An Lα X-ray is emitted when an electron jumps from the M to the L shell, and a Kα X-ray is emitted when an electron jumps from the L to the K shell. Less energy is involved when an electron jumps from the M to the L shell than when it jumps from  the L to the K shell. Also, wavelength of a photon is inversely related to its energy. Therefore, an Lα X-ray has higher wavelength than a Kα X-ray for the same material.