What is seen on the screen in Young's slits experiment with monochromatic light?

A series of equally spaced bright fringes separated by dark ones, brightest at the centre and fading gradually towards the edges of the pattern.

State the equation used to find the wavelength in Young's slits experiment, defining every symbol with its unit.

λ = ay/d, where λ is the wavelength in metres, a is the separation of the two slits in metres, y is the distance between the centres of adjacent bright fringes in metres and d is the distance from the slits to the screen in metres.

Why is the screen placed a long way from the slits in Young's experiment?

The fringe spacing is proportional to the distance to the screen, so a large distance spreads the pattern out and reduces the percentage uncertainty in the measurement of the fringe spacing.

Two slits 0.50 mm apart produce fringes 2.4 mm apart on a screen 2.00 m away. Calculate the wavelength of the light.

λ = ay/d = (5.0 × 10⁻⁴ × 2.4 × 10⁻³)/2.00 = 6.0 × 10⁻⁷ m, that is 600 nm, and 6.0 × 10⁻⁷ × 2.00 / 5.0 × 10⁻⁴ returns a fringe spacing of 2.4 mm as given.

CCEA A-Level Physics 1210

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Superposition, Interference and Diffraction

Young's double slit

3 things to know
Unit AS 2

Overview

Recall questions on the experiment and what is seen on the screen, the equation λ = ay/d with every symbol, why the slits must be close and the screen far, worked wavelength and fringe-spacing calculations, the effect of changing the slit separation, the screen distance and the colour, and why several fringes are measured at once.

What you need to know

Answer each one before you reveal it.

Your recallNot started

Key term

Describe Young's slits experiment for measuring the wavelength of monochromatic light.

Key term

Why must the two slits be very close together in Young's experiment?

Key term

How is the requirement for coherent sources met in Young's slits experiment with a lamp?

Questions from this subtopic

Each one with the model answer underneath.

1
Describe Young's slits experiment for measuring the wavelength of monochromatic light.
Model answer
Monochromatic light illuminates two narrow parallel slits a small measured distance apart, and the light from them falls on a screen a measured distance away, where a pattern of equally spaced bright and dark fringes is produced and the spacing of the bright fringes is measured.
2
What is seen on the screen in Young's slits experiment with monochromatic light?
Model answer
A series of equally spaced bright fringes separated by dark ones, brightest at the centre and fading gradually towards the edges of the pattern.
3
State the equation used to find the wavelength in Young's slits experiment, defining every symbol with its unit.
Model answer
λ = ay/d, where λ is the wavelength in metres, a is the separation of the two slits in metres, y is the distance between the centres of adjacent bright fringes in metres and d is the distance from the slits to the screen in metres.
4
Why must the two slits be very close together in Young's experiment?
Model answer
The fringe spacing is inversely proportional to the slit separation, so slits that are far apart give fringes too close together to measure, while slits a fraction of a millimetre apart give a workable spacing.
5
Why is the screen placed a long way from the slits in Young's experiment?
Model answer
The fringe spacing is proportional to the distance to the screen, so a large distance spreads the pattern out and reduces the percentage uncertainty in the measurement of the fringe spacing.
6
Two slits 0.50 mm apart produce fringes 2.4 mm apart on a screen 2.00 m away. Calculate the wavelength of the light.
Model answer
λ = ay/d = (5.0 × 10⁻⁴ × 2.4 × 10⁻³)/2.00 = 6.0 × 10⁻⁷ m, that is 600 nm, and 6.0 × 10⁻⁷ × 2.00 / 5.0 × 10⁻⁴ returns a fringe spacing of 2.4 mm as given.

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