Light and Optics: Revision Notes (Form 4 SPM Physics)

What this chapter covers

• 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

Key ideas explained

• 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.

Formulas to memorise (none are given in the exam)

• n = sin i / sin r, Refractive index (Snell's law), n = refractive index (no unit, ratio) (); i = angle of incidence (°); r = angle of refraction (°)
• 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)
• n = 1 / sin c, Critical angle, n = refractive index (no unit, ratio) (); c = critical angle (°)
• 1/f = 1/u + 1/v, Thin lens formula, f = focal length (m); u = object distance (m); v = image distance (m)
• 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)

Experiments the DSKP names

• 6.1.4 · Determine the refractive index n of a glass block or perspex
• 6.1.6 · Determine the refractive index using real depth and apparent depth
• 6.4.1 · Investigate object distance u vs image distance v for a convex lens; find focal length via the lens formula

Terms you must define precisely

• 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.
• Critical angle, The angle of incidence in the denser medium for which the angle of refraction in the less dense medium is 90°.
• Total internal reflection, The complete reflection of light back into the denser medium when the angle of incidence exceeds the critical angle.
• Converging lens, A lens that is thicker at the centre and brings parallel rays of light together at a focal point.
• Focal length, The distance between the optical centre of a lens (or mirror) and its focal point.
• Thin lens formula, The relationship linking object distance u, image distance v and focal length f of a thin lens: 1/f = 1/u + 1/v.
• Linear magnification, The ratio of the height of the image to the height of the object, equal to image distance divided by object distance.

A revision order that works

1. Rewrite every definition in one sentence with the correct unit; definitions are the cheapest marks in Paper 2.
2. Derive each formula once from its meaning, then practise rearranging it before substituting numbers.
3. For each experiment, write the manipulated, responding and constant variables from memory, then the graph you would plot.
4. Finish with a timed set of calculations with units on every line.
5. Read the 6 standards above and tick the ones you can already explain aloud without notes.

More Light and Optics resources

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