I got A and C but the markscheme says C only any explanations?
The angle of the first order beam will have a sine which is half of the sine of the second order beam. It is the sines that will halve, not the angles.
The angle of the first order beam will have a sine which is half of the sine of the second order beam. It is the sines that will halve, not the angles.
maybe you're thinking about a small angle approximation. halving sin (Θ) implying halving Θ is a good approximation... but only when Θ is a small angle, the question doesn't say anything about the size of Θ (and the diagram doesn't make Θ look small tbh)