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26 October, 04:25

69. A grating with lines uniformly spaced at 775 nm is illuminated by a sodium discharge lamp.

For what range of wavelength will there only be three principal lines generated?

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  1. 26 October, 04:38
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    Only two diffraction orders + 1 and - 1 are possible

    Explanation:

    The diffraction net is a system with many uniformly spaced lines, which is described by the expression

    d sin θ = m λ m = 1, 2, 3, ...

    Where d is the spread between the lines, λ the wavelength and m is an integer that corresponds to the order of diffraction, all wavelengths

    For this case the spacing is

    d = 775 nm

    The sodium emission lines are 589.00 nm and 589.59 nm

    With this data we can calculate how many diffraction orders are visible

    θ = sin⁻¹ (m λ / d)

    m = 1

    λ1 = 589.00 nm

    θ1 = sin⁻¹ (1 589/775)

    θ1 = 49.46 °

    λ2 = 589.59 nm

    θ2 = sin⁻¹ ((1 589.59 / 775)

    θ2 = 49.53 °

    m = 2

    λ1 = 589.00 nm

    θ1 = sin⁻¹ (2 589/775)

    θ1 = sin⁻¹ (1.5)

    This order is not possible because the sin cannot be greater than 1

    m = - 1

    λ1 = 589 nm

    θ1 = sin⁻¹ (-1 589/775)

    θ1 = - 49.46

    θ = sin⁻¹ (-1 589.59/775)

    λ2 = 589.59 nm

    θ2 = - 49.53 °

    m = - 2

    λ1 = 589 nm

    θ1 = sin-1 (-2 589/775)

    This order does not occur because the breast cannot be greater than 1

    Only two diffraction orders + 1 and - 1 are possible
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