Dielectrics at optical frequencies — assertion–reason on εr and ionic polarization Assertion (A): For ionic dielectrics, the optical dielectric constant (εr measured at optical frequencies) is less than the static dielectric constant. Reason (R): At optical frequencies, the ionic polarization component Pi is effectively zero (only electronic polarization responds).

Introduction / Context:The frequency dispersion of permittivity arises because different polarization mechanisms have different response times. Electronic polarization responds up to optical frequencies; ionic and orientational polarizations cannot follow very rapid fields, leading to a reduced εr at high frequency.

Given Data / Assumptions:

• Ionic dielectric with multiple polarization mechanisms: electronic, ionic, and possibly orientational.
• Optical frequency range where lattice ions cannot follow the field.
• Linear response regime.

Concept / Approach:Total polarization P is the sum of contributions. At low (static) frequencies, both electronic and ionic components contribute significantly, giving a higher εr. At optical frequencies, the ionic displacement cannot keep pace, so Pi ≈ 0, leaving primarily electronic polarization. Consequently, εr,optical < εr,static. Therefore, R explains A.

Step-by-Step Solution:Recognize mechanism time scales: electronic (fast), ionic (slower), orientational (slowest).At optical frequencies, only electronic polarization remains effective.Hence static εr (electronic + ionic + orientational) exceeds εr at optical frequencies (essentially electronic only).

Verification / Alternative check:Typical dispersion curves show stepwise decreases of εr with increasing frequency at relaxation or resonance bands; the highest-frequency plateau corresponds to electronic polarization alone.

Why Other Options Are Wrong:Any option denying the reduction of εr at optical frequencies contradicts standard dielectric dispersion theory.

Common Pitfalls:

• Confusing optical dielectric constant with static values used in low-frequency capacitor design.
• Overlooking temperature effects that can alter low-frequency orientational polarization.

Final Answer:Both A and R are true and R is correct explanation of A