[填空题]
A He-Ne gas laser which produces monochr;2 7taista7jkomatic light of a known wavelpo 39m7xp7kmrength $\lambda\;=\;6.328\times10^{-7 }\;m$ 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$
