## Wave Optics  Important Questions for CBSE Class 12 Physics Huygens Principle

1.Wave Optics Describes the connection between waves and rays of light. According to wave theory of light, light is a form of energy which travels through a medium in the form of transverse wave.

2.Wave front The locus of all those particles which are vibrating in the same phase at any instant is called wave front. Thus, wave front is a surface having same phase of vibrating particles at any instant at every point on it.

3.Phase Speed Phase speed is the speed with which wave front moves and it is equal to wave speed.

4.The shape of wavefront due to a
(i)point source is spherical
(ii)line source is cylindrical
(iii) source at infinity is a plane.

5.A line perpendicular to a wave front is called a ray. The direction of rays are always perpendicular to the wavefront along the direction of propagation of wave.

6.Huygens’ Principle Huygens’ principle is essentially a geometrical construction, which gives the shape of the wave front at any time, allows us to determine the shape of the wavefront at a later time. According to Huygens’ principle,
(i)Every point on a wave front behaves like a light source and emits secondary wavelets.
(ii)The secondary wavelets spread in all directions in space (vacuum) with the velocity of light.
(iii) The envelope of wave front of secondary wavelets, after a given time, along forward direction gives the new position of wave front.

7.The laws of reflection and refraction can be verified using Huygens’ wave theory.   ### 1 Mark Questions

1.When light travels from a rarer to a denser medium, the speed decreases. Does this decrease in speed imply a decrease in the energy carried by the light wave? Justify your answer.[All India 2010]
Ans.speed decreases due to decrease of wavelength of wave but energy carried by the light wave depends on the amplitude of electric field vector

2.What type of wave front will emerge from a
(i)point source
(ii)distant light source? [Delhi 2009]
Ans.(i)When source of light is a point source, the  wavefront is spherical.
(ii) At very large distances from the source, a portio of spherical or cylindrical waver at appears to be plane.

3.Draw a diagram to show refraction of  a plane wave front incident on a convex lens and hence draw the refracted wave front.                       [Delhi  2009]
Ans.The refraction of a plane wavefront is shown in the figure below 4.Differentiate between a ray and a  wave front.[Delhi 2009]
Ans.Ray A ray always remains perpendicular to the wave front and directed along the direction of propagation of wave.
Wave front The locus of all those particles which are vibrating in the same phase at any instant is called wave front.

5.Draw the wave front coming out from a convex lens, when a point source of light is placed at its focus. [Foreign 2009]
Ans.The wavelength in the given condition is shown in figure below 6.Sketch the shape of wave front emerging from a point source of light and also mark the rays. [Foreign 2009]
Ans. 7.Define a wave front. [Foreign 2009]
Ans.When light is emitted from a source, then the  particles present around it begins to vibrate. The locus of all such particles which are vibrating in the same phase is termed as wave front.

### 3 Marks Questions

8.Define a wavefront. Use Huygens’  geometrical construction to show the propagation of plane wave front from a rarer medium (1) to a denser medium (2) undergoing refraction, hence derive Snell’s law of refraction.     [Foreign 2012]
Ans.When light is emitted from a source, then the  particles present around it beginsto vibrate. The locus of all such particles which are vibrating in the same phase is termed as wave front.  9.(i) Use Huygens’ geometrical  construction to show the  behaviour of a plane wave front,
(a)passing through a biconvex lens
(b)reflected by a concave mirror,
(ii) When monochromatic light is  incident on a surface separating two media, why does the refracted light have the same frequency as that of the incident light?  [Foreign 2012]
Ans.  (ii)The frequency and time period of an electromagnetic wave depends only on the source which produces it. The frequency is independent of the medium through which it travels. But the speed and wavelength depends on the medium through which the wave travels. Because of this, the frequency and time period of sound wave do not change due to change in medium

10.Using Huygens’ geometrical construction of wave  front, show how a plane wave g^is reflected from a surface. Hence, verify laws  of reflection.           [All India 2011]
Ans.When light is emitted from a source, then the  particles present around it beginsto vibrate. The locus of all such particles which are vibrating in the same phase is termed as wave front.  11.Use Huygens’ principle to verify the  laws of refraction.  [Delhi 2011]
Ans.When light is emitted from a source, then the  particles present around it beginsto vibrate. The locus of all such particles which are vibrating in the same phase is termed as wave front.  12.Using Huygens’ principle, draw a  diagram showing how a plane wave gets refracted, when it is incident on the surface separating a rarer medium from a denser medium. Hence, verify Snell’s laws of refraction.           [All India 2011]
Ans.When light is emitted from a source, then the  particles present around it beginsto vibrate. The locus of all such particles which are vibrating in the same phase is termed as wave front.  13.How is a wave front defined? Using  Huygens’ construction, draw a figure showing the propagation of a plane wave refracting at a plane surface separating two media. Hence, verify Snell’s law of refraction.               [Delhi 2008]
Ans.When light is emitted from a source, then the  particles present around it beginsto vibrate. The locus of all such particles which are vibrating in the same phase is termed as wave front.  ### 5 Marks Questions

14.(i) Use Huygens’ geometrical  construction to show how a plane wave front at t = 0 propagates and produces a wave front at a later time.
(ii) Verify, using Huygens’ principle, Snell’s law of refraction of a plane wave propagating from a denser to a rarer medium.
(iii)When monochromatic light is incident on a surface separation two media, the reflected and refracted light both have the same frequency. Explain why?[Delhi 2013 C]
Ans.  (iii) The reflection and refraction phenomenon occur due to interaction of corpuscles of incident light and the atoms of matter on receiving light energy, the atoms are forced to oscillate about their mean positions with the same frequency as incident light. According to Maxwell’s classical theory, the frequency of light emitted by a charged oscillator is same as its frequency of oscillation. Thus, the frequency of reflected and refracted light is same as the incident frequency.

15.State Huygens’ principle. Using.  this principle draw a diagram to show how a plane wavefront incident at the interface of the two media gets refracted when it propagates from a rarer to a denser medium. Hence, verify Snell’s law of refraction.           [Delhi 2013]
Ans.Huygens’ Principle

• Each point on the primary wavefront acts as a source of secondary wavelets, sending out disturbance in all directions in a similar manner as the original source of light does, (l)
• The new position of the wavefront at any  instant (called secondary wavefront) is the envelope of the secondary wavelets at that instant.

When light is emitted from a source, then the  particles present around it beginsto vibrate. The locus of all such particles which are vibrating in the same phase is termed as wave front.  16.(i) A plane wavefront approaches a  plane surface separating two media. If medium 1 is optically denser and medium 2 is optically rarer, using Huygens’ principle, explain and show how a refracted wavefront is constructed?
(ii)Verify Snell’s law.
(iii)When a light wave travels from a rarer to a denser medium, the speed decreases. Does it imply reduction in its energy? Explain.[Foreign 2011]
Ans.(i) Let a plane wavefront AB is incident at the  interface XY separating two media such that medium 1 is optically denser than medium 2. Let time t is taken by the wave to reach from B to C    