Answer:
500 nm.
Step-by-step explanation:
The wavelength of harmonic oscillator can be calculated by the formula,
![\lambda_(1) =(\lambda)/(n)](https://img.qammunity.org/2020/formulas/physics/college/ae8l6pkv94k04rt1txjupakesppq0drzsf.png)
Here,
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,
![\lambda_(1) =(1000 nm)/(2)\\\lambda_(1) =500 nm](https://img.qammunity.org/2020/formulas/physics/college/bfmrza7dtb53uab207cuhc5nkg69r16sgf.png)
Therefore, the second longest wavelength is 500 nm.