Question

What is meant by interference and diffraction of light ? Write the conditions of interference.

Hint:

The main difference to remember is that interference is due to the superposition of waves from two (or more) different coherent sources, while diffraction is due to the superposition of secondary wavelets from different parts of the same wavefront.

Answer 1

Step 1: Definition of Interference and Diffraction: 
Interference of Light: 
Interference is the phenomenon in which two or more light waves, originating from coherent sources, superimpose to produce a resultant wave of greater, lower, or the same amplitude. This superposition results in a modification of the distribution of light energy in the region of superposition, creating a pattern of alternating bright and dark fringes.
Constructive Interference: Occurs when waves meet in phase, resulting in a bright fringe (maximum intensity).
Destructive Interference: Occurs when waves meet out of phase, resulting in a dark fringe (minimum intensity).
Diffraction of Light: 
Diffraction is the phenomenon of the bending of light waves as they pass around the edges of an obstacle or through a narrow aperture. This bending causes the light to spread into the regions of the geometrical shadow. Diffraction is a characteristic property of all waves and becomes more pronounced when the size of the obstacle or aperture is comparable to the wavelength of the light. 
Step 2: Conditions for Sustained Interference: 
For interference to be observed and for the fringe pattern to be stable (sustained), the following conditions must be met: \begin{enumerate} Coherent Sources: The two sources of light must be coherent. This means they must emit light waves of the same frequency (or wavelength) and must have a constant phase difference between them. This is the most crucial condition. 
Monochromatic Light: The light from the sources should be monochromatic (of a single wavelength) to obtain a clear and distinct interference pattern. If white light is used, the fringes will be colored and will overlap. 
Equal Amplitudes: The amplitudes of the interfering waves should be equal or nearly equal. This ensures that the intensity at the dark fringes is close to zero, providing good contrast between the bright and dark fringes. 
Sources Close Together: The two coherent sources must be very close to each other. This is necessary to produce interference fringes of sufficient width to be observable. \end{enumerate}