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Homework Statement
In Fig. 6.1 the distance from the two slits to the screen is 1.8 m. The distance CP is
2.3 mm and the distance between the slits is 0.25 mm.
Calculate the wavelength of the light provided by the laser.
Homework Equations
$$asinθ = nλ$$
$$w = \frac{λD}{a}$$
The Attempt at a Solution
$$θ = tan^{-1}(\frac{2.3×10^{-3}}{1.8}) = 0.0732°$$
Since P is first order, n = 1.
$$ λ = (0.25×10^-3)(sin(0.0732°)) = 319.4 nm $$
According to the mark scheme it is incorrect:
$$ λ = ax / D $$
$$ = \frac{2 × 2.3 × 10^{–3} × 0.25 ×10^{–3}}{1.8} $$
$$ = 639 nm $$Why must I multiply my wavelength by two?
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