The fringe width in Young's double slit experiment is given by β=dλD.
When the experiment is performed in a medium of refractive index μ, the wavelength of light becomes λ′=μλ.
The new fringe width is β′=dλ′D=μdλD=μβ.
Substituting the given values, β=2.4 μm and μ=1.2:
β′=1.22.4=2 μm.
Answer: 2