Wave Optics


Suppose there is a dark room, a completely dark room and through the window, there is a small hole. When light enters through that tiny hole, what happens? We see that through the small hole light enters but instead of just bright light, we see a region of light and dark bands. This is nothing but the diffraction of light. Let us study diffraction in detail.

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Every one of us knows what diffraction is. It is the bending of light around the corner of an obstacle. Reflected light produces fridges of light, dark or colored bands. At times diffraction of sunlight in clouds produces a multitude of colors. Example of diffraction in nature is diamond rays in the solar eclipse.


Types of Diffraction

There are two types of diffractions

  • Fresnel Diffraction
  • Fraunhofer Diffraction


From the above figure, we observe that the source is located at a finite distance from the slit, and the screen is also at a finite distance from the slit. The source and the screen are not very far from each other. So this is a Fresnel diffraction. Here, if suppose the ray of light comes exactly at the edge of the obstacles, the path of the light is changed. So the light bends a little and meets the screen.

A beam of width α travels a distance of α2/λ , called the Fresnel distance before it starts to spread out due to diffraction. But when the source and the screen are far away from each other, and when the source is located at the infinite position, then the ray of light coming from that infinite source are parallel rays of light. So this is Fraunhofer diffraction.

Here we have to make use of the lens. But why do we use the lens? Because in Fraunhofer diffraction, the source is at infinity so the rays of light which pass through the slit are parallel rays of light.

So in order to make these rays parallel to focus on the screen, we, make use of the converging lens. The zone which we get in front of the slit is the central maxima. On either side of central maxima, there is bright zone i.e 1st maxima.

Download Wave Optics Cheat Sheet by clicking on the button below

wave optics

Browse more Topics under Wave Optics

 Fresnel Diffraction  Fraunhofer Diffraction
Here the wavefront used is spherical. Here the wavefront used is plane.
An image is formed at a finite distance. An image is formed at an infinite distance.
A lens is not required. A lens is required.

Questions For You

Q1. In a single slit diffraction  λ= 500mmwith and a lens of diameter 0.01mm, width of central maxima, obtain on screen at a distance of 1 m will be

  1. 5mm
  2. 1mm
  3. 10mm
  4. 2.5mm

Answer: C. The angle subtended by two minima in the slit = α = 2λ/w, where w is the slit width. Here the lens diameter would act as slit width. The width of central maxima is the distance between the two minima = dα, where d is the distance between slit and screen = 1m. Thus the width of central maxima = 10mm

Q2. Yellow light is used in single slit experiment with slit width 0.6 mm. If the yellow light is replaced by X-rays, then the pattern will reveal

  1. no  pattern
  2. that the central maxima narrower
  3. less number of fringes
  4. more number of fringes

Answer: D. A wavelength of X rays ranges from 0.1 to 10mm and yellow light is the range 570 to 590mm.  Fringes are formed at nλD/d. Hence the fringes of x-rays are formed close to each other.

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