Why every textbooks put the light source in the horizontal line?

[Twukwuw]

in the diffraction experiment, the light source and the slit are in the same horizonatal line.

My question is, if the light source is placed at another position which is no longer in the previous horizontal line, would we still get the same diffraction pattern?
Will the diffraction pattern shift?

Thanks,
Twukwuw.

 

[quantumdude]

in the diffraction experiment, the light source and the slit are in the same horizonatal line.

Well if you rotate your book 90 degrees then they'll be in the same vertical line instead.

My question is, if the light source is placed at another position which is no longer in the previous horizontal line, would we still get the same diffraction pattern?
Will the diffraction pattern shift?

No, you won't get the same pattern. If the beam comes in obliquely then the wavefronts in the beam all travel different distances by the time they hit the screen. The path length differences will result in phase differences.

Sending the beam in perpendicular to the screen ensures that all wavefronts travel the same distance to the screen, and hence are in phase. You have to have that in order to get the nice clean diffraction pattern that is described so well by the equations in your textbook.