Is this formula given in the exam?

No. SPM Physics papers provide no formula sheet, so this must be recalled.

How do I tell a and x apart?

a is the separation between the two slits, usually a very small distance like 0.5 mm. x is the fringe separation, the distance on the screen between two neighbouring bright fringes. They are different lengths in the pattern; swapping them turns λ = ax / D upside down.

The question gives the width of ten fringes. What do I use for x?

Divide the total width by the number of gaps, not the number of fringes. Ten bright fringes have nine gaps between them, so x = total width / 9. Using one fringe spacing keeps the formula λ = ax / D correct.

Why must all the distances be in metres?

The formula is consistent only when λ, a, x and D share the same unit. Since wavelength is naturally in metres, convert a and x from millimetres (1 mm = 10⁻³ m) and D from any other unit to metres before substituting.

Interference of waves Formula, SPM Physics

Interference of waves

λ = ax / D

Not given in the exam, recall it.

What it is for

Interference of waves is used in Waves (SPM Physics Form 4). Keep the SI units in every line of working. Waves

Symbols and SI units

Symbol	Meaning	SI unit
`λ`	wavelength	m
`a`	slit separation	m
`x`	fringe separation	m
`D`	distance from slits to screen	m

Rearrangements

x = λD / a
a = λD / x
D = ax / λ

Worked example

Slits a = 0.5 mm apart, fringes x = 2.0 mm apart, screen at D = 2.0 m. λ = ax / D = (5.0 × 10⁻⁴ × 2.0 × 10⁻³) / 2.0 = 5.0 × 10⁻⁷ m.

Common trap

Do not swap a (slit separation) with x (fringe separation). Convert all distances to metres before substituting.

Understanding two-source interference

For two-source (double-slit) interference of waves, λ = ax / D relates the wavelength to the geometry of the pattern. λ is the wavelength in metres (m), a is the slit separation in metres (m), x is the fringe separation, the distance between adjacent bright (or dark) fringes, in metres (m), and D is the distance from the slits to the screen in metres (m).

Bright fringes appear where waves from the two slits arrive in phase and reinforce. Closer slits or a more distant screen spread the fringes further apart, which is why x grows when a shrinks or D grows.

Rearrange as needed: for fringe separation x = λD / a, for slit separation a = λD / x, and for screen distance D = ax / λ.

A worked example, step by step

Two slits a = 0.4 mm apart are lit by light of wavelength λ = 6.0 × 10⁻⁷ m, with a screen D = 1.5 m away. Find the fringe separation x.

Convert the slit separation to metres: a = 0.4 mm = 4.0 × 10⁻⁴ m.

Rearrange to make x the subject and substitute with units:

x = λD / a = (6.0 × 10⁻⁷ m × 1.5 m) / (4.0 × 10⁻⁴ m) = 2.25 × 10⁻³ m.

So x = 2.25 × 10⁻³ m, or about 2.3 mm (2 significant figures). This spacing is large enough to see clearly, which is why a small slit separation is used in the demonstration.

Common mistakes and how this is tested

The trap is swapping a and x, the slit separation and the fringe separation play opposite roles, so mixing them up inverts the answer. Read the question carefully to see which distance is between the slits and which is between fringes.

Other errors: leaving a or x in millimetres instead of converting to metres; measuring the width of several fringes but forgetting to divide by the number of gaps to get one fringe separation; and confusing D (slits to screen) with a.

Under calculate show λ = ax / D or the rearranged form, substitute in metres and give the unit. Under explain you may need to say why the fringes spread out when the slits are moved closer.

Source: DSKP KSSM Physics Form 4 and 5 (Versi English), Sijil Pelajaran Malaysia: Format Pentaksiran mulai 2021, Fizik (4531) (Bahagian Pembangunan Kurikulum (BPK), KPM)

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