Diffraction of Light

1. Diffraction of Light

2. Diffraction of Light Bending of light waves around the small obstacle... Diffraction, which is the bending of waves as they pass by some objects or through an aperture. The phenomenon of diffraction can be understood using Huygens’s principle which states that Every unobstructed point on a wavefront will act a source of secondary spherical waves.

3. Is Diffraction a Good Thing? Radio waves can diffract around hills and buildings and can be received better in more places than short waves that don’t diffract as much.

4. Is Diffraction a Good Thing? Diffraction is bad when we want to see very small objects with microscopes. If the size of the small object is the same as the wavelength of light, the image will be blurred by diffraction.

5. Interference of Light 1) It is the result of interaction of light coming from two different wavefronts originating from the same source. 2) Fringes may or may not be of same width. 3) Fringes are of same intensity. 4) The points of minimum intensity are perfectly dark. hence the contrast between the fringes is good. Diffraction of Light 1) It is the result of interaction of light coming from two different parts of the same source. 2) Fringes in a particular pattern are not of same width. 3) Fringes are of decreasing intensity. 4) The points of minimum intensity are perfectly dark. But intensity of bright fringes decreases. Hence comparatively less contrast among fringes. Difference between Interference & Diffraction

6. Fresnel Diffraction 1) Distance of slit from source and screen is Finite. 2) Wavefront incident on the slit is spherical or cylinderical. 3) Wavefront incident on the screen is spherical or cylinderical. 4) There is path difference between the rays before entering the slit which depends on the distance between source and slit. 5) Path difference between the rays forming the diffraction pattern depends on distance of slit frpm source as well as screen and the angle of diffraction. Hence mathematical treatment is complicated. 6) Lenses are not required to observe Fresnel diffrcation in the laboratory. Fraunhoffer Diffraction 1) Distance of slit from source and screen is Infinite. 2) Wavefront incident on the slit is plane. 3) Wavefront incident on the screen is plane. 4) There is no path difference between the rays before entering the slit. 5) Path difference depends on angle of diffraction. Hence mathematical treatment is comparatively easier. 6) Lenses are required to observe Fraunhofer diffraction in the laboratory. Difference between Fresnel & Fraunhoffer Diffraction

7. Single Slit Fraunhoffer Diffraction When monochromatic light strikes a single slit, diffraction from the edges produces an interference pattern as illustrated.

8. Single Slit Fraunhoffer Diffraction (cont..) • According to Huygen’s principle, each portion of the slit acts as a source of waves • The light from one portion of the slit can interfere with light from another portion • The resultant intensity on the screen depends on the direction θ. • a= dy1+dy2+.....+dyn, phase difference in ray 1 & 2 • ΔΦ =(2Π/λ).dy.sinθ • Φ = (2Π/λ).a.sinθ total Phase in 1 &5 • let α = Φ/2

9. Single Slit Fraunhoffer Diffraction (cont..) Resultant Amplitude at angle θ Eθ = E0 { sin α /α } Intensity of the beam is governed by Iθ = I0 { sin α /α }2 Where α = (π / λ) a sin θ

10. Single Slit Fraunhoffer Diffraction (cont..)

11. Single Slit Fraunhoffer Diffraction (cont..) Iθ = I0 { sin α /α }2 Case-1:Central/Principal maxima where θ = 0 ====>>waves travels in straight lines. therefore α = (π / λ) a sin θ= 0 that means sin α /α =0/0 that is undefined. here we can say waves travels in straight line without diffraction. Equal distance travelled by the waves hence no phase difference.

12. Single Slit Fraunhoffer Diffraction (cont..) case-2: Minima( minimum Intensity) the intensity at point ‘P’ will be ZERO when sin α =0 where θ ≠ 0 (θ =0 is condition for central maxima) α = (π / λ) a sin θ= ±mπ or a sin θ = ±mλ (m=1,2,3,......) case-3: Secondary Maxima ( Brightness of decreasing Intensity) half way between the two minima it can be obtained for α = ±(m+1/2)π , where m=1,2,3,....{3π/2, 5π/2, 7π/2….) for m=1,2, and 3 Iθ/I0=0.045,0.016 and 0.08