Wave Optics
Class 12 · Physics
4 ideas in Class 12 Physics, explained in plain words, with games and quizzes to practise.
Huygens Principle and Wavefronts
Part of: Huygens, Coherence and Interference
Huygens pictured every point on a wavefront as a fresh source of tiny wavelets, with the new front as their envelope. This predicts equal angles for reflection, Snell's law for refraction through slower media, and the vanishing of the refracted front beyond the critical angle.
Why it matters. Lens shapes, mirages and fibre guiding all follow from wavelet geometry. Forgetting that frequency stays fixed across media breaks every refraction proof.
Formulas
- Reflection: angle in = angle out
- Refraction: speed ratio sets the bend via Snell
- Frequency unchanged across media
Key words
- Wavefront
- : Surface joining points of a wave that oscillate in step.
- Secondary wavelets
- : Tiny waves spreading from each point of a wavefront.
- Huygens principle
- : New wavefront is the envelope of secondary wavelets.
Try it in two minutes
- Draw a wavefront touching a boundary.
- Grow a wavelet from each contact point.
- Join their envelope for the new front.
Try at home. Explain why a straw looks bent using slower light in water.
Coherence and Young's Double Slit
Part of: Huygens, Coherence and Interference
Stable fringes need coherent sources with a fixed phase link, usually one source split in two. Bright fringes sit where paths differ by whole wavelengths and dark ones at half-integer differences, spaced evenly by wavelength times screen distance over slit separation.
Why it matters. Thin-film colours, holograms and interferometers all use path differences. Using independent sources, or mixing bright with dark conditions, destroys fringe answers.
Formulas
- Fringe width: B = lambda D / d
- Bright: path difference = n x lambda
- Dark: path difference = (n + 1/2) x lambda
Key words
- Coherent sources
- : Sources with a constant phase difference, giving steady fringes.
- Fringe width
- : Spacing between neighbouring bright or dark fringes.
- Path difference
- : Extra distance one wave travels over the other to a point.
Try it in two minutes
- Split one source so the pair stays in step.
- Find path difference from geometry.
- Set it to whole or half wavelengths for bright or dark.
Try at home. Shine a laser through two close slits and measure fringe gaps.
Diffraction by a Single Slit
Part of: Diffraction and Polarisation
Light bends around slit edges, and waves from across the slit cancel at angles where path difference is a whole wavelength, giving dark minima at a sin theta = n lambda. The middle band stays brightest and twice as wide as the rest, widening as the slit narrows.
Why it matters. Telescopes, eyes and cameras are all limited by diffraction, not just lenses. Confusing diffraction minima with interference maxima swaps dark and bright.
Formulas
- Minima: a sin(theta) = n x lambda
- Central width: 2 x lambda D / a
- Narrower slit gives wider pattern
Key words
- Diffraction
- : Bending and spreading of waves around edges and through openings.
- Central maximum
- : Brightest, widest middle band of the single-slit pattern.
- Secondary maxima
- : Fainter bright bands flanking the central maximum.
Try it in two minutes
- Divide wavelength by slit width for the first minimum angle.
- Double that angle times distance for central width.
- Narrow the slit and watch the pattern widen.
Try at home. Squint at a distant lamp and notice rays spreading from your lashes.
Polarisation of Light
Part of: Diffraction and Polarisation
Polarisation filters vibrations to one direction, possible only for transverse waves, which proves light is transverse. A polaroid halves unpolarised light, and Malus law cuts it further by cosine squared of the angle; at Brewster's angle reflected light is fully polarised.
Why it matters. Sunglasses, LCD screens and glare control all use polaroids. Treating light as longitudinal, or forgetting the square in Malus, breaks every intensity answer.
Formulas
- Malus: I = I0 cos^2(theta)
- One polaroid halves unpolarised light
- Brewster: tan(theta) = n
Key words
- Polarisation
- : Restriction of transverse vibrations to one direction.
- Malus law
- : Transmitted intensity equals incident times cosine squared of angle.
- Brewster angle
- : Incidence giving fully polarised reflected light.
Try it in two minutes
- Pass light through one polaroid and note halving.
- Rotate a second one and watch cosine-squared dimming.
- Tilt glass to Brewster's angle for full polarisation.
Try at home. Cross two sunglass lenses and watch the light vanish.
What your child will practise
- Coherence and Young's Double Slit: Ocean Quick Fire (Quick Fire)
- Diffraction by a Single Slit: Desert Puzzle (Clue Puzzle)
- Diffraction by a Single Slit: Jungle Quick Fire (Quick Fire)
- Huygens Principle and Wavefronts: Candy Quick Fire (Quick Fire)
- Polarisation of Light: Ocean Puzzle (Clue Puzzle)
- Quiz: Coherence and Young's Double Slit (8 questions)
- Quiz: Diffraction by a Single Slit (8 questions)
- Quiz: Huygens Principle and Wavefronts (8 questions)
- Quiz: Polarisation of Light (8 questions)