Chapter 10 Wave Optics
NCERT Class 12 Physics - Chapter 10: Wave Optics
Topics Covered
- Huygens' Principle
- Refraction and Reflection of Plane Waves
- Coherent and Incoherent Addition of Waves
- Interference of Light Waves
- Young’s Double Slit Experiment
- Diffraction
- Diffraction Due to a Single Slit
- Polarisation
Huygens' Principle
Huygens' Principle states that every point on a wavefront acts as a source of secondary wavelets, which spread out in all directions with the same speed as the wave. The new wavefront is the tangent to these secondary wavelets.
Example
Using Huygens' Principle, we can explain the bending of light around obstacles and the formation of new wavefronts in different media.
Refraction and Reflection of Plane Waves
When plane waves encounter a boundary between two different media, they undergo refraction (bending) and reflection. Refraction occurs due to the change in speed of the waves in different media, while reflection occurs when the waves bounce back into the original medium.
Example
A light wave traveling from air into water bends towards the normal due to the decrease in speed, demonstrating refraction. Part of the wave also reflects back into the air.
Coherent and Incoherent Addition of Waves
Coherent waves have a constant phase difference and the same frequency, leading to constructive or destructive interference. Incoherent waves have random phase differences and do not produce a stable interference pattern.
Example
Light from two coherent sources, such as lasers, can produce clear interference patterns, while light from incoherent sources, like bulbs, does not.
Interference of Light Waves
Interference occurs when two or more light waves superpose, leading to regions of constructive interference (bright fringes) and destructive interference (dark fringes). The overall intensity distribution depends on the phase difference between the waves.
Example
Interference patterns can be observed using two coherent light sources, such as in the Young’s double slit experiment.
Young’s Double Slit Experiment
Young’s double slit experiment demonstrates the wave nature of light. Light passing through two closely spaced slits produces an interference pattern of bright and dark fringes on a screen. The fringe spacing depends on the wavelength of light and the distance between the slits and the screen.
Example
If the slit separation is 0.5 mm and the wavelength of light used is 600 nm, the resulting interference pattern will have specific fringe spacings determined by the setup geometry.
Diffraction
Diffraction is the bending of light around obstacles and the spreading of light waves when they pass through small apertures. It occurs when the size of the obstacle or aperture is comparable to the wavelength of light.
Example
Light passing through a narrow slit spreads out and forms a diffraction pattern on a screen.
Diffraction Due to a Single Slit
When light passes through a single slit, it forms a diffraction pattern with a central bright fringe and alternating dark and bright fringes on either side. The intensity and width of the fringes depend on the slit width and the wavelength of light.
Example
Using a slit of width 0.1 mm and light of wavelength 500 nm, a diffraction pattern with specific fringe spacings and intensities can be observed.
Polarisation
Polarisation is the phenomenon where light waves oscillate in a particular direction. Unpolarised light has oscillations in all directions perpendicular to the direction of propagation, while polarised light has oscillations in one direction.
Example
Polarised sunglasses reduce glare by blocking horizontally polarised light reflected from surfaces like water or roads.
Word Meanings
Wavefront
A surface over which an optical wave has a constant phase.
Secondary Wavelets
Small wavelets originating from every point on a wavefront, as per Huygens' Principle.
Phase Difference
The difference in the phase of two waves at a point.
Constructive Interference
The phenomenon where two waves superpose to give a resultant wave with greater amplitude.
Destructive Interference
The phenomenon where two waves superpose to give a resultant wave with lower amplitude.
Coherent Sources
Sources that emit waves with a constant phase difference.
Incoherent Sources
Sources that emit waves with random phase differences.
Diffraction Pattern
The pattern of dark and bright regions resulting from the diffraction of light.
Fringe
A bright or dark band caused by interference or diffraction of light.
Polarisation
The orientation of oscillations in a light wave in a particular direction.
Unpolarised Light
Light waves oscillating in all directions perpendicular to the direction of propagation.
Polarised Light
Light waves oscillating in one direction.
Amplitude
The height of a wave crest or depth of a wave trough from the rest position.
Wavelength
The distance between successive crests or troughs of a wave.
Interference Pattern
The pattern of alternating bright and dark fringes resulting from the superposition of waves.
Slit Separation
The distance between two slits in an interference experiment.
Intensity
The power carried by a wave per unit area.
Optical Path
The product of the refractive index of a medium and the actual path length traveled by light.
Geometric Path
The actual distance traveled by light in a medium.
Frequently Asked Questions (FAQs)
1. What is Huygens' Principle?
Huygens' Principle states that every point on a wavefront acts as a source of secondary wavelets, which spread out in all directions. The new wavefront is the tangent to these secondary wavelets.
2. How does refraction occur?
Refraction occurs when light passes from one medium to another with a different refractive index, causing the light to bend due to the change in speed.
3. What is the difference between coherent and incoherent waves?
Coherent waves have a constant phase difference and the same frequency, leading to stable interference patterns. Incoherent waves have random phase differences and do not produce stable interference patterns.
4. What is interference of light waves?
Interference is the phenomenon where two or more light waves superpose to form regions of constructive (bright fringes) and destructive (dark fringes) interference.
5. What is Young’s double slit experiment?
Young’s double slit experiment demonstrates the wave nature of light by producing an interference pattern of bright and dark fringes using two closely spaced slits.
6. What causes diffraction?
Diffraction occurs when light waves encounter an obstacle or aperture with a size comparable to the wavelength of the light, causing the waves to spread out.
7. What is the diffraction pattern of a single slit?
The diffraction pattern of a single slit consists of a central bright fringe with alternating dark and bright fringes on either side, caused by the bending of light around the slit edges.
8. What is polarisation of light?
Polarisation is the orientation of the oscillations of light waves in a specific direction. It can be achieved by passing light through polarising filters.
9. How do polarised sunglasses work?
Polarised sunglasses reduce glare by blocking horizontally polarised light reflected from surfaces like water or roads, allowing only vertically polarised light to pass through.
10. What is the phase difference between two waves?
The phase difference is the difference in the phase angles of two waves at a given point, determining the type of interference (constructive or destructive).
11. How do you calculate fringe spacing in Young’s double slit experiment?
Fringe spacing (β) is calculated using the formula: β = λD/d, where λ is the wavelength of light, D is the distance between the slits and the screen, and d is the distance between the slits.
12. What is the significance of the central maximum in a diffraction pattern?
The central maximum is the brightest and widest fringe in a diffraction pattern, indicating the region where light waves interfere constructively at the center.
13. How does a diffraction grating work?
A diffraction grating consists of multiple closely spaced slits that disperse light into its component wavelengths, producing a spectrum of colors.
14. What is the difference between diffraction and interference?
Diffraction is the bending of light around obstacles, while interference is the superposition of two or more waves resulting in a pattern of constructive and destructive regions.
15. How does the width of a slit affect the diffraction pattern?
A narrower slit produces a wider diffraction pattern, while a wider slit produces a narrower pattern. The intensity and spacing of fringes are also affected by the slit width.
16. What is the Brewster angle?
The Brewster angle is the angle of incidence at which light is perfectly polarised upon reflection. It is given by: tanθ = n^2/n^1, where n^1 and n^2 are the refractive indices of the two media.
17. What is the principle of a polariser?
A polariser allows light waves oscillating in a specific direction to pass through while blocking waves oscillating in other directions, producing polarised light.
18. How is polarisation used in 3D glasses?
3D glasses use polarising filters to present two slightly different images to each eye, creating a perception of depth when viewed together, simulating a three-dimensional effect.
19. What is the role of a wavefront in wave optics?
A wavefront represents the locus of points having the same phase in a wave. It helps in understanding the propagation and interaction of waves, such as refraction and diffraction.
20. How does the wavelength of light affect diffraction?
Longer wavelengths result in more significant diffraction, causing waves to spread out more. Shorter wavelengths result in less diffraction and more defined wavefronts.