Hey Sreejita

Dipole moment is the product of the magnitudes of either charge that makes up the dipole and the distance between them. In chlorobezene and fluorobenzene, there is partial double bond character between halide and benzene ring due to resonance effect which is shorter than the single bonds between the hydrocarbon chain and chlorine.Chlororbenzene/fluorobenzene has sp2 carbons whereas methyl halides don't  and have sp3 carbon, which are less electronegative than sp2 carbons. The electron pulling effect of Cl/ F is reduced by benzene ring and hence accumulate lesser charge.

So, they have lesser dipole moment than methyl halides.

Feel free to ask any other query!!