Why is induced electric field due to bound charges in dielectric always less than the external electric field?
Answer
When a dielectric is placed in an external electric field $\vec E_{\rm external}$ a change is induced on the surface of the dielectric as shown in the diagram below in red.
This may be due to the formation of temporary electric dipoles in the dielectric or the rotation of permanent electric dipoles.
The number if induced charges cannot exceed the number of external charges and is usually less.
When the electric fields are shown you will see that the induced charges on the dielectric produce an induced electric field within the dielectric $\vec E_{\rm induced}$.
By superposition the electric field within the dielectric $\vec E_{\rm external} + \vec E_{\rm induced}$ will be less than $\vec E_{\rm external}$.
The very best that can be done is to reduce the field in the dielectric to zero.
This occurs when the number of external charges is equal the number of induced charges ie $|\vec E_{\rm external}|=|\vec E_{\rm induced}|$.
You might no longer call the material a dielectric but rather call it a conductor.
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