Light and Optics: Paper 2 Answering Guide (Form 4 SPM Physics)

Paper 2 at a glance

• Section B: 2 items worth 20 marks, answer one

"Define" items from this chapter

One precise sentence each; the unit is part of the definition where there is one.

• Linear magnification, The ratio of the height of the image to the height of the object, equal to image distance divided by object distance.
• Real image, An image formed where light rays actually meet, which can be captured on a screen.
• Virtual image, An image formed where light rays only appear to come from, which cannot be captured on a screen.
• Refraction of light, The bending of light as it passes from one transparent medium to another because of a change in its speed.
• 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.
• Angle of incidence, The angle between an incident ray and the normal at the point where the ray meets a surface.

"Explain" items from this chapter

Give the physics reason, not a description. Model answers in one breath:

• 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.
• 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.
• 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.

"Calculate" items from this chapter

Formula, substitution with units, answer with unit, and the trap examiners set for each one.

• n = sin i / sin r, Refractive index (Snell's law), n = refractive index (no unit, ratio) (); i = angle of incidence (°); r = angle of refraction (°) · Angles are measured from the normal, not the surface. The angle in the less dense medium (usually air) is i.
• n = real depth / apparent depth, Refractive index by depth, n = refractive index (no unit, ratio) (); real depth = real depth (m); apparent depth = apparent depth (m) · Real depth is always greater than apparent depth (the object looks shallower). Use the same unit for both depths.
• n = 1 / sin c, Critical angle, n = refractive index (no unit, ratio) (); c = critical angle (°) · Rearrange to sin c = 1/n first. Total internal reflection only happens going from a denser to a less dense medium when the angle exceeds c.
• 1/f = 1/u + 1/v, Thin lens formula, f = focal length (m); u = object distance (m); v = image distance (m) · Apply the sign convention (real distances +, virtual −). Take the reciprocal at the end to get f, not 1/f.
• m = v / u = hᵢ / hₒ, Linear magnification, m = linear magnification (no unit, ratio) (); v = image distance (m); u = object distance (m); hᵢ = image height (m); hₒ = object height (m) · m is a ratio with no unit. m > 1 means enlarged, m < 1 means diminished. Keep u and v in the same unit.

Other command words

• Describe, say what happens, in order, with the observable evidence; keep "why" for an explain mark.
• Sketch, axes labelled with units, the shape that matters (curve vs straight line, intercept, origin), no plotted numbers needed.

Where the marks are in this chapter

The 6 content standards of Light and Optics can each be asked as a definition, an explanation, a calculation or a diagram. We do not predict which will appear, prepare all of them.

• 6.1 Refraction of Light
• 6.2 Total Internal Reflection
• 6.3 Image Formation by Lens
• 6.4 Thin Lens Formula
• 6.5 Optical Instruments
• 6.6 Image Formation by Spherical Mirrors

A three-step answer routine

1. Underline the command word and the quantity asked for.
2. Write the principle or formula before any numbers.
3. Close with the unit, the direction (if a vector) and a one-line conclusion.

More Light and Optics resources

• Chapter overview: Light and Optics
• Revision Notes
• Common Mistakes
• Practice Questions
• 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)