Questions
1. Calculate: Light enters glass with i = 60° and r = 35°. Find the unknown using n = sin i / sin r.
n = sin i / sin r = sin 60° / sin 35° = 0.866 / 0.574 = 1.51.
2. Calculate: A coin in water has real depth = 8.0 cm and apparent depth = 6.0 cm. Find the unknown using n = real depth / apparent depth.
n = real depth / apparent depth = 8.0 / 6.0 = 1.33.
3. Calculate: A glass has refractive index n = 1.5. Find the unknown using n = 1 / sin c.
sin c = 1 / n = 1 / 1.5 = 0.667, so c = sin⁻¹(0.667) = 41.8°.
4. Calculate: A convex lens forms a real image: u = 20 cm, v = 30 cm. Find the unknown using 1/f = 1/u + 1/v.
1/f = 1/u + 1/v = 1/20 + 1/30 = 5/60 = 1/12, so f = 12 cm.
5. Calculate: An image forms at v = 30 cm for an object at u = 15 cm. Find the unknown using m = v / u = hᵢ / hₒ.
m = v / u = 30 / 15 = 2.0 (the image is twice as tall).
6. State the manipulated, responding and constant variables for: To determine the refractive index of a glass block by measuring the angle of incidence and the angle of refraction.
Manipulated: Angle of incidence, i. Responding: Angle of refraction, r. Constant: The same glass block (same medium).
7. State the manipulated, responding and constant variables for: To determine the refractive index of glass by comparing the real depth and the apparent depth of a mark seen through glass blocks.
Manipulated: Real depth of the mark, H (using glass blocks of different thickness). Responding: Apparent depth of the mark, h. Constant: The same type of glass and the same viewing method.
8. State the manipulated, responding and constant variables for: To investigate the relationship between the object distance u and the image distance v for a convex lens and to find its focal length.
Manipulated: Object distance, u. Responding: Image distance, v. Constant: The same convex lens (same focal length).
9. Explain: why a straw looks bent in water.
Light bends as it passes from water into air because it changes speed. Your eye traces the bent rays back in straight lines, so the underwater part of the straw appears shifted.
10. Explain: how optical fibres trap light.
Light hitting the wall of a fibre at a steep enough angle is completely reflected back inside, not refracted out. This total internal reflection lets light travel long distances along the fibre.
11. Explain: how a magnifying glass makes things bigger.
A magnifying glass is a convex lens. Held close to an object, it forms an enlarged, upright, virtual image that your eye sees behind the object, so the object appears bigger.
12. Define refractive index.
A measure of how much a medium slows down light, equal to the speed of light in a vacuum divided by its speed in the medium.
13. Define angle of incidence.
The angle between an incident ray and the normal at the point where the ray meets a surface.
14. Define critical angle.
The angle of incidence in the denser medium for which the angle of refraction in the less dense medium is 90°.
15. Define total internal reflection.
The complete reflection of light back into the denser medium when the angle of incidence exceeds the critical angle.
16. Define converging lens.
A lens that is thicker at the centre and brings parallel rays of light together at a focal point.
17. Define focal length.
The distance between the optical centre of a lens (or mirror) and its focal point.
Marking yourself
- Calculation: formula written, values substituted with units, final answer with unit and sensible significant figures.
- Variables: manipulated = what you change; responding = what you measure; constant = what you keep the same.
- Definition: one sentence, correct physical quantity, correct unit.
Exam tip
More Light and Optics resources
- Chapter overview: Light and Optics
- Revision Notes
- Common Mistakes
- Paper 2 Answering Guide
- Paper 3 Guide
- Key Terms
- Calculation practice sets
Source: DSKP KSSM Physics Form 4 and 5 (Versi English), Sijil Pelajaran Malaysia: Format Pentaksiran mulai 2021, Fizik (4531) (Bahagian Pembangunan Kurikulum (BPK), KPM)