Refraction Through Glass Slab

Question

A ray of light passes through a rectangular glass slab with parallel faces immersed in air, and a student describes how the emergent ray relates to the incident ray.

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When light passes through a glass slab with parallel faces, NCERT shows that refraction at the first surface bends the ray toward the normal, and refraction at the second parallel surface bends it back by an equal and opposite amount. Because the two normals are parallel, the net angular change is zero, so the emergent ray runs parallel to the incident ray. However, the ray is displaced sideways by a lateral shift that grows with the slab thickness and the angle of incidence. The option claiming a perpendicular emergence is wrong because parallel faces cannot rotate the ray by ninety degrees. The option claiming the same straight line with no shift is wrong because, although the direction is unchanged, the finite slab thickness always introduces a lateral displacement. The option about progressive bending from internal reflection is wrong because ordinary transmission involves refraction, not repeated reflections. As stated in NCERT Class 12, Chapter 9, a parallel-sided slab produces only lateral displacement without net deviation. A geometry check confirms that equal and opposite refractions at parallel surfaces restore the original direction while offsetting the path.

Refraction Through Glass Slab

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