At temperature T, the average kinetic energy of any particle is (3/2) KT. The de Broglie wavelength follows the order:

Kinetic energy of any particle= $\frac{3}{2} KT$

Also K.E.= $\frac{1}{2} {mv}^{2}$

$\frac{1}{2} {mv}^{2} = \frac{3}{2} KT v = \sqrt{\frac{3 KT}{m}}$

De-broglie wavelength

= $λ = \frac{h}{mv} λ = \frac{h}{\sqrt{3 KTm}} λ α \frac{1}{\sqrt{m}}$

mass of thermal neutron > mass of thermal electron > mass of visible photon

$∴$ order of debroglie wavelengths is Visible photon > Thermal electron > Thermal neutron

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