Science (086) — AI-generated practice question

Observation 1 — Slab at an angle (line appears bent):
When one edge of the glass slab is inclined to the drawn line, light from the line travels obliquely from air into glass (denser medium) and bends towards the normal (angle of refraction < angle of incidence). At the opposite face it passes from glass back into air and bends away from the normal. According to Snell's law, $\frac{\sin i}{\sin r} = \text{constant}$. Because the ray strikes the surface at an oblique angle, the lateral (sideward) shift is visible, making the line appear bent/displaced sideways at the edges of the slab.

Observation 2 — Slab perpendicular to the line (line appears raised, not bent):
When the slab edge is exactly perpendicular to the line, light travels normally (straight down) through the slab. A ray incident normally (i = 0°) passes without any change in direction, so there is no bending or lateral shift. However, the slab is denser than air, so light slows down inside it. The portion of the line beneath the slab appears raised (shifted upward) due to the difference in optical density — exactly like printed letters appearing raised when a thick glass slab is placed over them.

Conclusion:
The key difference is the angle of incidence: oblique incidence causes lateral displacement (bent appearance); normal incidence causes only apparent depth shift (raised appearance) with no bending.

Source: Chapter 9, Section 9.3 Refraction of Light; Activity 9.9; Section 9.3.1 Refraction through a Rectangular Glass Slab

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What examiners look for (5 marks):

• 1 mark: Identifying that Case 1 involves oblique incidence → refraction at both faces → lateral shift/bending.
• 1 mark: Correctly applying Snell's law / bending towards/away from normal.
• 1 mark: Identifying that Case 2 involves normal (perpendicular) incidence → i = 0 → no bending.
• 1 mark: Explaining why the line still appears raised (apparent depth / denser medium effect).
• 1 mark: Clear concluding contrast between the two cases.

Key points to remember:

• A ray at 0° incidence passes straight through — no lateral displacement, no bending.
• Apparent raising of objects seen through a slab is due to the denser medium slowing light, not due to oblique refraction.
• The textbook (Activity 9.9) explicitly contrasts these two placements — always link your answer to that activity.