# Class 12 Physics Chapter 10 Important Questions Wave Optics

Q.1:- What is a wavefront? Ans:- A wavefront is a surface of constant phase. A wavefront is also defined as the locus of all the medium particles

Science is a complex and challenging subject, as it involves so many principles and concepts that are difficult to memorize. Those student who opt for science have to face many challenges and work hard to get good marks in the exam. In this lesson, students will learn about Wave Optics. The best solution of the problem is to practice as many Physics Class 12 Chapter 10 Important Questions as possible to clear the doubts.

Q.1:- What is a wavefront?
Ans:- A wavefront is a surface of constant phase. A wavefront is also defined as the locus of all the medium particles which at any instant are vibrating in the same phase condition.

Q.2:- Name the shape of the wavefront originating from a - (a) point source, and (b) line source.
Ans:-

• (a) Wavefront originating from a point source is a spherical wavefront.
• (b) A cylindrical wavefront originates from a line source.

Q.3:- Differentiate a ray and a wavefront.
Ans:- A wavefront is surface of constant phase but a ray is a line drawn normal to the wavefront and pointing in the direction of propagation of the wave or wavefront.

Q.4:- What is the direction and speed of propagation of a wavefront?
Ans:- In a homogeneous isotropic medium, a wavefront is propagated parallel to itself with the speed of wave motion (in case of light waves with the speed of light).

Q.5:- If a wave undergoes (a) refraction, and (b) reflection at a surface, what happens to its phase?
Ans:-

• (a) When a wave undergoes refraction from one transparent medium to another, whether denser or rarer, its phase does not undergo any change, i.e., the phase remains the same.
• (b) When a wave undergoes reflection from a denser medium surface, its phase changes by π radian. However, there is no phase change due to reflection at the surface of a rarer medium.

Q.6:- What is interference of light?
Ans:- Interference of light is the phenomenon of redistribution of light energy and formation of alternate bright and dark fringes in space when light waves from two coherent light sources superpose on each other.

Q.7:- What are coherent sources of light?
Ans:- Two sources of light are said to be coherent if they emit monochromatic light waves of exactly the same wavelength (or same frequency) such that the originating (initial) phase difference between the waves is either zero or having a constant value.

Q.8:- State the essential conditions which must be satisfied for two light sources to be coherent.
Ans:- Two essential conditions for two light sources to be coherent are:

• (a) Two sources must emit light of exactly the same wavelength (or same frequency).
• (b) The initial phase difference between light waves emitted by the two sources must be either zero or having a constant value which does not change with time.

Q.9:- What is a sustained interference pattern? State the essential condition to obtain sustained interference.
Ans:- A sustained interference pattern is that in which positions of bright and dark fringes obtained on the screen do not change with time. To obtain a sustained interference pattern, the two sources of light must be coherent one.

Q.10:- State two conditions for sustained interference of light. Draw the variation of light intensity with position in the interference pattern of Young's double-slit experiment.
Ans:- Two essential conditions for sustained interference of light are:

• (a) The two light sources must be coherent.
• (b) Two light sources must be very narrow and lie close to each other.

Q.11:- How does the angular separation of interference fringes change in Young's experiment if the distance between the slits is increased?
Ans:- The angular separation of interference fringes decreases on increasing the distance d between the slits because angular fringe width,

a = λ/d.

Q.12:- How does the fringe width in Young's double-slit experiment change when the distance of separation between the slits and screen is doubled?
Ans:- If the distance of separation D between the slits and screen in Young's double slit experiment is doubled, the fringe width is also doubled because ß ∝ D.

Q.13:- Give an illustration of interference of light from your daily routine.
Ans:- Beautiful colours exhibited by extremely thin films (e.g., thin oil film, soap bubbles, etc.) when seen in white light are a very good illustration of interference of light.

Q.14:- What is diffraction of light?
Ans:- Diffraction of light is the phenomenon of bending of light round the sharp corners or edges of narrow apertures and obstacles and its spreading even into the regions of geometrical shadow.

Q.15:- State the condition for diffraction of light to occur.
Ans:- The size of the slit or obstacle should be comparable to the wavelength of light used.

Q.16:- What should be the approximate size of a rectangular slit to observe diffraction with it?
Ans:- The approximate size of rectangular slit be few times the wavelength of light used. Thus, width of a slit should be of the order of 0.1 mm or even less.

Q.17:- Give one basic difference between diffraction and interference of light.
Ans:- Interference is the result of superposition of light waves coming from two different coherent sources but diffraction is the result of superposition of light wavelets from a single coherent source.

Q.18:- In interference and diffraction patterns, we obtain bright and dark fringes. Does it mean that energy has been gained or lost? Give reason.
Ans:- In interference as well as diffraction phenomena, there is no gain or loss of energy. In both these phenomena, light energy is simply redistributed. If it reduces in one region producing a dark fringe, then it correspondingly increases in another region producing a bright fringe. Thus, total energy remains conserved.

Q.19:- What do you mean by 'limit of resolution' and 'resolving power' of an optical instrument?
Ans:- The least linear or angular separation between two point objects so that they appear as just resolved when viewed through the given optical instrument is called its 'limit of resolution'. The reciprocal of the limit of resolution of an optical instrument is called its resolving power.

Q.20:- Which physical phenomenon proves the transverse nature of light waves?
Ans:- Polarisation of light proves the transverse nature of light waves.

Q.21:- What is linearly polarised (or plane polarised) light?
Ans:- The light in which the vibrations of light (i.e., the vibrations of electric vector) are restricted in a particular direction perpendicular to the direction of propagation of light is called linearly polarised or plane polarised light.

Q.22:- Why is that, light waves can be polarised but sound waves cannot be polarised?
Ans:- Light waves are transverse waves and can be polarised. However, sound waves are longitudinal waves and they cannot be polarised. It is because only transverse waves can be polarised.

Q.23:- Which of the following waves can be polarised (a) heat waves (b) sound waves? Give reason to support your answer.
Ans:- Heat waves can be polarised because these are transverse waves. However, sound waves cannot be polarised because these are longitudinal waves.

Q.24:- State Brewster's law.
Ans:- According to Brewster's law, when light is incident at an angle i at the surface of a transparent plane surface of a medium of refractive index n, such that tan i = n, then the reflected light is completely plane polarised light.

Q.25:- What do you mean by the term "crossed polaroids"?
Ans:- By the term "crossed polaroids" we mean two polaroids whose pass axes are mutually at right angles to each other.

Q.26:- What is the main drawback/limitation regarding wave nature of light?
Ans:- A mechanical wave requires a medium for its propagation but light can travel even through vacuum. Huygens assumed the existence of all pervading media called 'Aether' to overcome this problem but the aether medium has not been detected by any experimental means.

Q.27:- Why do we fail to observe the diffraction from a wide slit illuminated by monochromatic light?
Ans:- We know that for diffraction at a single slit, central diffraction maxima extends from θ = 0 to θ = ±(λ/a), where a is the width of slit. For a wide slit, a is large and so value of θ is extremely small. Hence, diffraction fringes cannot be observed.

Q.28:- A parallel beam of monochromatic light falls normally on a narrow slit and the light coming out of the slit is obtained on a screen, kept behind, parallel to the slit plane. What kind of pattern do we observe on the screen, and why?
Ans:- We obtain a diffraction pattern on the screen which consists of a central broad maximum and few diffraction fringes of poor contrast and rapidly decreasing intensity on either side of it. The diffraction pattern is observed due to superposition of wavelets originating from the wavefront incident on the narrow slit.

Q.29:- What information does one obtain from polarisation phenomenon about the nature of light?
Ans:- Only transverse waves can be polarised. As light waves exhibit the polarisation phenomenon, it shows that light waves are transverse in nature.

Q.30:- Can a person recognise by the unaided eye whether a given light is polarised or not?
Ans:- No, one cannot recognise by unaided eye whether a given light is polarised or unpolarised.

Q.31:- When an unpolarised light is incident on a plane glass surface, what should be the angle of incidence so that the reflected and refracted waves are perpendicular to each other?
Ans:- The unpolarised light is incident on a plane glass surface at an angle i such that tan i = n, where n = refractive index of glass. In that case, the reflected and refracted waves are perpendicular to each other and the reflected light is completely plane polarised.

Q.32:- When light travels from a rarer to a denser medium, the speed decreases. Does the reduction in speed imply a reduction in the energy carried by the light wave?
Ans:- No, decrease in speed of light on entrance in a denser medium does not imply decrease in the energy carried by the light wave. It is because energy of a wave depends on its amplitude and does not depend on speed of wave.

Q.33:- Two slits in Young's double-slit experiment are being illuminated by two different sodium lamps emitting light of same wavelength. Is an interference pattern formed on the screen, and why?
Ans:- No interference pattern is obtained on the screen because two different sodium lamps emitting light of same wavelength behave as two noncoherent sources. As a result, their phase changes continuously and the intensity on the screen will be simply a sum of intensities of light due to two slits. Thus, there will be uniform illumination on the screen and no interference fringes are formed.

Q.34:- Comment about the statement that "light added to light may produce darkness".
Ans:- Yes, the statement is possible. If two coherent light waves superpose and are in mutually opposite phase, then the resultant intensity of light at the given point will be the least and we shall get a dark fringe. Effectively, it means that light added to light is producing darkness.

Q.35:- Coloured spectrum is seen when one looks towards the filament of a clear electric lamp through a fine muslin cloth. Explain why.
Ans:- The fine muslin cloth is made of very fine threads and the space between any two consecutive threads behaves as a fine slit. Hence, one observes a diffraction pattern. Since the light of lamp is white light, diffraction maxima or minima for different colours occur at different angles of diffraction. Hence, a coloured spectrum is seen by an observer.

Q.36:- Why does the light from a clear blue portion of the sky show a rise and fall of intensity when viewed through a polaroid which is rotated in its own plane?
Ans:- The light from a clear blue portion of the sky is polarised light on account of polarisation due to scattering when sunlight is scattered in atmospheric air molecules. Consequently, the light from a clear blue portion of the sky shows variation in intensity when viewed through a polaroid on rotating it in its own plane.

Q.37:- How can you distinguish between an unpolarised light beam, a linearly polarised beam and a partially polarised beam of light by using a polaroid?
Ans:- To identify the light, we place a polaroid in the path of the given light beam and slowly rotate it in its own plane. Observe the intensity of transmitted light. If the intensity goes on changing and in one complete rotation of polaroid two times the light intensity is maximum and two times the light intensity is altogether zero (i.e., it is complete dark), then light falling on polaroid is "plane polarised light".

If on rotating the polaroid in its own plane, the intensity of light remains the same throughout, the given light is an 'unpolarised light'. However, if the intensity of transmitted light fluctuates but it is never complete dark, then it means that the light falling on the polaroid is a "partially polarised light".