why does the electric field inside a dielectric medium decreases wen it is placed in an external electric field.??

A dielectric material gets polarised when it is placed inside an electric field. The field produced due to the polarisation of material minimise the effect of electric field. Hence,the electric field inside a dielectric medium decreases when it is placed in an external electric field.

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The “resultant” electric field inside dielectric decreases because dipoles (present in the form of polar covalent bonds) in the dielectric get oriented owing to the applied external electric field. And unlike ideal conductors where there are a lot of free electrons to completely nullify the effect of applied external fields (and that’s why ideal conductors are always equi-potential i.e. electric field intensity within them is zero), electrons in most dielectrics are bonded so they can only nullify the effect of external field to a limited extent.
The external field induces dipole moment by stretching or re-orienting the molecules of dielectric. Net effect of these induced molecular dipole moment is the production of a field that opposes the external field. So, dielectric gets polarised in opposite direction and, electric field inside dielectric decreases.
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Ro go
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