# Important Questions and Answers: Interference

1. What  is interference of light? What are the conditions for  interference?

Ans: Interference of light is the non uniform distribution of light energy (wave) due to the superposition of coherent sources of light. The conditions for interference of light are as follows:

• The source must be coherent.
• The source must be close to each other.
• The two sources should be monochromatic.
• The light wave should be passed through very narrow slit.

2. Can two independent source of light produce interference?

Ans: No, two independent source of light cannot produce interference because two sources cannot be coherent as they have different frequency and wavelength. So they will have different phase difference due to the random emission of light and interference pattern cannot be obtained.

3. Why two sources of light should be nearer to each other for the production of good interference pattern?

Ans: In interference the fringe width is inversely proportional to the distance between the light sources given by the relation below.

$$\beta = \frac{\lambda \text{D}}{\text{d}}$$

So, to produce a sustained interference, two light sources must be nearer to each other.

4. What is constructive and destructive interference?

Ans: When two waves meet each other, then they may either cancel each other or add up together. If the two waves are added together, it is called constructive interference.  If they cancel each other, it is called destructive interference. In other words, during interference, dark and bright fringes (spots) are seen in the space which is called destructive interference and constructive interference respectively.

5. Differentiate between interference and diffraction.

 Diffraction Interference The phenomenon of interaction of light coming from different parts of the same wave front is called diffraction. The phenomenon of non uniform distribution of light energy (wave) due to the superposition of coherent sources of light is called interference. In diffraction, the widths of fringes are not equal. In interference, the width of fringes are equal. Bands are very less in number. Bands are very large in number. Dark fringes in diffraction are not completely dark. Dark fringes in interference are perfectly dark.

6. What is the effect on fringe width when Young’s apparatus is immersed in water?

Ans: The fringe width of the Young’s apparatus is given by:

$$\beta = \frac{\lambda \text{D}}{\text{d}}$$

Here, λ is the wavelength of light, D is the distance of screen from the slits and d is the distance between two slits. As the wavelength of light is less in water than in air and as fringe width has direct relation to the wavelength, so fringe width decreases.

7. Young’s double slits experiments involve both diffraction and interference phenomena. Explain.

Ans: The light wave arriving at each slits are diffracted first and the diffracted waves interfere to give interference pattern. So, both phenomenons are involved in Young’s double slits experiment.

8. What happens to the fringe width when the separation between the slits is doubled and the distance between the slits and the screen is halved in Young’s double slit experiment?

Ans: We know that the fringe width of the Young’s apparatus is  given by:

$$\beta = \frac{\lambda \text{D}}{\text{d}}$$

Here, λ is the wavelength of light, D is the distance of screen from the slits and d is the distance between two slits. If d = 2d and D = D/2, Now:

$$\beta = \frac{\lambda \cdot \frac{\text{D}}{2} }{\text{2d}} = \frac{ \lambda \text{D}}{2.2d} = \frac{1}{4} \beta$$

So, the fringe width decreases by four times.

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