In a physics lab, light with a wavelength of 520 nm travels in air from a laser to a photocell in a time of 16.6 ns . When a slab of glass with a thickness of 0.850 m is placed in the light beam, with the beam incident along the normal to the parallel faces of the slab, it takes the light a time of 21.6 ns to travel from the laser to the photocell. What is the wavelength of the light in the glass?