A beam of monochromatic light is refracted from vacuum into a medium of refractive index 1.5, the wavelength of refracted light will be

From  $\mu =\frac{c}{v}=\frac{n{\lambda }_v}{n{\lambda }_m},{\lambda }_m=\frac{{\lambda }_v}{\mu }$
Here, c = velocity of light in medium and
v = velocity of light in vacuum; ${\lambda }_m$ = refractive index of the medium.
Hence, wavelength in medium  ${\lambda }_m<{\lambda }_a \left(\because \mu >1,given\right)$
So, the required wavelength decreases.
Alternatively,
$v=f\lambda$ , On refraction, the frequency, f do not change. When light is refracted from vacuum to a medium, the velocity, v decreases. Therefore, $\lambda$  also decreases.