[填空题]
A He-Ne gas laser which produmn:ap+i 4e k4uces monoz9/7okibdx:s,s5 bg/h;jm c ychromatic light of a known wavelength $\lambda\;=\;6.328\times10^{-7 }\;m$ is used to calibrate a reflection grating in a spectroscope. The first-order diffraction line is found at an angle of 21.5$^{\circ}\,$ to the incident beam. How many lines per meter are there on the grating? $ \times10^{ 5}\;lines/m$
