# What is the explanation for bright lines in light-diffraction by a straight edge?

I’m just studying diffraction by a straight edge. I did not find any explanation for the origin of the bright lines l' and l". The line l' is visible in relation to the laser beam at an angle >30°, while the line l" is visible up to 180°. This can be seen if a circular screen is placed instead of a flat screen.

In this regard, I am interested in the following questions:

How is the origin of bright lines l' and l” interpreted?

Are they just a continuation of the diffraction pattern f or is l' created by diffraction and l" by the reflection of the incident light beam?

How is it that the line l" stretches (bends) at such a wide angle (180°)? Maybe partly diffraction and partly reflection?

What is their usual name? Can it be said that these are diffraction lines?

## 2 Answers

The incident EM wave has a plane wavefront and within the extent of the beam it $$induces$$ an essentially homogeneous current along the edge of the obstacle. That edge current in turn acting as an antenna radiates a cylindrical wave. The two waves, the incident plane wave and the indirectly induced cylindrical wave interfere; the result is that the field extends in the geometrical optics shadow with a fluctuating amplitude as is shown below in @Farcher's intensity diagram.

• I find that a great explanation I was looking for. However, the following is not clear to me: Since one part of the incident EM wave (usually about half of the cross section) continues to move in a straight line, does this mean that interference is possible only in a narrow area within the cross section of the incident beam (drawing - area m and in the field of view)? Everything else should not be caused by diffraction. What about the line l 'which is in the region of the optical shadow? The line l' must be caused by diffraction. – Spigel May 7 at 14:23
• If you a have a slit then "lines" in the geometric optical shadow along $\ell'$ and $\ell''$ result from being the sum of two edge waves interfering. For a single edge diffraction there is no such pair of "lines" instead you have a smoother shadow region as shown in Farcher's diagram to the left of edge, the ripple visible in the bright side, area "m" (to the right side of the edge) is the sum of the edge wave and the plane incident wave. – hyportnex May 7 at 15:35
• Should the range -4 to 0 on the diagram be l'? According to the diagram, the intensity in this area drops sharply to zero. In reality the length l 'is greater than 1.5m, when the distance between the flat edge and the screen is 3m. The intensity does not fall so sharply. The interference itself is barely visible and amounts to max. 1-3 cm on the right, outside the geometric shadow. This paper describes this phenomenon mathematically, but not a simple physical explanation. – Spigel May 7 at 17:14

The diagram is a bit misleading.

The intensity variation due to diffraction at an edge is as follows, and what you might see on a screen even with a laser pointer and a razor blade edge. The extended lines are light which has been reflected off the edge imperfections.