People with bad eyesight use glasses made of high index glass. If a ray in air strikes the glasses at 31.2 deg, and enters the glass at 17.6 deg, what is the index of refraction of the glass?

Light from a sodium lamp passes through a diffraction grating that has 1000 slits per millimeter. the interference pattern is viewed on a screen 1.000 m behind the grating. the first (m = 1) two bright yellow fringes that are visible are 0.7288 m and 0.7300 m from the central maximum. what are the wavelengths of these two fringes?