Diffraction of light is the bending of light waves around the edges of opaque objects and the penetration of light into the region of geometric shadow.
Diffraction of light is a characteristic of all waves and is observed in cases where:
- The size of the obstacle (or the openings through which the wave propagates) is comparable to the wavelength.
- The distance from the obstacle to the observation point is much greater than the size of the obstacle.
The condition for observing diffraction is given by:
r ≥ d2 / λ
Where:
r – distance from the screen to the obstacle.
d – size of the obstacle.
λ – wavelength.
If you pass a beam of light through the aperture AB from the source S, you will observe a light spot on the screen.
The diameter of the spot corresponds to the width of the light beam incident on the screen.
By decreasing the aperture AB, we will observe that the spot, or the pattern of light on the screen, becomes smaller, indicating a narrowing of the light beam.
However, starting from a certain size of the aperture, further reduction results in an increase in the size of the spot. At the same time, the spot loses its clarity, becomes more extended, and unevenly illuminated.
However, starting from a certain size of the aperture, further reduction results in an increase in the size of the spot. At the same time, the spot loses its clarity, becomes more extended, and unevenly illuminated.
French physicist Augustin-Jean Fresnel explained the presence of stripes on the screen by stating that the light waves arriving from different points converge into a single point on the screen and interfere with each other.
Huygens-Fresnel Principle
All secondary sources located on the surface of a wavefront are coherent with each other. The amplitude and phase of a wave at any point in space result from the interference of waves emitted by secondary sources.
The Huygens-Fresnel principle provides an explanation for the phenomenon of diffraction:
Secondary waves from points on the same wavefront (a wavefront is a multitude of points where oscillation has reached at a given moment) are coherent since all wavefront points oscillate with the same frequency and phase.
The diffraction phenomenon imposes limitations on the application of geometric optics laws:
The law of rectilinear propagation of light, as well as the laws of reflection and refraction, are quite accurate only when the dimensions of obstacles are significantly larger than the wavelength of light.