Understanding the Role of Convex Lenses in Fraunhofer Diffraction

In summary, the conversation discusses the placement of a monochromatic light source at the principal focus of a convex lens, which results in the formation of a parallel beam of light or a plane wave front incident on a narrow slit. The teacher demonstrated the diffraction of the light rays passing through the slit and showed how they converge at a point, contradicting the expected behavior of the second convex lens which appears to be diverging the rays instead of converging them. The confusion arises as most books support the teacher's explanation, but the image clearly shows the left lens converging and the right lens diverging the rays. The conversation ends with further clarification that the parallel rays are indeed converging and the teacher's demonstration was not wrong.
  • #1
Prashasti
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Homework Statement


A monochromatic source of light 'S' is placed at the principal focus of a convex lens 'L1'. A parallel beam of light or a plane wave front is incident on the narrow slit represented by AB (its width - 'a')

Homework Equations


So, what my teacher did was,
She showed diffraction of the light rays as they pass through the slit.
And then she made the rays to meet at a point 'P'.
Look at the image. Doesn't the convex lens L2 seem to be actually 'diverging' the rays? Shouldn't it 'converge' them?
I'm literally confused about it, because all the books seem to support my teacher.
singleslitexpt.png
 
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  • #2
Prashasti said:
Doesn't the convex lens L2 seem to be actually 'diverging' the rays? Shouldn't it 'converge' them?
Looks like it's converging to me. In this image, the left lens is converging and the right lens is diverging.

foclen.gif

Image compliments of http://hyperphysics.phy-astr.gsu.edu/hbase/geoopt/foclen.html
 
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  • #3
Yes the parallel rays are converging. They were parallel and are now drawing together.
 
  • #4
Does it mean why my teacher did was wrong?
 
  • #5
Jilang said:
Yes the parallel rays are converging. They were parallel and are now drawing together.
I 'm talking about the second lens...
 

1. What is Fraunhofer diffraction?

Fraunhofer diffraction is a phenomenon in which light waves passing through a narrow slit or aperture are diffracted, or bent, resulting in a diffraction pattern. This effect was first observed by German physicist Joseph von Fraunhofer in the early 19th century.

2. How is Fraunhofer diffraction different from other types of diffraction?

Fraunhofer diffraction occurs when the diffracting object (such as a slit or aperture) and the observer are at a large distance from each other, resulting in a parallel beam of light. This is in contrast to other types of diffraction, such as Fresnel diffraction, where the observer is closer to the diffracting object and the resulting diffraction pattern is curved.

3. What factors affect the diffraction pattern in Fraunhofer diffraction?

The diffraction pattern in Fraunhofer diffraction is affected by the wavelength of the light, the size and shape of the diffracting object, and the distance between the object and the observer. Additionally, the intensity and polarization of the light can also affect the diffraction pattern.

4. How is Fraunhofer diffraction used in practical applications?

Fraunhofer diffraction is commonly used in various fields such as optics, astronomy, and spectroscopy. It is used to study the properties of light, measure the size and shape of small objects, and analyze the chemical composition of materials through spectroscopy.

5. What is the significance of Fraunhofer diffraction in the field of optics?

Fraunhofer diffraction is significant in optics because it helps us understand the behavior of light as it passes through small openings or around small objects. This phenomenon has also led to the development of various optical instruments and techniques used in modern optics, making it a fundamental concept in the field.

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