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**Question**-

*A certain crystal has a set of planes spaced 0.376 nm apart. A beam of neutrons strikes the crystal at normal incidence and the first maximum of the diffraction pattern occurs at a scattering angle (i.e angle between incoming neutrons and reflected beam) of 41.4 degrees. What is the de Broglie wavelength of the neutrons in units nm and to 3 decimal places.*

**Usefull equations**-

2d sin theata = nλ

where d = plane spaceing, n is the order in this case =1

**My Attempt-**

useing above eqn,

2*0.0376E-9 (m)*sin(41.4)=4.973E-11 m

I have been told that this solution is wrong as the question states "at normal incidence" which apparantly complicates things and makes a need to change the angle theata. Any suggestions? thanks in advance.