Young's Double Slit

Objectives

Lesson outcomes

Explain how two coherent slits produce a stable interference pattern.
Describe the conditions needed to observe double-slit fringes.
Use lambda = ax / D to calculate wavelength, slit separation, fringe spacing, or screen distance.
Connect bright and dark fringes to path difference and phase difference.

Syllabus

CIE 9702 syllabus points

6 linked

8.2.1 explain the meaning of the term diffraction
8.2.2 show an understanding of experiments that demonstrate diffraction including the qualitative effect of the gap width relative to the wavelength of the wave; for example diffraction of water waves in a ripple tank
8.3.1 understand the terms interference and coherence
8.3.2 show an understanding of experiments that demonstrate two-source interference using water waves in a ripple tank, sound, light and microwaves
8.3.3 understand the conditions required if two-source interference fringes are to be observed
8.3.4 recall and use λ = ax / D for double-slit interference using light

Lesson Notes

Student guidance and lesson notes

Overview

Young’s double slit turns the interference model into a measurable pattern. You should understand how the fringes form and be able to use the double-slit equation accurately.

What You Need to Know

A single light source is used so that the two slits act as coherent sources with a constant phase relationship.
Light diffracts at each slit. The diffracted waves overlap and interfere on the screen.
Bright fringes are formed where the waves arrive in phase and interfere constructively.
Dark fringes are formed where the waves arrive in antiphase and interfere destructively.
For double-slit interference using light, use lambda = ax / D.
In this equation, a is the slit separation, x is the fringe spacing, and D is the distance from the slits to the screen.
Measuring across several fringe spacings and dividing by the number of gaps gives a more reliable value of x.

How to Work Through It

Start by recalling coherence, constructive interference, and destructive interference.
Sketch the double-slit arrangement and identify the source, slits, screen, and fringe spacing.
Explain why diffraction at the slits is needed before the two waves can overlap.
Practise calculations using lambda = ax / D, including unit conversions and rearrangements.

Check Your Understanding

Why must the two slits act as coherent sources?
What does x represent in the double-slit equation?
What happens to fringe spacing if the slit separation is increased?
Why is it better to measure several fringes instead of one fringe spacing?

Common Mistakes

Using the width of one slit instead of the separation between the two slits for a.
Leaving millimetres or centimetres unconverted when calculating wavelength.
Treating a bright fringe as a place where only one wave arrives.
Forgetting that diffraction at the slits allows the two sets of waves to overlap.

Next Steps

Complete double-slit calculation practice with clear symbols and units.
Revisit the link between diffraction and interference before moving to diffraction gratings.