DC Pandey Interference and Diffraction of Light Solutions

DC Pandey Optics and Modern Physics Solutions Chapter 28 ‘Interference and Diffraction of Light’ are prepared to provide assistance in JEE Physics preparation along with Class 12 board exams. The important topics that are covered in these DC Pandey Solutions include an introduction to the interference of light, energy distribution in interference and the essential conditions for the interference of light. Young’s Double Slit Experiment and diffraction of light are other crucial topics of your syllabus which are covered in our solutions. 

There are 82 questions in this chapter divided into 2 exercises. The questions are arranged in level 1 and level 2 according to the difficulty and complexity of the problems. Our solutions for DC Pandey Interference and Diffraction of Light gradually introduce complicated topics. With the help of our solutions, you will be able to find maxima and minima in the diffraction of light, the wavelength of first-order bright and dark fringe, and phase difference easily. We have also explained the phasor method elaboratively for you. 

At the Instasolv platform, you will get ample opportunity to brush up your knowledge with constant reminders of the concepts in a step by step manner in each question of the chapter. The Instasolv experts provide you with 100% accurate solutions for DC Pandey Optics and Modern PhysicsInterference and Diffraction of Light”. Practising with these solutions will make you acquainted with the pattern of questions asked in Class 12 exams as well as JEE. 

Important Topics in DC Optics and Modern Physics Solutions Chapter 28: Interference and Diffraction of Light

Introduction to Optical Interference

When the wavelengths of light tend to be larger than the dimensions of the apertures of lenses and mirrors, the rules of geometric optics learnt so far fail while the phenomenon of wave nature of light is observed that is studied under physical optics. The interference of waves that we have studied in previous chapters will hold true for light as well as light is an electromagnetic wave. The colours that appear on the films of oils or soap bubbles are a result of this topic of physical optics.


When the light bends over the corners of openings when the width is as small of the order of the wavelength of the light, the bending is termed as Diffraction. All types of waves undergo the process of Diffraction.

Energy Distribution in Interference

We have studied earlier that when two or more waves incident and reflecting waves overlap over one another then the properties are governed using the principle of superposition. 

In the interference of light waves takes place, redistribution of the light energy happens. If there is only one source in the medium then the distribution of energy is uniform. 

But if there are 2 sources, even if they are similar, the distribution of energy becomes non-uniform. At some points, energy becomes maximum while at some points it becomes a minimum. Such redistribution is known as the interference of light. 

For instance, if the crest of two waves overlaps, then the resultant displacement becomes the sum of the amplitudes of both and the intensity will become 4 times as I α A2.

Condition for Interference

For a visible and sustained pattern of interference to occur, the following are the essential conditions:

  • Coherent Sources:

The sources should emit light waves that must maintain a constant phase difference with each other. In case of light waves, unlike in the mechanical waves, the atoms gain extra energy due to thermal agitation or accelerated electrons, different sources radiate light in a random phase relationship with a completely undefined phase relationship. 

Therefore, to make it a coherent source, we split one light source into two, to achieve a constant phase difference and hence, a sustained interference pattern.

  • Same Frequency or Wavelength:

The two coherent sources emitting the light wave must have the same frequency to maintain a constant phase difference.

  • Equal Amplitudes: 

The amplitudes of the waves meant to interfere should equal or nearly equal. If the difference happens to be very small between the maxima and minima of the intensities, the interference will be less visible.

Young’s Double Slit Experiment

Thomas Young demonstrated the interference of light through the double-slit experiment for the first time in 1801. He made a narrow vertical slit S for a monochromatic source of light. The light from S falls on two narrow and closely spaced slits S1 and S2 which are parallel to one another with a distance of d between them. Thus, S1and S2act as two coherent sources. If the slits are too narrow, the overlapping can be observed through alternate bright and dark equally spaced vertical bands.

If the two light waves meet at a point P at a distance D from the coherent sources, the two waves are nearly parallel with a path difference,

, is the angle of light rays with the normal on the screen on which slits are made.

Exercise Discussion for DC Pandey Optics and Modern Physics Solutions Chapter 28: Interference and Diffraction of Light

  • There is only one Introductory Exercise in this chapter that comprises 8 questions in which you will be required to calculate the fringe width, maxima and minima of the intensities, etc. in certain situations of interference of light waves.
  • Then there are two exercises for around 80 JEE-based questions. These exercises include 2 levels of subjective questions and multiple-choice questions. The topics covered in the questions of each section include young’s double-slit experiment, conditions necessary for the interference of light, diffraction and energy distribution in interference of light waves.

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