[填空题]
Light of wavelength tj nvdp.b.r i.lx4 y:(xdyj ()z/e1p 2sogx7 9o-p6u.yw fig a4x8wuu$400\,nm$ and $700\,nm$ is incident on a diffraction grating that has 700 rulings per$mm$. A series of bright lines are formed on a screen. The angles for different maxima for the light are shown.
1.(1)In order to observe bright lines on the screen, the light incident on the diffraction grating must be coherent. What is meant by coherent light?
(2)Find the value for $\frac{sin θ_1}{sin θ_2}$ = .
2.(1)Determine the maximum number of bright fringes of the same colour that can be observed on the screen.
(2)For the same wavelength of light, what change can be made to the arrangement to increase the number of bright lines observed?
