Refraction: measure the bend from the normal
A practical ray-diagram guide distinguishing refraction, normal incidence and lateral displacement.
Refraction occurs when light changes speed between transparent media; an oblique ray can change direction at the boundary.
Draw the reference line first
NCERT’s light chapter measures ray angles from the normal: a line perpendicular to the boundary at the point where the ray meets it. In the air-to-glass example, the ray bends toward that normal; returning from glass to air, it bends away.
Optical density in this discussion is not the same as mass density. Avoid deciding a ray’s direction merely from how heavy a material feels.
Original angle-reading exercise
Suppose a drawn ray makes 35 degrees with the surface. The normal is perpendicular to that surface, so the angle with the normal is 90 − 35 = 55 degrees. A question asking for the incidence angle therefore needs 55 degrees, not 35.
Next imagine an oblique air-to-glass ray with an incidence angle of 40 degrees and a refracted angle of 25 degrees. The smaller angle to the normal shows bending toward it. These are illustrative drawing values, not a measured refractive-index table.
A parallel slab can shift a ray
For an oblique ray passing through a rectangular glass slab with air on both sides, NCERT shows an emergent ray parallel to the incident ray but displaced sideways. Parallel directions do not mean the light followed one unchanged straight line through both surfaces.
Sketch the original incident line as a dotted extension. Compare the final ray with that extension to see the displacement. This helps separate an angular change from a sideways shift.
Check the special case
At normal incidence, the ray meets the boundary along the normal and does not acquire the usual directional bend. A change of speed need not imply a change of direction.
Before answering, label the boundary, normal, incident ray and refracted ray. Then identify the two media and whether the ray is oblique or normal. This sequence prevents a memorised “toward” or “away” rule from being applied without its conditions.
Publication disclosure
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Frequently asked questions
Are incidence angles measured from the surface?
No. They are measured from the normal.
Can refraction occur without a directional bend?
Yes, at normal incidence the speed can change without bending the ray.