CBSE Class 12 Physics (2026–27)
Chapter 10: Wave Optics
20 Important Questions & Answers
Wave Optics in the CBSE Class 12 syllabus includes Huygens’ principle, wavefronts, interference, Young’s Double Slit Experiment (YDSE), coherent sources, diffraction, and polarization.
Q1. What is a wavefront? Name its types.
Answer:
A wavefront is an imaginary surface joining all points of a wave that are vibrating in the same phase at a given instant. It represents the locus of points having identical phase. Wavefronts help explain the propagation of light using Huygens’ principle. There are mainly three types of wavefronts. A spherical wavefront is produced by a point source. A cylindrical wavefront is generated by a linear source. A plane wavefront is obtained when the source is very far away, making the curvature negligible. The concept of wavefront is fundamental in understanding reflection, refraction, interference, and diffraction phenomena in wave optics.
Q2. State Huygens’ Principle.
Answer:
According to Huygens’ Principle, every point on a wavefront acts as a source of secondary wavelets that spread in all directions with the speed of light in the medium. After a certain time interval, the new wavefront is obtained by drawing a common tangent, called the envelope, to all these secondary wavelets. This principle successfully explains the propagation of light as a wave and provides a basis for understanding reflection and refraction. It also supports the wave nature of light and helps derive the laws of reflection and refraction using geometrical constructions.
Q3. How does Huygens’ Principle explain the law of reflection?
Answer:
Using Huygens’ Principle, every point on the incident wavefront acts as a source of secondary wavelets. When these wavelets strike a reflecting surface, they generate a reflected wavefront. Geometrical construction shows that the angle between the incident ray and the normal is equal to the angle between the reflected ray and the normal. Thus, the law of reflection, which states that the angle of incidence equals the angle of reflection, is obtained. The incident ray, reflected ray, and normal all lie in the same plane. This proves that Huygens’ Principle is consistent with the experimentally observed laws of reflection.
Q4. What are coherent sources of light?
Answer:
Coherent sources are two or more light sources that emit waves of the same frequency and maintain a constant phase difference with time. These sources are essential for producing a stable and sustained interference pattern. Independent light sources generally do not remain coherent because their phase changes randomly. In Young’s Double Slit Experiment, two coherent sources are obtained by illuminating two narrow slits with light from a single source. Coherence ensures that bright and dark fringes remain fixed and clearly visible on the screen, making interference possible.
Q5. What is interference of light?
Answer:
Interference is the phenomenon in which two or more coherent light waves superpose and redistribute light intensity in space. When the waves meet in phase, constructive interference occurs, producing bright fringes. When they meet out of phase, destructive interference occurs, producing dark fringes. Interference is a direct consequence of the wave nature of light and cannot be explained by particle theory alone. The phenomenon was conclusively demonstrated through Young’s Double Slit Experiment. Applications of interference include anti-reflection coatings, measurement of wavelength, and precision optical instruments.
Q6. State the conditions for sustained interference.
Answer:
For sustained interference, the interfering light sources must be coherent, meaning they should have the same frequency and a constant phase difference. The amplitudes of the waves should preferably be equal or nearly equal to obtain clear fringes. The sources should be monochromatic to avoid overlapping patterns of different wavelengths. The distance between the sources should be small, and the screen should be placed at a suitable distance. If these conditions are not satisfied, the interference pattern becomes unstable or indistinct. These requirements are fulfilled effectively in Young’s Double Slit Experiment.
Q7. Describe Young’s Double Slit Experiment.
Answer:
Young’s Double Slit Experiment consists of two narrow slits illuminated by a single monochromatic light source. The slits act as coherent sources and produce overlapping light waves on a screen. Due to interference, alternate bright and dark fringes appear. Bright fringes are formed by constructive interference, while dark fringes result from destructive interference. The experiment provided strong evidence for the wave nature of light and disproved the purely particle-based explanation of light. The fringe pattern obtained is stable because the two slits maintain a constant phase relationship. This experiment remains one of the most important demonstrations in wave optics.
Q8. Define fringe width and write its expression.
Answer:
Fringe width is the distance between the centers of two consecutive bright fringes or two consecutive dark fringes in an interference pattern. It is represented by the symbol β. In Young’s Double Slit Experiment, the fringe width is given by:
\beta=\frac{\lambda D}{d}
where λ is the wavelength of light, D is the distance between the slits and the screen, and d is the separation between the slits. The fringe width increases with wavelength and screen distance but decreases with slit separation. This relation is widely used in numerical problems.
Q9. How does fringe width change if wavelength is increased?
Answer:
The fringe width in Young’s Double Slit Experiment is directly proportional to the wavelength of light. Therefore, if the wavelength increases, the fringe width also increases. As a result, the distance between consecutive bright or dark fringes becomes larger. For example, red light, which has a longer wavelength than blue light, produces wider fringes. This relationship is useful in determining unknown wavelengths experimentally. The increase in fringe width makes the interference pattern more spread out and easier to observe on the screen.
Q10. Why does YDSE prove the wave nature of light?
Answer:
Young’s Double Slit Experiment proves the wave nature of light because it demonstrates interference, a phenomenon that occurs only when waves superpose. The appearance of alternate bright and dark fringes indicates constructive and destructive interference of light waves. If light behaved solely as particles, such a pattern would not be observed. The experiment showed that light possesses wave characteristics and can exhibit phase relationships. This discovery was a major milestone in physics and strongly supported the wave theory of light proposed by Huygens and later developed by other scientists.
Q11. What is diffraction of light?
Answer:
Diffraction is the bending and spreading of light waves when they pass through a narrow aperture or around an obstacle. It occurs when the dimensions of the aperture are comparable to the wavelength of light. Diffraction demonstrates that light behaves as a wave. Instead of traveling strictly in straight lines, light spreads into the region behind the obstacle. The phenomenon becomes more noticeable when the slit width is very small. Diffraction is important in optical instruments, spectroscopy, and the study of wave behavior. It also limits the resolving power of optical systems.
Q12. What is the diffraction pattern of a single slit?
Answer:
The diffraction pattern produced by a single slit consists of a broad central bright maximum surrounded by alternating dark and less intense bright fringes on both sides. The central maximum is the brightest and widest part of the pattern. The intensity decreases as one moves away from the center. Unlike interference fringes, diffraction fringes are not equally bright. The pattern is formed because different portions of the wavefront emerging from the slit interfere with one another. This observation provides strong evidence for the wave nature of light.
Q13. Why is the central maximum in diffraction wider than the others?
Answer:
In a single-slit diffraction pattern, the central bright maximum occupies the region between the first minima on either side. This region is approximately twice as wide as the regions occupied by the secondary maxima. As a result, the central maximum appears broader and brighter than the others. Most of the diffracted light energy is concentrated in this central region. The secondary maxima receive comparatively less intensity and are narrower. This characteristic feature helps distinguish diffraction patterns from interference patterns, where fringes are generally of equal width.
Q14. Distinguish between interference and diffraction.
Answer:
Interference arises due to the superposition of light waves from two or more coherent sources, whereas diffraction occurs due to the superposition of waves originating from different parts of the same wavefront. In interference, bright fringes are usually of equal intensity, while in diffraction the central maximum is brightest and widest. Interference requires at least two coherent sources, whereas diffraction can occur with a single slit. Interference produces equally spaced fringes, while diffraction patterns have varying intensities. Both phenomena demonstrate the wave nature of light but differ in their origin and appearance.
Q15. What is polarization of light?
Answer:
Polarization is the phenomenon of restricting the vibrations of light waves to a single plane perpendicular to the direction of propagation. Ordinary light vibrates in all possible directions and is called unpolarized light. When these vibrations are confined to one plane, the light becomes plane-polarized. Polarization provides strong evidence that light waves are transverse in nature because longitudinal waves cannot be polarized. The phenomenon is widely used in sunglasses, photography, LCD screens, and scientific instruments to reduce glare and improve image quality.
Q16. What is plane-polarized light?
Answer:
Plane-polarized light is light in which vibrations occur only in one particular plane perpendicular to the direction of propagation. It is obtained from ordinary light using polarizing devices such as Polaroids. The process of producing such light is known as polarization. Plane-polarized light exhibits properties different from ordinary light and is useful in many optical applications. The existence of polarization confirms that light is a transverse wave because only transverse waves can have vibrations restricted to a specific direction. This concept is important in understanding modern optical technologies.
Q17. State Brewster’s Law.
Answer:
Brewster’s Law states that when unpolarized light is incident on a transparent surface at a particular angle known as Brewster’s angle, the reflected light becomes completely plane-polarized. The law is expressed as:
\tan i_p=\mu
where (i_p) is Brewster’s angle and (μ) is the refractive index of the medium. At this angle, the reflected and refracted rays are perpendicular to each other. Brewster’s Law is useful in designing polarizing devices and reducing reflected glare.
Q18. Mention two uses of Polaroids.
Answer:
Polaroids are materials that produce plane-polarized light. They are widely used in everyday life and scientific applications. One important use is in polarized sunglasses, where they reduce glare from roads, water surfaces, and snow. Another use is in photography, where they improve image contrast and eliminate unwanted reflections. Polaroids are also employed in LCD displays, 3D movie glasses, optical instruments, and stress analysis studies. Their ability to control the direction of light vibrations makes them valuable in modern optical technology and communication systems.
Q19. Why cannot sound waves be polarized?
Answer:
Sound waves cannot be polarized because they are longitudinal waves. In longitudinal waves, particles of the medium vibrate parallel to the direction of wave propagation. Polarization requires vibrations to be restricted to a particular plane, which is possible only for transverse waves. Since longitudinal waves do not possess vibrations perpendicular to the direction of propagation, they cannot undergo polarization. This difference is important in distinguishing between transverse and longitudinal waves. The successful polarization of light confirms that light behaves as a transverse electromagnetic wave.
Q20. How does diffraction support the wave theory of light?
Answer:
Diffraction supports the wave theory of light because it demonstrates the bending and spreading of light around obstacles and through narrow openings. Such behavior is characteristic of waves and cannot be explained adequately by the particle theory alone. The diffraction pattern, consisting of a central maximum and secondary fringes, results from interference among different parts of the same wavefront. The phenomenon becomes more pronounced when the aperture size is comparable to the wavelength of light. Thus, diffraction provides strong experimental evidence that light possesses wave properties and behaves according to wave principles.