A diffraction grating has slits ruled on opaque material and has 20 slits per mm. The distance between the slit centres is twice the width of each slit. The grating is uniformly illuminated at normal incidence. The transmitted light passes through a lens of focal length 1 m and then onto a screen that lies 1 m from the lens. On the screen the first-order diffraction peak lies 10mm from the centre-line of the apparatus. Calculate the wavelength of the light and predict the location of the next visible diffraction peak.