Difference between interference and diffraction?

  1. Hi, I'm currently taking a Wave and modern physics class and we recently finished two chapt: interference and diffraction.

    I'm having a hard time grasping the difference. I understand that diffraction is the expansion of light when it goes through a small opening ( it "flares" out contrarily to geometrical optics). I also understand that interference is when two light waves "meet" and "create a new wave" ( in simple terms I guess). I get stuck at the part where my book says : "when monochromatic light from a distant source (or a laser) passes through a narrow slit and is then intercepted by a viewing screen, the light produces on the screen a diffraction pattern"

    How can diffraction create a pattern on a screen? We haven't spoken of Hyugen princple in class but I know its due to that. Only that all points on a wavefront serve as point source of spherical secondary wavelet. Really I don't understand how that helps me with diffraction :/

    I'm sorry this may sound really dumb and confusing but I hate solving problems without understanding what I am doing

    Thank you for anybody who take time to answer and I'm sorry if this is in the wrong section.
  2. jcsd
  3. Jano L.

    Jano L. 1,295
    Gold Member

    It is not very clear what is it you are asking. The diffraction pattern is a name for the system of fringes on the screen. This pattern is due to particular spatial distribution of light - diffracted light. This diffracted light itself is not simply plane wave because of the opaque screen with slit, containing electrons that scatter the primary light wave. The primary and secondary waves sum up (interfere) to produce the resulting diffracted light wave. So you can say that he relation between the diffraction and interference is that diffracted light has its explanation in interference of myriads of elementary waves, due to electrons in source and the opaque screen with the slit.
  4. My apologies for the unclear question. I guess the topic is very unclear to me. Are you saying that the wavefronts shift to an out of phase pattern to create the interference?
  5. Jano L.

    Jano L. 1,295
    Gold Member

    I do not understand "wavefronts shift to an out of phase pattern to create the interference?"
  6. Ugh sorry :/ I mean, when the light waves goes through the slit, how does it create interference shouldn't it just continue in the same direction ( so straight forward ) through the slit to just create a big light spot on the screen? I don't understand how we can observer destructive interference on a screen when only diffraction occurs with one slit. :/ I'm really sorry for my poor wording this isn't my first language.
  7. Jano L.

    Jano L. 1,295
    Gold Member

    The original wave indeed continues in the same direction, and would produce just a big light spot if it was the only wave there. But when this wave strucks the obstacle, it makes the electrons in it vibrate and produce secondary waves of the same frequency. These new waves have different points of origin - the electrons in the obstacle - and propagate to all directions (spherical elementary wave). What we observe at the screen is the action of the sum of the primary and many secondary waves; this sum is not plane wave any more, because the elementary waves from the electrons in the obstacle are not planar, but spherical waves.
  8. Ah, I think I understand, when the light hits the slit, it creates different waves that interfere together. So the path difference when reaching the screen can create a dark fringe. Thanks a lot ! You helped me a lot
  9. Jano L.

    Jano L. 1,295
    Gold Member

    You're welcome. In order to avoid confusion, I should add that this explanation is not based on Huyghens principle or its derivatives. The only sources of the light waves in the above picture are the primary source of light and the particles in the obstacle.
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