Answer:
6
Explanation:
Following are the possible transition
1 ) 4 to 3
2) 4 to 2
3) 4 to 1
4 ) 3 to 2
5 ) 3 to 1
6 ) 2 to 1 .
There are altogether 6 wavelengths of photons possible .
This result can be achieved with the help of permutation and combination.
No of ways in which 2 things can be selected out of 4 things , combination being all different.
₂C⁴ = (4 X 3) / 2 = 6.
