[填空题]
A He-Ne gas laser which produces mkzp h7 g-;4 sgpn*c3fdld h96wonochromakotb-ngp5x adx7k n1wh 6(,i4lsh.6 ptic 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$
