Light travels from water (refractive index 1.33) into glass (refractive index 1.52). (a) In which medium does light travel faster, and how does the difference in speed determine the direction in which the ray bends at the interface? (b) If this piece of glass is shaped into a concave lens and a ray parallel to the principal axis enters it from air, describe the complete path of the ray after it exits the lens and explain where an observer on the transmission side would perceive the ray to have originated.
Generated by claude-sonnet-4-6 · 2026-06-26 01:16 · grounding rag
Model Answer
(a) Light travels faster in water (n = 1.33) than in glass (n = 1.52), since a higher refractive index means lower speed. When light passes from water into glass (rarer to denser medium), it slows down and bends towards the normal at the interface.
(b) A ray parallel to the principal axis, after entering the concave lens from air, diverges (spreads out) after refraction. The diverging ray, when extended backwards on the transmission side, appears to meet at the principal focus on the same side as the incident ray (in front of the lens). An observer on the transmission side would perceive the ray as originating from this virtual, erect focus located between the optical centre and the lens.
Source: Chapter 9, Sections 9.3, 9.3.1, 9.3.2
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Explanation
- Part (a): The key link is refractive index ↔ speed: higher n = slower speed. Water→glass is rarer→denser, so the ray bends towards the normal. Many students confuse the direction; always use the rarer-to-denser rule.
- Part (b): A concave lens always forms a virtual, erect, diminished image. A ray parallel to the principal axis diverges after passing through a concave lens, and its backward extension passes through the focal point on the same side as the object (incident side). This focal point is virtual — the observer perceives the ray as coming from there. Examiners expect you to mention: divergence of ray, virtual focus, and observer's perception of origin.