In the given figure, a ray of light passes through four transparent media with refractive indices n1, n2, n3 and n4. The surfaces of all the media are parallel. If the emergent ray CD is parallel to the incident ray AB, we must have-
1
n1 = n2
2
n2 = n3
3
n3 = n4
4
n4 = n1
5
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