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Leaving Cert Physics: Diffraction

How often Diffraction 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 5 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.

Diffraction, Higher Level(9 marks)

Quick ones on Diffraction.

(a)What does the diffraction of light tell us about its nature?
  1. That light is made of particles only
  2. That light behaves as a wave
  3. That light always travels in straight lines
(b)Why can long-wave radio signals reach houses behind hills that shorter-wave signals miss?
  1. They carry more energy than shorter-wave signals
  2. They travel faster than shorter-wave signals
  3. Their long wavelength diffracts more around the hills
(c)Why do ships' foghorns use low-frequency sound?
  1. Long wavelengths diffract and spread out widely
  2. High frequencies cannot pass through water droplets
  3. Low frequencies travel faster through fog
Show the answers

(a) That light behaves as a wave

(b) Their long wavelength diffracts more around the hills

(c) Long wavelengths diffract and spread out widely

Your turn: Higher Level questions on Diffraction.

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
2017Q5(d)
2016Q3

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

More Diffraction questions

Diffraction, 3 marks

When is the diffraction of a wave most noticeable?

  1. When the gap is much wider than the wavelength of the wave used
  2. When the wave has a very high frequency and short period
  3. When the size of the gap is about the same as the wavelength
Show the answer

When the size of the gap is about the same as the wavelength

Sound diffracts around a doorway because its wavelength is about a metre. Light does not, because its wavelength is less than a micrometre.

Diffraction, 2 marks

Define diffraction.

  1. The bouncing back of waves when they meet a barrier or surface
  2. The spreading out of waves as they pass an obstacle or go through a gap
  3. The bending of waves as they pass from one medium into another
Show the answer

The spreading out of waves as they pass an obstacle or go through a gap

Bending on entering a new medium is refraction and bouncing off is reflection. Diffraction is greatest when the gap is about one wavelength wide.

Diffraction, 3 marks

Why does a diffraction grating give sharper, brighter maxima than a pair of slits?

  1. Light from many slits reinforces only at exact angles
  2. Each slit is wider, so more light gets through
  3. The grating focuses the light like a lens
Show the answer

Light from many slits reinforces only at exact angles

With hundreds of slits, the waves cancel almost everywhere except where the path difference is exactly nλ, so the bright lines are narrow and well separated.

Ordinary Level

Asked on 5 of the last 10 Ordinary Level papers, most recently in 2026.

Every paper, year by year

YearWhere it came up
2026Q11
2025Q8
2024Q14(c)
2023Not asked
2022Not asked
2021Not asked
2019Q5(g)
2018Q8
2017Not asked
2016Not asked

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

More Diffraction questions

Diffraction, 3 marks

Why can you hear someone around a corner but not see them?

  1. Sound travels faster than light, so it reaches you before the light does
  2. Light cannot be reflected by walls, but sound can
  3. Sound has a long wavelength and diffracts around the corner
Show the answer

Sound has a long wavelength and diffracts around the corner

Diffraction is greatest when the wavelength is similar to the gap. Sound wavelengths are about a metre; light wavelengths are tiny.

Diffraction, 2 marks

Which of these is an example of diffraction?

  1. An echo heard from a cliff face some distance away
  2. Waves spreading out after passing through a harbour entrance
  3. A pencil looking bent in a glass of water
Show the answer

Waves spreading out after passing through a harbour entrance

Diffraction is the spreading of waves through a gap or around an obstacle. The bent pencil is refraction; the echo is reflection.

Other Physics topics

All of Leaving Cert Physics