Answer:
500 nm.
Explanation:
The wavelength of harmonic oscillator can be calculated by the formula,
[tex]\lambda_{1} =\frac{\lambda}{n}[/tex]
Here, [tex]\lambda[/tex] is the longest wavelength, and n is the number of order.
Now given that the longest wavelength is 1000 nm.
Now calculate second longest wavelength,
[tex]\lambda_{1} =\frac{1000 nm}{2}\\\lambda_{1} =500 nm[/tex]
Therefore, the second longest wavelength is 500 nm.