The Double-Slit Experiment
The Double-Slit Experiment
- Light diffracts at each narrow slit. The two spreading waves overlap on the screen.
- Constructive interference makes bright fringes; destructive interference makes dark fringes.
From one laser to a fringe pattern
- A single slit before the double slit illuminates both slits from the same wavefront. The two slits therefore act as coherent sources.
- Use monochromatic light so one wavelength makes one stable, regularly spaced pattern.
- The central bright fringe lies opposite the midpoint because the two paths are equal there.
Measure wavelength from the fringes
Symbols
- = wavelength of the light (m)
- = separation of the slit centres (m)
- = separation of adjacent bright or adjacent dark fringes (m)
- = distance from the slits to the screen (m)
- Measure across several fringe spaces, then divide. From the first to the ninth dark fringe there are eight spaces, not nine.
- From : increasing wavelength or screen distance widens the pattern; increasing slit separation narrows it.
Worked example
Count the spaces before using the formula
The first and ninth dark fringes are 3.2 cm apart. The slit separation is 0.16 mm and the wavelength is 720 nm. Find .
- Eight spaces give .
- Rearrange: .
- Substitute SI values to obtain .
Answer
Your turn9702/22/O/N/25 Q4(b)
In a double-slit experiment, , and . Calculate the wavelength. [2] (9702/22/O/N/25 Q4(b))
Show worked answer
- Convert both millimetre values to metres.
- gives .
Answer