Diffraction and interference of light
What we'll do: We'll send light through one slit, two slits and a grating, and watch how it diffracts and forms interference fringes.
What you see on screen: Wavefronts passing through the slits and diffracting, plus the pattern on the screen — for two slits, regular bright and dark fringes; the sliders change the slit width and spacing and the wavelength, spreading the fringes apart. Readouts: slit configuration, wavelength and fringe spacing.
I'll give you a target fringe spacing — adjust the wavelength and slit spacing to reach it (Δy = λ·L/d).
What just happened?
After passing through a narrow slit, light did not give a sharp point but a blurred band of fringes — it diffracted. This is proof that light is a WAVE.
Diffraction:
a wave meeting a narrow slit or obstacle DIFFRACTS and spreads even "around the corner". The narrower the slit (or the longer the wave), the stronger the diffraction.
Interference (Young's experiment):
when light passes through TWO close slits, the waves from them superpose. Where "crest meets crest" they reinforce → a BRIGHT fringe; where "crest meets trough" they cancel → a DARK fringe. A regular fringe pattern forms.
Diffraction grating:
many slits give sharp, bright maxima in directions satisfying the condition:
(d — slit spacing, m — order, λ — wavelength). From this the wavelength is determined.
Why you need this for the exam:
diffraction, interference and the grating-maxima condition are the core of wave optics (advanced level). The trick: a bright fringe = waves reinforcing, dark = cancelling. Also worth remembering: .
Remember: light as a wave diffracts and superposes on itself (interference), giving bright and dark fringes; for a grating the maxima satisfy d·sinθ = m·λ.
Theory & formulas
The wave nature of light
At a narrow slit light diffracts, and behind two slits the waves from them superpose (interference), producing bright and dark fringes on the screen.
How fringes form
- waves reinforcing (in phase) → a bright fringe,
- cancelling (out of phase) → a dark fringe.
The condition for maxima
For two slits / a diffraction grating, bright fringes appear when: A longer wavelength and a smaller slit spacing spread the fringes apart.
Notation: — slit spacing (grating constant), — diffraction angle, — order of the maximum (), — wavelength.
Frequently asked questions
›What is diffraction of light?
The bending of light (a wave) at narrow slits or obstacles.
›Where do bright and dark interference fringes come from?
From the superposition of waves: reinforcement gives a bright fringe, cancellation a dark one.
›What is the condition for the maxima of a diffraction grating?
d·sinθ = m·λ, where d is the slit spacing, m the order of the maximum, and λ the wavelength.
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