase difference.

7b)

\displaystyle \begin{aligned}   (d\sin \theta &=n\lambda ) \\  \frac{0.001}{300}\sin \theta &=2(640\times {{10}^{-9}}) \\  \theta &=22.58{}^\circ  \end{aligned}

\displaystyle \begin{aligned}   \tan 22.58{}^\circ &=\frac{L}{2.1} \\  L&=87.3\text{ cm} \end{aligned}

COMMENT: Note that at angles of more than 20°, we cannot use small angle approximations. The fringe spacing is not constant, so the \displaystyle \Delta y=\frac{L\lambda }{d} formula is not valid.

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