Diffraction Caused by a Single Slit

We are all familiar with the Rectilinear propagation of light taught to us in early grades. The diffraction phenomenon is the bending and twisting of light around the corner of an object. When light falls incidentally on its sharp edge, a faint illumination is seen inside the geometrical shadow of the thing, indicating that light did bend around that corner. When a ray of light falls incidentally on a slit of the same size as light’s wavelength, one may notice a bright and dark alternating pattern. This is diffraction due to a single slit.

Explanation of the Single Slit Phenomenon

According to Huygen’s principle, each portion of the wavefront in the slit emits wavelengths. These look like rays that start in phases heading in many directions. Each ray is perpendicular to the wavefront of a wavelength. 

Suppose that, compared to the slit’s size, the screen is at a far distance. Thus, the rays headed towards a common destination will be parallel. They stay in-phase when they travel straight ahead, and the central maximum is observed.

However, suppose the rays travel at an angle θ relative to the original beam’s direction. In that case, each ray travels a different distance to reach the screen and thus can be in or out of the phase.

The conditions for diffraction are:

• The incidental light has to be monochromatic.

• The slit width must be comparable to the incident light’s wavelength.

Diffraction Formula

The formula for diffraction due to a single slit is:

mλ = Dsinθ

Where,

• m = Order of the minimum, represented by ±1, ±2, ±3, …

• λ= The light’s wavelength

• D = Slit width

• θ= Angle relative to the light’s initial direction

Young’s Single Slit Experiment

Thomas Young was an English physicist who posited that light was made of waves. His experiment showed that interference might occur at the intersection of two light waves. 

In Thomas Young’s experiment of a single slit, self-coloured or monochromatic light passes through a slit of limited breadth to produce a similar design on the face of the screen. The breadth and intensity in the single slit pattern reduce on moving away from the central maximum, which is not the case in the diffraction pattern of a double-slit.

Here are the steps for Young’s experiment of a single slit:

• Coherent light is produced when the sun’s rays are passed through a single slit. 

• This coherent light is projected on a second screen with twin slits, which helps in the diffraction process of the incident illumination when it gets through. 

• The interference between the light beams that get diffracted is seen as distributions of light intensity on a dark film. 

Young saw that when the slits were huge, placed far from each other and closer to the screen, there was the formation of two overlapping light patches on the screen. But when the size of the slits was reduced and placed closer to each other, the light passing from the slits and on the screen created prominent bands of colour separated by darker regions or phases serially. 

So, the concept of interference fringes was created. The only way the coloured bands could be created was if light behaved as a wave undergoing constructive or destructive interference. 

Conclusion

Diffraction of light is the bending of light around corners so that it spreads out, illuminating areas where shadows were supposed to form if light were to behave rectilinearly.

It is challenging to separate diffraction from interference; these processes occur together. We sometimes see a silver lining in the sky because of the diffraction of light. 

In single slit diffraction, one can witness the bending of light or diffraction phenomenon. This process causes light from a coherent source to interfere with itself, leading to the production of a distinct design on the screen called a diffraction pattern. This diffraction process is visible when the sources are equivalent to the wavelength of light incident upon the slit.