# Important Questions and Answers: Diffraction

1. What is diffraction of light ? Differentiate between diffraction and interference.

Ans: Diffraction of light is the phenomenon of bending of light round the corners and spreading into the regions of the geometrical shadow.

 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.

2. Diffraction grating is better than two slits set up  for measuring the wavelength of a monochromatic light. Explain.

Ans: In diffraction grating, the spectrum of the light obtained is very sharp and clear than that of two slit. This helps for precise measurement of wavelength of the monochromatic light.  So, diffraction grating is better than two slits set up for measuring the wavelength of monochromatic light.

3. What happens to the single slit experiment when the width of the slit is less than wavelength of the wave?

Ans: In single slit experiment, the first central maximum occurs at: $$\text{sin} \theta = \frac{ \lambda }{\text{d}}$$

Here, λ is the wavelength of light and d is the width of the slit. When the width of slit is less than the wavelength of the wave, the value of λ/d increases. So, diffraction pattern becomes wider.

4. Why is diffraction of sound waves more evident (common) than that of light waves?

Ans: We know that the wavelength of sound waves greater that of light waves. For a sustained diffraction, the size of the obstacles should be in the order of wavelength of the waves. The wavelength of sound is comparable to the size of the obstacles around which they bend. But, the wavelength of light is in the order of 10-6 m and thus cannot be compared with the size of obstacles. So, diffraction of light cannot be observed easily.

5. We cannot observe the diffraction pattern in a wide slit illuminated by monochromatic light. Why ?

Ans: We know that the size of central maximum is given by:

$$\text{2y} = \frac{ 2\lambda \text{D}}{\text{a}}$$

Where D is the distance between the slits and a is the slit width. So, as the slit width increases, the size of central maximum reduces and diffraction pattern cannot be seen. Even the variation of intensity of other maxima and minima become so small to distinguish. Hence, we cannot observe the diffraction pattern in a wide slit illuminated by monochromatic light.

6. What is the diffreence between Frensel’s diffraction and  Fraunhofer diffraction ?

Ans: During diffraction, If the source of light and screen are at finite distance from the obstacle, then it is called Frensel diffraction whereas if the source of light and screen are kept at infinite distance from the aperture or obstacle, it is called Fraunhofer’s diffraction.

Light does not need to be converged in Frensel’s diffraction. So, no lens is used to converge the beam light. But, lenses and mirrors are used to converge and modification of diffracted light in Fraunhofer’s diffraction.

7. What happens to the Fraunhofer diffraction of light, when it is placed in water?

Ans: The wavelength and decreases is less in water than in air. So, wavelength of light is less in water than in air. In diffraction, the width of central maximum has direct relation to the wavelength of the light used i.e., β∝ λ. So, when the apparatus is placed in water, the width of central bright maximum decreases.

8. Colored spectrum is seen when we see through muslin cloth. Why?

Ans: Muslin cloth is made up of very fine thread which acts like thin slits. When white light is passed through this cloth, the light get diffracted and interference of these diffracted beams give rise to a colored spectrum.

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#### 8 Responses to “Important Questions and Answers: Diffraction”

Radio wave differ act around building but light wave do not. why?

• saiyukta

Wavelength of wave should be comparable to the size of obstacle . since radio wave has longer wavelength than light it can easily diffract around the building

• Prakash panta

As we know that for diffraction the wavelength of wave is in order of the obstacles. Hence the radio waves have the wavelenngth in order of building so it easily diffracted.But light waves have high wavelength which is not in the order of the building so they don’t diffract.

3. awosanya

Enter your thoughts,In a two slit interference pattern at a point we observe 10th order maximum for λ = 7000A°. What order will the same of the light be visible if replaced by light of wavelength 500A°.,questions etc..

4. awosanya

In a two slit interference pattern at a point we observe 10th order maximum for λ = 7000A°. What order will the same of the light be visible if replaced by light of wavelength 500A°.

5. QUDUS