Young's double-slit experiment shows that light can behave like a wave by producing a pattern of bright and dark bands called interference fringes. A coherent light source shines through two very narrow slits, and the overlapping waves spread out toward a screen. Where the waves arrive in step, they make a bright fringe, and where they arrive out of step, they make a dark fringe.
This experiment matters because it gives direct evidence that light has wavelength and phase, not just particle-like motion.
Understanding Physics: Young's Double-Slit Experiment
Each narrow opening does more than select a small part of the incoming beam. It causes diffraction, meaning the light spreads sideways after passing through the opening. This spreading is essential.
If light travelled only in straight, separate lines, the two beams would make two simple patches on the screen. Instead, each slit sends light across much of the screen, so light from either route can reach the same point. The screen then records the combined effect.
This is why the slits must be very narrow and close together. Wider slits reduce the spreading and make the pattern less clear.
A stable relationship between the waves is needed for sharp fringes. Ordinary white light from a lamp contains many wavelengths and changes phase randomly. Its different colours make patterns with different spacings, while random phase changes wash out the bands.
A laser is useful because it produces light with a narrow range of wavelengths and a steady phase relationship over a useful distance. The central bright band is usually easy to identify because the two routes to it have equal length. Moving away from the centre changes the route lengths gradually, creating a regular sequence of bands when the geometry is simple.
The simple fringe formula is an approximation. It works best when the screen is far from the slits compared with their separation, and when the bands are close to the central line. At larger angles, the geometry must be handled more carefully.
Real experiments have another feature. Each individual slit creates a broad diffraction envelope that controls the overall brightness. The interference fringes sit inside this envelope.
Farther from the centre, the fringes become dimmer. At certain positions, the diffraction from each slit can make an interference maximum nearly disappear. Students often confuse diffraction with interference.
Diffraction describes spreading from one opening. Interference describes the combination of contributions from two or more openings.
Young's setup can be used to measure a wavelength. A student can measure the distance between several neighbouring bright bands, divide by the number of gaps, then use the measured slit separation and screen distance. Measuring across many gaps reduces the effect of ruler error.
Careful alignment matters. The screen should be perpendicular to the centre line, the slits should be vertical if horizontal bands are expected, and stray room light should be reduced. This experiment later connects to electron and atom experiments.
Even when particles arrive one at a time, many detected impacts build the same kind of interference distribution. That result shows that wave behaviour in physics is not limited to water waves or ordinary light beams.
Key Facts
- Path difference between the two slits controls interference: Δr = r2 - r1.
- Bright fringes occur when Δr = mλ, where m = 0, 1, 2, 3, ...
- Dark fringes occur when Δr = (m + 1/2)λ, where m = 0, 1, 2, 3, ...
- For small angles, fringe position on the screen is y = mλL/d for bright fringes.
- Fringe spacing is Δy = λL/d, where L is screen distance and d is slit separation.
- Increasing wavelength or screen distance spreads fringes farther apart, while increasing slit separation makes fringes closer together.
Vocabulary
- Coherent light
- Light waves are coherent when they maintain a constant phase relationship and usually have the same frequency.
- Interference
- Interference is the combination of overlapping waves that can produce larger or smaller total amplitudes.
- Constructive interference
- Constructive interference occurs when waves meet in phase and reinforce each other to make a bright fringe.
- Destructive interference
- Destructive interference occurs when waves meet out of phase and cancel to make a dark fringe.
- Path difference
- Path difference is the difference in distance traveled by waves from the two slits to the same point on the screen.
Common Mistakes to Avoid
- Using the slit width instead of the slit separation in y = mλL/d is wrong because d is the distance between the centers of the two slits, not the width of one slit.
- Forgetting to convert nanometers to meters is wrong because all quantities in the interference equations must use consistent units.
- Counting the central bright fringe as m = 1 is wrong because the central maximum has zero path difference and is labeled m = 0.
- Assuming dark fringes occur at Δr = mλ is wrong because that condition gives constructive interference, while dark fringes occur at half-integer multiples of the wavelength.
Practice Questions
- 1 A double-slit setup uses light with wavelength 600 nm, slit separation 0.20 mm, and screen distance 2.0 m. What is the distance from the central bright fringe to the first-order bright fringe?
- 2 In a double-slit experiment, adjacent bright fringes are spaced 4.5 mm apart on a screen 1.5 m away. If the slit separation is 0.25 mm, what is the wavelength of the light?
- 3 If the slit separation is increased while the wavelength and screen distance stay the same, what happens to the spacing between bright fringes? Explain using the fringe spacing equation.