In a double-slit measurement, ten fringe spacings measure 24 mm with slits 0.50 mm apart and a screen 2.0 m away. What is the wavelength of the light?

The fringe spacing is 24/10 = 2.4 mm, so λ = xd/D = (2.4 × 10⁻³ × 5.0 × 10⁻⁴)/2.0 = 6.0 × 10⁻⁷ m.

Which measurement usually limits the accuracy of a double-slit wavelength determination?

The slit separation limits it, because it is very small and is normally taken from the manufacturer's figure rather than measured.

How is a diffraction grating used to measure the wavelength of a laser?

The grating is set at right angles to the beam, the angle of a chosen order is measured from the straight-through direction, and the wavelength is found from λ = d sin θ / n.

OCR A-Level Physics B H557

Planning

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Waves and Quantum Behaviour

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Waves and Quantum Behaviour

Measuring the wavelength of light

3 things to know
Components 01, 02 and 03

Overview

Recall questions on the double-slit arrangement, why ten fringe spacings and a distant screen help, what limits accuracy, gratings and higher orders, angles measured both sides, and laser safety.

What you need to know

Answer each one before you reveal it.

Your recallNot started

Key term

How is the wavelength of laser light measured using a double slit?

Key term

Why is the width of ten fringe spacings measured rather than one in the double-slit experiment?

Key term

Why should the screen be placed as far from the double slit as the room allows?

Questions from this subtopic

Each one with the model answer underneath.

1
How is the wavelength of laser light measured using a double slit?
Model answer
The fringes are projected onto a screen a measured distance away, the width of several fringe spacings is measured and divided to give x, and the wavelength is found from λ = xd/D.
2
Why is the width of ten fringe spacings measured rather than one in the double-slit experiment?
Model answer
The fringes are only a few millimetres apart, so measuring across ten and dividing greatly reduces the percentage uncertainty in the spacing.
3
In a double-slit measurement, ten fringe spacings measure 24 mm with slits 0.50 mm apart and a screen 2.0 m away. What is the wavelength of the light?
Model answer
The fringe spacing is 24/10 = 2.4 mm, so λ = xd/D = (2.4 × 10⁻³ × 5.0 × 10⁻⁴)/2.0 = 6.0 × 10⁻⁷ m.
4
Why should the screen be placed as far from the double slit as the room allows?
Model answer
A larger slit-to-screen distance gives wider fringes, which can be measured with a smaller percentage uncertainty.
5
Which measurement usually limits the accuracy of a double-slit wavelength determination?
Model answer
The slit separation limits it, because it is very small and is normally taken from the manufacturer's figure rather than measured.
6
How is a diffraction grating used to measure the wavelength of a laser?
Model answer
The grating is set at right angles to the beam, the angle of a chosen order is measured from the straight-through direction, and the wavelength is found from λ = d sin θ / n.

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