1. Diffraction

2. Diffraction is a phenomenon of a wave that it can be emitted from a source or it can pass through an aperture. When the wave is propagating it is spread out. What causes diffraction is interference of an infinite number of waves that is emitted by source points.

3. Huygens principle sais that a large hole can be approximated by many small holes where each are a point source. The point source generating spherical waves is the source of diffraction.

4. There are different types of diffraction: -Fresnel diffraction -Fraunhofer diffraction For Fresnel diffraction the distance between the aperture and a screen is small.

5. For Fraunhofer diffraction there is a large distance between aperture and the screen. Fresnel and Fraunhofer diffraction:

7. The field generated by the source is propa- gating towards an aperture and there will be diffraction through the aperture. Irradiance: The figure shows a bending of light around a circular aperture:

10. We can see that the irradiance is higher when the opening is smaller and then that the irradiance is lower when the opening is larger. Fresnel diffraction at straight edge:

12. The intensity distribution shows that at the edge the intensity of the light is reduced to a quarter of the intensity and behind the edge it is falling monotonically to zero. Outside the intensity is increasing and it oscillates with increasing frequency.

13. Airy disk: This is a Fraunhofer diffraction.

14. At a circular aperture the intensity pattern is called an Airy disk. It is a ring system so that the plots are radial sections of a pattern possesing circular symmetry. When q = 1.22l/D ( q should equal the angle

15. theta and l the wavelength lambda) then the Airy disk has its first minimum. D is the diameter of the aperture.