The double-slit experiment Cambridge International AS & A Level Physics revision
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In plain words
Shine a laser at two narrow slits close together, and you don't get two bright lines on the wall. You get a whole row of evenly spaced bright and dark stripes. Measure how far apart they are, and you have measured the wavelength of light with a ruler.
Three things to know
- λ = ax ÷ D, where a is the separation of the slits, x is the fringe spacing (the distance between neighbouring bright fringes) and D is the distance from the slits to the screen.
- A bright fringe is where the path difference from the two slits is a whole number of wavelengths. A dark fringe is where it is an odd number of half wavelengths.
- Rearranged, x = λD ÷ a: the fringes are further apart if the wavelength is longer, the screen is further away, or the slits are closer together.
Worked example
In a double-slit experiment the slits are 0.50 mm apart and the screen is 2.0 m away. The fringe spacing is 2.4 mm. Find the wavelength of the light.
- λ = ax ÷ D, with every length in metres.
- = 0.50 × 10⁻³ × 2.4 × 10⁻³ ÷ 2.0.
- = 6.0 × 10⁻⁷ m (600 nm).
Tips and tricks
- Measure across several fringes and divide: ten fringe spacings are much easier to measure than one.
- Put every length into metres before you start. Slit separations are usually given in millimetres.
It lands in your notebook with its questions as flashcards.
The double-slit experiment: 5 questions and answers
These are the quiz’s questions. Do the quiz first, then come back here for the ones that got you.
In λ = ax ÷ D, what is x?
The distance between neighbouring bright fringes.
The two slits are moved closer together. What happens to the fringes?
x = λD ÷ a, so a smaller a gives a larger x.
The screen is moved further from the slits. What happens to the fringes?
The fringe spacing is proportional to D.
What is seen at the centre of the pattern, the same distance from both slits?
The path difference is zero, so the waves arrive in phase.
The slits are 1.0 mm apart, the screen is 2.0 m away and the wavelength is 5.0 × 10⁻⁷ m. What is the fringe spacing?
5.0 × 10⁻⁷ × 2.0 ÷ 1.0 × 10⁻³.
Quiz
5 questions
Tap an answer and you’ll see straight away whether it’s right, and why.
Worksheet
3 questions, 7 marks. Write your answers on paper, then check them.
The double-slit experiment
Cambridge International AS & A Level Physics 9702 · 7 marks · papermunch.org
Name ______________________________ Date ______________
Light of wavelength 6.0 × 10⁻⁷ m passes through two slits 0.30 mm apart and falls on a screen 1.5 m away. Find the fringe spacing.[3]
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3.0 mm. 6.0 × 10⁻⁷ × 1.5 ÷ 0.30 × 10⁻³.
Ten fringe spacings measure 25 mm across. Find the fringe spacing.[2]
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2.5 mm.
The red laser in a double-slit experiment is swapped for a blue one. State and explain what happens to the fringes.[2]
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They get closer together. Blue light has a shorter wavelength, and the fringe spacing is proportional to the wavelength.
Answers: The double-slit experiment
- 1. 3.0 mm. 6.0 × 10⁻⁷ × 1.5 ÷ 0.30 × 10⁻³.
- 2. 2.5 mm.
- 3. They get closer together. Blue light has a shorter wavelength, and the fringe spacing is proportional to the wavelength.