CBSE Class 12 Physics Revision Notes Chapter 10

Class 12 Physics Chapter 10 Notes- Wave Optics

Physics is the study of the behaviour of the universe. One gets to know how the universe functions through the understanding of Physics and its related concepts. Hence, it has been primarily used in scientific research and thus plays a crucial  role in higher education.

The main topics covered in Class 12 Physics Chapter 10 include Wave Front, Huygen’s Principle, Principle of Superposition, Interference of Light, Young’s Double Slit Experiment (YDSE), Illustrations of Interference, Doppler’s Effect in Light, Diffraction of Light, Polarisation of Light, Validity of Ray Optics, Resolving Power, Laws to Support Wavelength Properties and all the topics related to Wave Optics.

Students can get access to all  the topics covered  in the Chapter 10 Physics Class 12 notes available on the Extramarks’ website. It contains all the concepts covered in a point-wise manner. These notes are written in a systematic and organised manner  along with highlighted main points to aid students in learning main points quickly. As a result, students need not look elsewhere to supplement their studies to better their performance. In fact  students can also find various resources at Extramarks’ website to  test their knowledge and understanding to nail the exams. 

The resources provided by Extramarks has built credibility over the years and it’s trusted  by both  students and the teachers alike. It is one of the fastest-growing educational  platforms with all the NCERT-related study material.  As reiterated earlier, students can also look for extra questions to practise on our website. By registering on  Extramarks’ website, students can access  resources like NCERT textbooks, NCERT solutions, NCERT Exemplar, CBSE past year papers and NCERT-based mock tests which are prepared by experienced faculty while adhering to the latest CBSE updates regarding the examination pattern.

Key Topics Covered in Class 12 Physics Chapter 10 Notes

Wave optics is the study of the  wave model of the light, which is demonstrated with the help of a theory in the chapter. The wave model of light has some contradictions compared to the corpuscular model of light, which is covered in detail in this chapter. Students will also learn about the various behaviours of light in the form of waves. This chapter lays a strong foundation for the optics section of Class 12 Physics.

Extramarks has covered the important topics like the Doppler effect, Principle of Superposition, Young’s Double Slit Experiment, Polarisation of light, Validity of Ray optics and various principles and laws related to Wave optics in a  structured manner for students to grasp all the concepts quickly and excel in their examinations. 

After completing this chapter, students will be able to distinguish between the ray optics and the wave optics. They will be able to apply all the concepts, theories, formulas, laws, and principles related to it with complete confidence. In this way, Class 12 Physics Chapter 10 Notes can help students to recall the crucial points in the exam and these notes will come handy during the last-minute revision.

Introduction

You have already studied the corpuscular model of light. Using the model, you have already learnt to derive Snell’s law. You also know very well about the predictions of the corpuscular model, which states that if the ray of light on refraction bends towards the normal, then the speed of light would be more significant in the second medium.

In this chapter, we will get to know about the wave theory of light. It satisfies all the principles of reflection and refraction but has certain contradictions when compared with the corpuscular model of light. The wave theory of light helped to confirm that the speed of light in water is less than the speed of light in air. 

It was due to the interference experiment of Thomas Young. The fact that light is indeed a wave phenomenon was concluded. After the interference experiment, many more experiments were carried out involving interference and diffraction of light waves, and this was only possible with the wave theory of light. Hence, by the middle of the eighteen century, the wave theory was firmly established. Since we know a wave needs a medium for propagation, the dilemma among scientists was regarding its propagation in a vacuum. Maxwell explained this through his electromagnetic theory of light.

In this chapter, we will discuss the original interpretations of Huygens’s Principle and derive the laws of reflection and refraction. We will also discuss the interference phenomenon based on the principle of superposition. You will also get to know the phenomenon of diffraction, which is based on the Huygens-Fresnel principle, and the phenomenon of polarisation, which is based on the fact that the light waves are transverse electromagnetic waves.

If you are looking for complete study material of  the chapter wave optics, then the Extramarks’ website is the right place. You may access  the complete chapter along  with its solutions in  Class 12 Physics Chapter 10 Notes available on the Extramarks’ website.

Huygen’s Principle

To understand Huygens’s Principle in a better way, let us first know what a wavefront is.

When you throw a pebble in still water, due to the disturbance generated you can find patterns formed on the surface in the form of circular rings. It can be noted that the disturbance on circular rings is maximum. All the points in a circle oscillate in a phase as they cover equal distances from the source. This locus of points is called the wavefront. Thus, the wavefront is defined as a surface of the constant phase.

The direction of light’s propagation is perpendicular to the wavefront. Each point on the given wavefront acts as a source of a new disturbance called secondary wavelets, which move in all the directions with the speed of the light in the medium. A surface touching those secondary wavelets tangentially in the forward direction at a given time, gives the new wavefront called a secondary wavefront.

In 1678, Huygen put forward that the light propagates in the form of the wavefront. According to Huygens’s Principle, “each point of the wavefront is a source of secondary disturbance, and the wavelength emanating from these points spreads out in all directions at the speed of the wave. These wavelets emanating from the wavefront are usually referred to as secondary wavelets. If we draw a common tangent to all these spheres, we obtain the new position of the wavefront at a later time.”

You can strengthen your  conceptual understanding of Huygen’s Principle by solving various  types of questions with graded difficulty levels  in the CBSE Class 12 Physics Chapter 10 Notes available on the Extramarks’ website.

Refraction and Reflection of Plane Wave using Huygens Principle

Reflection is the returning of light, sound or heat by a body or surface without absorbing it. It is also the shift in the direction of a wavefront at an interface between two types of media so that the wavefront returns to the medium from which it initiated. 

Refraction is the shift in the direction of a wave passing from one medium to another or through the medium of varying densities caused by its change in speed. It is also a shift in the direction of propagation of any wave as a result of its travelling at different velocities at different points along the wavefront.

We will use different applications of reflection and refraction to draw laws and conclusions on the wave theory of light.

It is very important to know the difference between reflection and refraction for the proper understanding of the chapter  wave optics. You can find a detailed  explanation in the Class 12 Physics Chapter 10 Notes on the Extramarks’ website.

• Refraction of a Plane Waves

In this section, we will apply the Huygens Principle to derive the law of refraction with the help of experiments. Suppose there is a surface XY separating mediums 1 and 2. Assume  V1 and V2 be the speed of the light in the medium 1 and 2 with considerable wavefront AB propagating in the direction of incidence at an angle i. Thus, using the observation of the experiment, we obtain the formula as

Sin i/sin r=v1 /v2.

Now let C represents the speed of the light in the vacuum when

n1=C/ v1

and 

n2=C/ v2.

This is known as the refractive index of medium 1 and 2, respectively.

n1 sin i = n2 sin r. 

This is Snell’s law of refraction. Further, if we add the wavelength of the light in mediums 1 and 2, we get 

 v1/λ1 = v2/λ2.       

The above equation shows that when a wave gets refracted into a denser medium, the wavelength and the speed of the propagation decrease, but the frequency remains the same.

Reflection of the plane waves is an important topic in the chapter wave optics. The entire reflection of plane waves is covered in the Class 12 Physics Chapter 10 Notes available on the Extramarks’ website.

• Reflection on a Rarer Medium

In this section, we will cover the reflection of a plane wave at a rarer medium.  As mentioned above , we will proceed in a similar manner for a refracted wavefront.  In this case, the angle of refraction will be greater than the angle of incidence. However, Snell’s law will be valid in the same way. The formula for the reflection at a rare medium is given by

sin ic =  n2 /n1. 

The angle Ic is also the critical angle and is greater than all other angles of the incident. No other waves are being refracted; hence, we will know about a phenomenon known as total internal reflection

Reflection of a rarer medium can be understood thoroughly only if you solve lots of numerical problems based on it. You can find additional questions to practise in the notes of Class 12 Physics Chapter 10 available on the Extramarks’ website.

• Reflection of a Plane Waves by a Plane Surface.

 We will learn  about the behaviour of the wavefronts as they undergo reflection or refraction in this section.

(a) We will take a plane wave passing through a thin prism. Since the speed of light waves is less in glass, the lower portion of the incoming wavefront, which moves through the greatest thickness of glass, will get delayed resulting in a tilt in the emerging wavefront.

(b) When a plane wave is an incident on a thin convex lens: the central part of the incident plane wave traverses the thickest portion of the lens and is delayed the most. The emerging wavefront has a depression at the centre, and therefore, the wavefront becomes spherical and converges to point F, which is known as the focus.

(c) A plane wave is an incident on a concave mirror, and on reflection, we have a spherical wave converging to the focal point F. Similarly, we can understand refraction and reflection through concave lenses and convex mirrors.

An in-depth explanation of this topic is required. The explanation of the reflection of plane waves by a plane surface is included in the Class 12 Physics Chapter 10 Notes available on the Extramarks’ website.

• The Doppler Effect

 In the previous  section, we learned about the behaviour of the wavefront. The change in the frequency of the light due to relative motion between the source of light and the observer is the study of the Doppler effect of light when the actual frequency changes to the apparent frequency. 

The speed of the source is determined on the basis of a stationary observer. There are two cases viz when the source is moving towards the observer, and the source is moving away from the observer. Due to different cases, there is a shift in the radiation of the spectrum when the source moves towards the observer, the spectrum of radiation shifts towards the violet end, and when the source shifts away from the observer, the spectrum of radiation shifts towards the red end.

Application of Doppler effect

1. Determination  of the speed of aeroplane and submarine in radar and Sonar
2. Calculation of lotus velocity is stars and galaxy by spectrum  swift
3. Tracking of satellite

All the basic principles of the Doppler effect, and its applications are covered in the Class 12 Physics Chapter 10 Notes available on the Extramarks’ website.

Coherent and Incoherent addition of waves

In this  section, we will learn about the interference pattern produced by the superimposition of two waves. Two or more waves superimpose over each other. Then the resulting displacement is equal to the vector sum of their displacements produced by the individual waves at a point.

It is given by y= y1+ y2.

When the waves emitted from two sources have the same frequency and constant phase difference, they are said to be coherent. When the waves superimpose, the position of maxima and minimum is fixed for a coherent source which forms a sustained interference pattern. The coherent  source is produced by prism lenses mirror of certain specifications.

Conventional right sources are used to produce coherent sources that emit light waves having different frequency wavelengths, and the phase is called an incoherent source. The changes between the energy levels in an item completely lay random in these waves. There is no control over the energy loss in the form of radiation. Tungsten filament lamps are coherent sources.

The Class 12 Physics Chapter 10 Notes available on the Extramarks’ website include  differences between  coherent and  incoherent sources. One can get the NCERT solutions from the Extramarks’ website.

Interferences of light waves and Young’s experiment

When two waves of the equal frequency (coming from two coherent sources) travel in a medium, in the same direction consequently. Because of their superposition, at some points intensity of light is considered maximum, while  at other points, intensity is minimum. This concept of light is known as interference of light.

(i) Constructive interference ( case of maximum intensity ): 

(a) Phase difference, for , Ø = 0° or 2nπ, here n = 1, 2, 3 …

(b) Path difference, for ∆x = nλ, i.e., even factor of  λ/2 and n = 1, 2, 3,…

(c) Resultant amplitude, Amax = a1+ a2. 

If a1=a2, then Amax=2a0, where a₁ =amplitude of the first wave, a₂ = amplitude of the second wave

(d) Resultant intensity,

Inet= I1+ I2+ 2√(I1 . l₂) cos (Ø), where I₁ = intensity of the first wave, I2=intensity of the second wave

Therefore, Imax = I1+ I2+2√/I1I2 = (√I₁ + √I₂) 2  If I0 =  I1 = I2, if Imax = 4I0

(ii) Destructive interference ( case of minimum intensity):

(a) Phase difference for Ø = 180° or (2n-1)π, here n = 1, 2, 3,… or (2n + 1)π

(b) Path difference for ∆ = (2n-1).λ/2, i.e., odd factors of λ/2

(c) Resultant amplitude, Amin = a1 – a2, If a1= a2  then Amin = 0

(d) Resultant intensity, Imin = I1+ I2 – 2√I₁I2 = (√I₁ – √I₂)2. If I₁ = I2 = Io ⇒ Imin = 0

(iii) Interference pattern is not created with the wave  superpositions of the random phase difference. The intensity thus produced is the sum of the two intensities. I = I1+ I2

Extramarks has covered all the topics and problems related to  the interference of light waves in the Class 12 Physics Chapter 10 Notes.

 In case of Young’s Double Slit experiment, monochromatic light is used.

Where, 

• d = distance between the slits, 
• D= distance between any slits and screen,
• λ = wavelength of monochromatic light coming out from the source

(i) Central fringe is bright, because at any central position Ø = 0° or ∆x = 0, where Ø = phase difference, ∆x = geometrical difference of the path

(ii) If the slit widths are  unequal, the minima will not be  completely dark.

(iii) Geometrical path difference, ∆x = S2P – S1P = d.sinØ, if d <<< D or ∆x =dy/D if y« D, where θ = angle made by line S1P with the horizontal axis passing through ‘0’.

(a) for maxima, if ∆x = nλ or y = nẞ, here, n = 0, 1, 2… for central maxima

(b) for minima, if ∆x = (2n-1)λ/2, here n = ±1, ±2, ±3… .or y = (2n-1)β/2, here fringe width β = λD/d

(iv) Highest order maxima, nmax = [d/λ], where [ ] represents greatest integer function.

(v) Total number of maxima = 2nmax +1

(vi) Highest order minima for, nmax= [ d/λ + ½ ]  represents greatest integer function.

(vii) Total number of minima = 2nmax

(viii) Intensity on screen, I = I1 + I2 +2√I1I2 cos (Ø), where Ø = 2π/λ.∆x

(ix) If I₁ = I2, Io = 4I, cos² (Ø/2)

(x) The nearest point to the central maxima when the bright fringes coincide, y = n1ẞ1= n2ß2= LCM of B1 and B2

(xi) The nearest point to the central maxima when the two dark fringes coincide, y = ( n1− 1/2)B1 = (n₂ –1/2 ) B2

(xii) Optical path difference

(a) ∆xopt = μ∆x

(b) Ø = 2π/λ. ∆x = 2π/λ.∆vacuum. ∆xopt , where  ∆xopt  = (μ-1)t, λ = wavelength of the light in a medium,   λvacuum = wavelength of the light in a vacuum

(xiii). In YDSE, if n1 fringes are seen in the field of the view with the light of wavelength λ₁, while n₂ with the light of wavelength λ2 in the same field, then n₁λ₁ = n₂λ2

All the important points related to YDSE are there  in  Class 12 Physics Chapter 10 Notes available on the Extramarks’ website.

Diffraction

The bending of the light around the corners of an obstacle of the exact size of the wavelength of the light is known as diffraction.

(i) Fresnel diffraction:

If source or screen or both are placed which are at a distance that is visible from a device which is diffracting (obstacle or aperture), the diffraction is known as Fresnel type. Common examples are-  diffraction at the straight edge, narrow wire or small opaque disc, etc.

(ii) Fraunhofer diffraction:

Here, the source, as well as the screen, are effectively at an infinite distance from the diffracting device. Examples are diffraction at the single slit, double slit etc.

• The single slit.

We draw the following conclusions after observing the interference pattern with that seen for a coherently illuminated single slit, usually called the single slit diffraction pattern.

(i) The interference pattern has multiple equally spaced bright and dark bands. The diffraction pattern has a bright central maximum which is twice as large as the other maxima. The intensity decreases on either side as we go to successive maxima away from the centre.

(ii) We calculate the interference pattern by superposing two waves initiating from the two narrow slits. The diffraction pattern of the superposition of a regular family of waves initiating from each point on a single slit. 

(iii) For a single slit width a, the first null of the interference pattern occurs at an angle of λ/a. At the same angle of λ/a, we get a maximum (not a null) for two narrow slits separated by a distance ‘a’.

You can learn more about this section from the Class 12 Physics Chapter 10 Notes on the Extramarks website.

• Seeing the single slit diffraction pattern

  You can notice the single slit diffraction pattern with an experiment using two razor blades and an electric glass bulb to perform it. The two blades are kept in a manner that they are placed parallel to each other, and a narrow split is formed in between. With the adjustment of the slit, the pattern of bright and dark bands can be seen. The position of all bands depends on the wavelength; hence, they will show different colours in the slit experiment. The filament plays the role of a single slit, and the eye focuses on the pattern of the screen.

We can draw the following conclusion from the given experiment:

In interference and diffraction, light energy is redistributed. If it reduces in one region, producing a dark fringe, it increases in other regions, producing a bright fringe. There is no gain or loss of energy which is consistent with the principle of conservation of energy.

• Resolving power of optical instruments

              (i) Resolving power of telescope: 

(a) The angular separation between two objects must be ∆θ = 1.22 λ/d, where λ = wavelength of light used, d = the diameter of the telescope objective

(b) Resolving power = 1/∆θ = d/1.22λ

(ii) Resolving power of microscope:

(a) Distance between two objects that can be just resolved = ∆d = λ/2nSinθ, where λ = wavelength of light used, n = refractive index of the medium between the object and the objective, θ = half-angle of the cone of light from one of the objects.

              (b) Resolving power = 1/∆d= 2nSinθ/λ

• The Validity of Ray Optics  

 A slit or hole of size ‘a’ is illuminated by a parallel beam at an angle of approximately λ/a

 This is the angular size of the bright  central maximum

The deflected beam acquired the pattern of zλ/a, where z is the distance of travelling.

When we equate the size of the slit and the width, we get

Z = a2/λ

Also, we define a quantity ZF called the Fresnel distance through the above equation, which is

ZF = a2/λ.

The equation z= a2/λ shows that Ray optics is valid in the limit tending to zero.

 Numerous examples based on  it may be found in  Class 12 Physics Chapter 10 Notes on  Extramarks’ website. 

Polarisation    

Light propagates as transverse electromagnetic waves. The magnitude of the electric field is much larger as compared to the magnitude of the magnetic field.

Electric field vectors are distributed in all directions in ordinary lights. It is called unpolarised light, whereas the electric field vector vibrates in one direction in a plane perpendicular to the direction of light propagation. It is called polarised light.

• Polarisation by Scattering

Scattered light in the direction perpendicular to the direction of incident light is completely plane polarised while transmitted light is unpolarised. In all other directions, light is partially polarised.

• Polarisation by Reflection 

If unpolarised light is incident on the boundary between two transparent media, the reflected light is polarised with its electric vector perpendicular to the plane of incidence when the refracted and reflected rays make a right angle with each other. Thus, we have seen that when a reflected wave is perpendicular to the refracted wave, the reflected wave is a totally polarised wave.

The formula for polarisation by reflection is given by

μ= sin ip/sin r =sin ig\sin(1/2-in).

You can find quick revision study material in the Class 12 Physics Chapter 10 Notes available on the Extramarks’ website.

Class 12 Physics Chapter 10 Notes Exercise &  Solutions.

Physics is a vast subject with complex formulas,definitions and problems. Solving numerical in Physics is quite important, especially  solving various types of questions with graded difficulty levels will help you to assess your understanding of the topic and step your preparation to get excellent results. Hence, the subject experts have designed Class 12 Physics Chapter 10 Notes keeping in mind all the concepts and the  numerical problems, understand every concept and answer any question easily. This encourages the students to master the topic and increase their confidence in achieving a higher grade.

 You can find  solutions to every concept covered as well as answers to all the questions given in the NCERT textbook. Students must practise chapter end exercises and additional exercises. In case they need any assistance, they can go through  all the exercises related to extra questions in the solutions provided on the Extramark’s website.

Mistakes while solving sums are obvious. To reduce this, we have provided various tips and tricks for students to  achieve greater accuracy while solving these problems. This will help them to develop an interest in the subject. In this way, they can make significant improvements  with less effort and be more confident.

Click on the  links below to view exercise-specific questions and solutions for Class 12 Physics Chapter 10 Notes:

• Chapter 10: Exercises

Along with Class 12 Physics Chapter 10 Notes, you may  explore NCERT notes on Extramarks’ website for solutions to all primary and secondary Classes from 1 to 12.

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NCERT Exemplar Class 12 Physics

Physics requires  a lot of patience and understanding.  And that level of  understanding comes with complete conceptual clarity and regular  practice of all the topics covered in the syllabus.Students can practice on their own  if they have access to a set of questions designed as per their curriculum. NCERT Exemplar Class 12 Physics covers all these topics at an advanced level. It’s advisable for students to begin with NCERT books first and then switch to extra study material to enhance their confidence during exams. 

Students can find tricky and challenging questions comparatively easier, once they start solving problems from the NCERT Exemplar. Extramarks has been providing educational resources for more than a decade. It has built that credibility because of  its wide range of questions  with proper solution sets. and  it is trusted by  teachers and students alike. The various tricks and tips provided in the book help students to improve their score  during their problem solving, and hence they will definitely find  significant improvement  in their academic results.

One can find questions from all the core topics and fundamental concepts of the NCERT textbook in the NCERT Exemplar. Subject  experts recommend that  all the students must include NCERT Exemplar in their study material since it is reliable and accurate. Experts have diligently restructured the information into different formats to enable a smooth and deep learning experience so that students need not look elsewhere to supplement their studies to better their performance. Extramarks provides one stop solutions to all your problems. To enjoy the maximum benefit of these resources, students just need to register themselves at Extramarks official website and stay ahead of the pack.

Key Features for Class 12 Physics Chapter 10 Notes

In order to master the topics, students need to be thorough with their concepts. Hence, Class 12 Physics Chapter 10 Notes covers the entire chapter. Following are the key features of chapter notes: 

• Students can find notes in easy to learn and understand format. Once you follow the systematic approach to grasp the topic, you will be able to crack those advanced level questions easily. 
• The notes include mind maps, flowcharts, tables and pie diagrams, making it interesting for the students so they start learning without being stressed.
• After referring to Class 12 Physics Chapter 10 Notes, students will see a significant improvement  in their study pattern and overall grades.