Wave optics (Class 12): interference, diffraction and polarisation
Light shows interference, diffraction and polarisation, which only a wave can do. In Young's experiment two narrow slits give bright and dark fringes whose spacing depends on the wavelength. These labs let you change the slit gap, the screen distance and the colour.
NCERT Class 12 Physics, Chapter 10 (Wave Optics). Every lab below runs in 3D in your browser or on a phone, in English and Hindi.
Try the labs
Young's double slit experiment · plan · Chapter 10
To observe interference of light from two coherent sources (Young's experiment) and determine the fringe width and wavelength.
Single slit diffraction · plan · Chapter 10
To study the diffraction pattern of a single slit and verify the position of minima and the width of the central maximum.
Polarisation, Malus' law and Brewster angle · plan · Chapter 10
To study the polarisation of light, verify Malus' law and find the Brewster angle of glass.
Huygens' principle: reflection and refraction of wavefronts · plan · Chapter 10
To understand Huygens' principle of secondary wavelets and derive the laws of reflection and refraction from it.
Diffraction grating: orders, dispersion and resolving power · plan · Chapter 10
To study the diffraction grating: positions of the principal maxima, dispersion and resolving power, and to find an unknown wavelength.
Fresnel biprism: fringe width and wavelength · plan · Chapter 10
To determine the wavelength of light using a Fresnel biprism by measuring fringe width, source separation and distance to the screen.
Newton's rings: radius of curvature and wavelength · plan · Chapter 10
To observe Newton's rings and determine the radius of curvature of a plano-convex lens or the wavelength of light.
Resolving power and the Rayleigh criterion · plan · Chapter 10
To understand diffraction-limited resolution of a telescope and the Rayleigh criterion for resolving two point sources.
Thin film interference: soap film, oil film and anti-reflection coating · plan · Chapter 10
To understand interference in thin films: colours of soap bubbles and oil slicks, and how an anti-reflection coating works.
Interference in Young's experiment
Light from two coherent slits a distance d apart reaches a screen at distance D. Where the path difference is a whole number of wavelengths there is a bright fringe, where it is an odd number of half wavelengths there is a dark one.
The fringes are equally spaced. A larger wavelength or a larger D widens them, a larger d narrows them.
β = λD / dpath difference = d sin θ = nλ (bright)I = 4 I₀ cos²(φ/2)
Diffraction at a single slit
A slit of width a gives a wide central maximum with weaker maxima on both sides. The first minimum occurs at a sin θ = λ. The central maximum is twice as wide as the others.
a sin θ = nλ (minima)angular width of central maximum = 2λ / a
Polarisation
Only a transverse wave can be polarised, which proved light is transverse. Polaroid passes the component of the electric field along its axis, so unpolarised light loses half its intensity and polarised light follows Malus's law. Reflected light is fully polarised at the Brewster angle.
I = I₀ cos²θ (Malus)tan iB = n
Frequently asked questions
What is the condition for sustained interference?
The two sources must be coherent: same frequency and a constant phase difference. Two separate bulbs are not coherent, so Young used one source and two slits.
What happens to the fringes in water?
The wavelength becomes λ/n, so the fringe width decreases by the factor n.
Difference between interference and diffraction?
Interference is the superposition of waves from a small number of coherent sources; diffraction is the bending of a wave at an obstacle or slit, which is itself an interference of many secondary wavelets.
Why is the central fringe bright?
At the centre the path difference is zero, so the waves arrive in phase.