Young's double slit: why fringes gets dimmer and inconsistent?

In summary: This means that the "interference pattern" is basically the "diffraction pattern" modulated by a sinc function. This is a bit like the "envelope" of a radio signal, and shows that the interference does indeed modulate the diffraction.In summary, the conversation discussed two questions about Young's double slit experiment. The first question was about the dimming of the bright fringes as they move away from the center, and it was explained that this is due to the amplitude decreasing as it moves further from the center. The second question was about the inconsistency between the formula for double slit interference and the actual pattern observed, and it was explained that this is due to the small angle approximation used to derive the formula. Overall,
  • #1
Simon George
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Hi all!

In Young's double slit experiment, there are two things I cannot explain. Any help is appreciated!

The first one is why the bright fringes get dimmer as you get further from the central/brightest spot. My theory, after looking in the two books I have, is that each single slit decreases the amplitude the further it goes from the center. I find this explanation kind of "too easy" and simple. Is that just it? and does that relate to any equation of intensity for double slit?

The second one is why the bright fringes get inconsistent with the formula for double slit (dsin(theta)=m*walength) as we get further form the center. For that, I have absolutely no idea and can't seem to find the answer anywhere.

Thank you!
 
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  • #2
Search about double slit interference in khan academy.They have explained it well very clearly
 
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harambe said:
Search about double slit interference in khan academy.They have explained it well very clearly
Thank you! Still don't get it why it gets dimmer. It seems vague
 
  • #5
Simon George said:
My theory, after looking in the two books I have, is that each single slit decreases the amplitude the further it goes from the center.
That's basically it. Do a Google search for "single slit diffraction" and you'll find many graphs of the intensity pattern for a single slit. For double slits (try searching for "double slit diffraction") the basic two-slit interference pattern "modulates" the single-slit pattern by "chopping it up" into narrower maxima. The "interference maxima" are narrower than the "diffraction maxima" because the distance between the slits is (normally) much larger than the width of each slit.
 

1. Why do the fringes become dimmer in Young's double slit experiment?

The fringes in Young's double slit experiment become dimmer because of interference patterns. When two coherent light waves from the two slits overlap, they can either reinforce or cancel each other out, resulting in areas of bright and dark fringes. As the distance between the slits increases, the overlapping area decreases, causing the fringes to become dimmer.

2. Why do the fringes appear inconsistent in Young's double slit experiment?

The fringes in Young's double slit experiment may appear inconsistent due to various factors such as the distance between the slits, the wavelength of light, and the distance from the slits to the screen. These factors can affect the interference patterns and result in variations in the brightness and location of the fringes.

3. Can changing the distance between the slits affect the brightness of the fringes in Young's double slit experiment?

Yes, changing the distance between the slits can affect the brightness of the fringes in Young's double slit experiment. As the distance between the slits increases, the overlapping area decreases, resulting in dimmer fringes. Conversely, decreasing the distance between the slits can make the fringes brighter.

4. How does the wavelength of light affect the fringes in Young's double slit experiment?

The wavelength of light can affect the fringes in Young's double slit experiment by determining the spacing of the fringes. The distance between two adjacent fringes is inversely proportional to the wavelength of light. Therefore, changing the wavelength can change the spacing of the fringes and make them appear closer or further apart.

5. Why do we use coherent light sources in Young's double slit experiment?

We use coherent light sources in Young's double slit experiment because they produce light waves with a constant phase difference. This allows for consistent and predictable interference patterns, making it easier to observe and measure the fringes. Incoherent light sources, on the other hand, produce light waves with random phase differences, causing inconsistent and unpredictable interference patterns.

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