Can w = λD/s be used with a single slit?

...and also can it be used with diffraction gratings.

Heyaa~

AQA A Level physics student here

w = λD/s is used for young’s double slit questions only. You can remember this my knowing that ‘s’ stands for slit SEPARATION, implying there’s two to be separated apart. You can’t use only one slit for this equation.

For a single slit, therefore, there’s another equation. Depending on your exam board, you may or may not need to know it. Consult with your specification to see what equations you need. For answering single slit questions, however, they don’t require an equation anyway.
Remember that the closer the single slit width is to the value of the wavelength, the more the light diffracts. This often means to reduce the single slit width since light wavelengths are VERY small in relation to how wide single slit widths are. Greater diffraction means more fringes in the pattern.

For diffraction gratings, only this equation is used:

nλ = dsinθ

n (number of diffraction order)(you’ll learn what this means)

λ (The wavelength)

d (m per line/ i.e. distance between each slit from one another)(you’re almost always given this value in ‘lines per mm’ in questions, which you need to manipulate to change to m per line by finding its reciprocal and multiplying by 10^3)(you’ll learn this as time goes on)

sinθ (the angle that the light diffracts to reach whatever diffraction order is needed)

Hope this helped

~Haneul_Sol

I'm AQA too May I ask one more question, do we need to know the equation W (fringe width) = λD/a. I was doing Edexcel last year (long story) and I think this was mentioned, but not sure if it's needed for AQA.