Are formulas given in the exam?

No. You must recall SPM Physics formulas.

Is a formula sheet given for these calculations?

No. SPM Physics (4531) provides no formula sheet, recall each formula and write units on every line.

What does the easy band mean?

Foundation items use the formula directly; marks come from showing the method.

Can a tutor mark my working?

Yes, in a one-to-one lesson the teacher checks each line and fixes the exact step that loses marks. From RM50/hr, paid trial available.

Light and Optics Calculations (Easy), SPM Physics

How to approach

These are direct plug-in problems: identify the formula and substitute. Get these automatic first.

The method

List what is given, with units.
Choose the formula and rearrange for the unknown.
Substitute with units on every line.
Round to the correct significant figures.

Worked examples

Refractive index (Snell's law)

n = sin i / sin r

Light enters glass with i = 60° and r = 35°. n = sin i / sin r = sin 60° / sin 35° = 0.866 / 0.574 = 1.51.

Refractive index by depth

n = real depth / apparent depth

A coin in water has real depth = 8.0 cm and apparent depth = 6.0 cm. n = real depth / apparent depth = 8.0 / 6.0 = 1.33.

Critical angle

n = 1 / sin c

A glass has refractive index n = 1.5. sin c = 1 / n = 1 / 1.5 = 0.667, so c = sin⁻¹(0.667) = 41.8°.

Thin lens formula

1/f = 1/u + 1/v

A convex lens forms a real image: u = 20 cm, v = 30 cm. 1/f = 1/u + 1/v = 1/20 + 1/30 = 5/60 = 1/12, so f = 12 cm.

Linear magnification

m = v / u = hᵢ / hₒ

An image forms at v = 30 cm for an object at u = 15 cm. m = v / u = 30 / 15 = 2.0 (the image is twice as tall).

Where marks are lost

Skipping units, rounding too early, and writing only the final answer (which loses method marks in Paper 2).

Practise these

Try each formula above with your own numbers, showing every line. Bring anything you get stuck on to a one-to-one lesson.

Formulas you use for Light and Optics calculations

These foundation problems draw on the 5 relationships in Light and Optics. None is printed in the exam, so recall each and write its symbols with units before substituting:

n = sin i / sin r, Refractive index (Snell's law)
n = real depth / apparent depth, Refractive index by depth
n = 1 / sin c, Critical angle
1/f = 1/u + 1/v, Thin lens formula
m = v / u = hᵢ / hₒ, Linear magnification

A worked example, with units on every line

Using n = sin i / sin r (n = refractive index (no unit, ratio) (); i = angle of incidence (°); r = angle of refraction (°)):

Light enters glass with i = 60° and r = 35°.
n = sin i / sin r = sin 60° / sin 35° = 0.866 / 0.574 = 1.51.

Where the marks are, and the common mistakes

In Paper 2, calculation marks are awarded line by line: the formula, the substitution with units, and the final answer with its unit and sensible significant figures. Watch this trap: Angles are measured from the normal, not the surface.

The angle in the less dense medium (usually air) is i. For foundation items, the numbers are direct, the marks are in showing the method, not just the answer.

Source: DSKP KSSM Physics Form 4 and 5 (Versi English) (Bahagian Pembangunan Kurikulum (BPK), KPM)

Written by the spmphysics.com.my editorial team.· Updated 5 Sept 2026