2. 在折射率<img src="https://tihai-oss-cloud.itihey.com/img/54bc07f130360e9c00924c5565bea429.png">的照相机镜头表面涂有一层折射率<img src="https://tihai-oss-cloud.itihey.com/img/125a6901b1115da043c36f9e40b8e3c7.png">的Mg<img src="https://tihai-oss-cloud.itihey.com/img/45068d68231cb77c8284d8e637d0fc9b.png">增透膜,若此膜仅适用于波长<img src="https://tihai-oss-cloud.itihey.com/img/1918d65b5b15ac6ae048979607ad9c8a.png">=550nm的光,则此膜的最小厚度为多少