Assertion: Standard optical diffraction cannot be used for discrimination between different X-ray wavelengths.
Reason: The grating spacing is not of the order of X-ray wavelengths.

Options

A. (a) If both the assertion and reason are true and reason explains the assertion assertion.
B. (b) If both assertion and reason are true but reason does not explain the assertion
C. (c) If assertion is true but reason is false.
D. (d) If assertion is false but reason is true.

Answer

A

Explanation

X-rays are electromagnetic radiations whose wavelengths are of the order of `1Å(=10^(-10)m)`. A standard optical diffraction grating cannot be used to discriminate between different wavelenghts in the X-ray wavelength range. For `lambda=1Å(=0.1nm)` and `d=3000nm`, for example, the first order maximum occurs at
`theta=sin^-1((mlambda)/(d))=sin^-1` ((1)(0.1))/(3000)=0.0019^(@)`
This is too close to the central maximum to be practicle. A grating with `d~~lambda` is desirable, but since X-ray wavelength are about equal to atomatic diameter, such gratings cannot be constructed directly.
Hence, option (a) is true.