Based directly on the provided information regarding Fresnel diffraction, zero lenses are used in front of the diffracting obstacle.
In the study of diffraction, the arrangement of light sources, obstacles, and screens plays a crucial role in determining the observed pattern. Different classes of diffraction are defined by these arrangements.
Lenses in Diffraction Setups
While various diffraction experiments might utilize lenses for specific purposes, such as focusing light or projecting patterns, the provided reference specifies a particular scenario: Fresnel diffraction.
According to the reference:
In Fresnel's class of diffraction the source of light and the screen on which the diffraction pattern in observed are at finite distance from diffracting obstacle. In this no lenses are used and the incident wavefrom is either spherical or cylindrical.
This statement is definitive for the specific conditions of Fresnel diffraction described. In this type, the characteristic setup involves the light source and the observation screen being relatively close to the obstacle, and the light waves spreading naturally (spherical or cylindrical wavefronts) without external focusing by lenses immediately before the diffracting element.
Key Characteristics of Fresnel Diffraction (Based on Reference)
- Source and Screen Distance: Finite distance from the diffracting obstacle.
- Lenses Used: None in front of the obstacle in this specific class.
- Incident Wavefront: Spherical or cylindrical.
This contrasts with other types of diffraction (like Fraunhofer diffraction), which often involve lenses to simulate infinite source/screen distances or to focus parallel light onto the obstacle. However, the question specifically asks based on the provided reference, which exclusively describes Fresnel diffraction and its setup regarding lenses.
Therefore, focusing solely on the information given, the exact answer for the described context is that no lenses are used.
