Light and Optics: Common Mistakes (Form 4 SPM Physics)

The mistakes, and the fix

1. n = sin i / sin r, Angles are measured from the normal, not the surface. The angle in the less dense medium (usually air) is i.
2. n = real depth / apparent depth, Real depth is always greater than apparent depth (the object looks shallower). Use the same unit for both depths.
3. n = 1 / sin c, 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.
4. 1/f = 1/u + 1/v, Apply the sign convention (real distances +, virtual −). Take the reciprocal at the end to get f, not 1/f.
5. m = v / u = hᵢ / hₒ, m is a ratio with no unit. m > 1 means enlarged, m < 1 means diminished. Keep u and v in the same unit.
6. In the Determining the refractive index of a glass block practical, skipping this: Use a sharp pencil so the rays and outline are thin and accurate.
7. In the Determining the refractive index of a glass block practical, skipping this: Mark the ray positions with dots far apart (or pins upright) to draw the ray accurately.
8. In the Determining the refractive index of glass by real depth and apparent depth practical, skipping this: Focus the microscope carefully and avoid parallax when taking each reading.
9. In the Determining the refractive index of glass by real depth and apparent depth practical, skipping this: Keep the microscope vertical so all readings are measured along the same line.
10. In the Object distance and image distance for a convex lens practical, skipping this: Line up the centres of the object, lens and screen at the same height.
11. In the Object distance and image distance for a convex lens practical, skipping this: Adjust the screen for the sharpest possible image before measuring.
12. Mixing up Total internal reflection and Converging lens. 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.
13. Mixing up Focal length and Thin lens formula. 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.
14. Mixing up Linear magnification and Real image. 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.
15. Dropping the unit on the final line, an answer without its SI unit is incomplete.
16. Rounding early: keep full calculator values and round only the final answer to sensible significant figures.

How a 1-to-1 teacher uses this list

In lessons the teacher marks a student's own work against this list, then re-teaches only the items that actually appeared, which is why 1-to-1 fixes habits that group classes miss.

More Light and Optics resources

• Chapter overview: Light and Optics
• Revision Notes
• 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)