1. What Young’s experiment shows
Young’s Double Slit Experiment is one of the clearest demonstrations of the wave nature of light. When light passes through two narrow, closely spaced slits, it produces a pattern of bright and dark fringes on a screen. This pattern can only be explained if light behaves like a wave and undergoes interference.
2. Basic idea of the experiment
The setup is simple but very clever. A single light source is used to illuminate two very narrow slits. These two slits act as coherent sources, meaning they emit waves of the same frequency and have a constant phase difference.
The light waves coming from the two slits overlap on the screen, and their superposition creates bright and dark fringes.
3. Why coherence is important
To get a stable interference pattern, the light reaching the screen must maintain a constant phase difference. Two independent bulbs cannot create this effect because their phases vary randomly. But when both slits are illuminated from the same source, coherence is naturally achieved.
4. How the bright and dark fringes form
At each point on the screen, waves from the two slits travel different distances to reach that point. This creates a path difference. Depending on whether this path difference leads to constructive or destructive interference, we see bright or dark fringes.
4.1. Constructive interference (bright fringes)
Bright fringes occur where the path difference is a whole number of wavelengths:
\( \Delta x = n\lambda \)
4.2. Destructive interference (dark fringes)
Dark fringes occur where the path difference is an odd multiple of half-wavelengths:
\( \Delta x = (2n + 1)\dfrac{\lambda}{2} \)
5. Fringe width and how to calculate it
The distance between two consecutive bright or two consecutive dark fringes is called the fringe width (\(\beta\)). It depends on the wavelength of light, the distance between the slits and the screen, and the separation between the slits.
5.1. Formula for fringe width
The fringe width is given by:
\( \beta = \dfrac{\lambda D}{d} \)
Here:
- \(\lambda\) = wavelength of the light used
- \(D\) = distance between the slits and the screen
- \(d\) = separation between the two slits
This formula shows that using light with a longer wavelength or increasing the distance to the screen spreads the fringes out.
6. Position of bright and dark fringes
The central bright fringe (called the central maximum) occurs where the path difference is zero. As I move away from the centre:
6.1. Bright fringe positions
\( y_n = n\beta \)
6.2. Dark fringe positions
\( y_n = \left(n + \dfrac{1}{2}\right)\beta \)
7. Why fringes are equally spaced
Because the two slits act like twins emitting identical waves, the path difference changes uniformly along the screen. This makes the bright and dark fringes appear at regular intervals.
8. Effect of changing the slit separation
If the distance between the slits (\(d\)) increases, the fringes get closer together. If \(d\) decreases, the fringes spread out more. This agrees with the formula \(\beta = \dfrac{\lambda D}{d}\).
9. Effect of using light of different colours
Different colours have different wavelengths. Since fringe width is directly proportional to \(\lambda\):
- Red light → larger fringe width (more spaced out)
- Violet light → smaller fringe width (closer fringes)
10. Young’s experiment and the proof of wave nature
The regular bright and dark bands produced in YDSE cannot be explained if light were made of particles travelling independently. These results arise only because light waves interfere and superpose — clear evidence that light behaves like a wave.
11. Where interference patterns appear in daily life
Even though YDSE is done in labs, interference happens everywhere:
- Soap bubbles showing colourful patches
- Thin oil layers on water forming rainbow patterns
- CD or DVD surfaces showing bands of colours
All these effects are everyday examples of the same wave interference that Young demonstrated.