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Leaving Cert Physics: Interference

How often Interference comes up on the Physics papers, every year it was asked, and questions to try.

HL Asked on 10 of the last 10 Higher Level papers, most recently in 2026. banker

OL Asked on 3 of the last 10 Ordinary Level papers, most recently in 2026.

June 2027 is the first exam on the new Physics course. These come from the 2016–2026 papers, so read them as a guide.

Interference, Higher Level(8 marks)

Quick ones on Interference.

(a)Equation used for a diffraction grating?
  1. nλ = d cos θ
  2. nλ = d sin θ
  3. λ = nd sin θ
(b)Distance between the slits of a diffraction grating with 500 lines per mm?
  1. 2 × 10⁻³ m
  2. 5 × 10⁻⁴ m
  3. 2 × 10⁻⁶ m
(c)What is the path difference for destructive interference?
  1. An odd number of half wavelengths
  2. Always zero
  3. A whole number of wavelengths
Show the answers

(a) nλ = d sin θ

(b) 2 × 10⁻⁶ m

(c) An odd number of half wavelengths

Your turn: Higher Level questions on Interference.

Higher Level

Asked on 10 of the last 10 Higher Level papers, most recently in 2026. banker

Every paper, year by year

YearWhere it came up
2026Q3
2025Q8
2024Q14(d)
2023Q3
2022Q13
2021Q8
2019Q10
2018Q3
2017Q9
2016Q3, Q11

Links open the State Examinations Commission’s paper for that year.

More Interference questions

Interference, 2 marks

What is constructive interference?

  1. Two waves meet out of phase and cancel each other
  2. A wave spreads out after passing through a narrow gap
  3. Two waves meet in phase and reinforce, giving a larger amplitude
Show the answer

Two waves meet in phase and reinforce, giving a larger amplitude

Crest meets crest, so the displacements add (superposition). It happens where the path difference is a whole number of wavelengths, giving a bright band.

Interference, 3 marks

A grating with more lines per millimetre is used with the same light. What happens to the pattern?

  1. The pattern is unchanged but brighter
  2. The orders spread out to larger angles
  3. The orders move closer together
Show the answer

The orders spread out to larger angles

More lines per mm means a smaller slit spacing d. From nλ = d sin θ, a smaller d gives a larger sin θ, so each order is further from the centre.

Ordinary Level

Asked on 3 of the last 10 Ordinary Level papers, most recently in 2026.

Every paper, year by year

YearWhere it came up
2026Q11
2025Not asked
2024Q14(c)
2023Not asked
2022Not asked
2021Not asked
2019Not asked
2018Q8
2017Not asked
2016Not asked

Links open the State Examinations Commission’s paper for that year.

More Interference questions

Interference, 2 marks

What is destructive interference?

  1. Two waves meet in step and add up to a bigger amplitude
  2. A wave bounces back off a barrier and returns the way it came
  3. Two waves meet out of step and cancel, giving a small or zero amplitude
Show the answer

Two waves meet out of step and cancel, giving a small or zero amplitude

When a crest meets a trough they cancel. When crest meets crest they reinforce, which is constructive interference.

Interference, 3 marks

Two loudspeakers play the same note from the same signal generator. Walking past them you hear?

  1. A note that gets steadily higher in pitch
  2. Loud and quiet spots one after another
  3. The same loudness everywhere you walk
Show the answer

Loud and quiet spots one after another

Loud spots are where the waves meet in step (constructive); quiet spots are where they cancel (destructive). This shows interference.

Other Physics topics

All of Leaving Cert Physics