Wavelength of light

[JoeParkinson]

What is the best way to accurately measure the wavelength of a light source that has been filtered eg. red light
I have researched diffraction gratings - and found n x lambda = dsin(theta) where n is the order of diffraction and d is the slit spacing, also looked at young double slit experiment, but im not sure. Any suggestions would be great, thanks

[Teddio]

Calculate the line spacing of the diffraction grating (d) Diffraction grating are usually defined in "lines per millimetre" Convert this to lines per metre (multiply by 1000) then take the reciprocal to get d. For a monochromatic light source the first order image will be a line, assuming the light has been passed through a slit. Measure the angle between the zero order line and the first order line, this gives angle theta.
The wavelength= d multiplied by the sine of angle theta.

[JoeParkinson]

thanks a lot teddio, big help mate