Generated by claude-sonnet-4-6 · 2026-06-15 06:48 · grounding rag
Model Answer
(i) Ray diagram — Refraction through a rectangular glass slab:
```
N
|
E | Air
\ |
\ | (incident ray)
\ |i
------O----------- AB (air-glass surface)
\
r\ Glass
\
O'
---------|-------- CD (glass-air surface)
\r'
\ Air
\
H
|
M'
```
- Ray EO is the incident ray (oblique, in air).
- OO′ is the refracted ray (bends towards normal inside glass; angle of refraction < angle of incidence).
- O′H is the emergent ray (bends away from normal; parallel to EO but laterally displaced).
- NN′ ⊥ AB and MM′ ⊥ CD are the normals at the two surfaces.
(ii) Snell's Law:
The ratio of the sine of the angle of incidence to the sine of the angle of refraction is a constant for a given colour of light and a given pair of media:
$$\frac{\sin i}{\sin r} = \text{constant (refractive index)}$$
(iii) Differences between virtual images formed by convex and concave lenses:
| Basis | Convex Lens | Concave Lens |
|---|---|---|
| (I) Object distance | Object must be placed between the optical centre O and focus F₁ (u < f). | Object can be placed anywhere between optical centre O and infinity; virtual image is always formed. |
| (II) Magnification | Magnification is greater than 1 (image is enlarged). | Magnification is less than 1 (image is diminished). |
Source: Chapter 9, Section 9.3.1 (Refraction through a Rectangular Glass Slab) and Section 9.3.4 (Image Formation by Lenses)
---
Explanation
- (i) Draw the diagram neatly and label: incident ray, refracted ray, emergent ray, normals (NN′, MM′), angles i and r, surfaces AB and CD, and media (Air/Glass). The emergent ray must be shown parallel to the incident ray (lateral displacement). Examiners award marks for correct bending direction (towards normal entering glass, away from normal leaving glass) and correct labelling.
- (ii) State both parts of Snell's law clearly and write the formula. Many students write only the formula and lose the statement mark.
- (iii) This is a 2-mark differentiation, so a table with two clear rows (one per basis) is the cleanest format. Key facts to remember: convex lens forms a virtual image only when the object is inside the focal length (m > 1); a concave lens always forms a virtual, erect, diminished image (m < 1) for any object position.