SINGLE SLIT DIFFRACTION USING RAZOR BLADES

In the NCERT  Physics textbook of class 12,there was only one activity the writers asked us to undertake and it quite caught my attention,it was written that the diffraction pattern that we study about could be observed on something as simple as in the slit between two razor blades.

So I took two razor blades and shone a laser pointer through the slits between them.,It did give a fair diffraction pattern.and the intensity of the maximums were as predicted with a central bright fringe, and intensity wearing off with a distance.

I used a laser pointer with wavelength of 650 nm incident on the single slit.If the slit size is estimated to be 0.1mm,and the distance of slit and screen was 12 cm,the first minima should occur at 0.78mm,which is quite close to the distance of first minima obtained(approx. 1 mm)

It was exciting to achieve the result using a self made apparatus.as it is my first attempt on my own most probably.

CONIC SECTION OBTAINED IN PENUMBRA

So,one day while playing around with a small torch in a dark room,I suddenly observed something.On projecting the light against the wall,the shadow(penumbra)forms a parabolic shape.Then when I started moving it by along the wall,the shadow started changing shapes:

• When the rays are perpendicular,the penumbra is absolutely circular.
  

• On tilting the torch by a bit,the penumbra becomes an ellipse,whose dimensions change with more tilting.
  
  
  
  
• At one point,the closed nature of the penumbra is lost and it forms a parabola,the boundary seems to become horizontal at infinity(extreme condition).
  
• On tilting further it takes a hyperbolic shape.
  
• And when almost parallel to the wall,it forms a pair of straight line(extreme condition).
  

This phenomenon could be explained by considering the ray of light to be one nappe of the circular cone,and the wall to be the plane whose intersection with the wall gives the the shadow in form of the various conic sections.

photo:NCERT Maths textbook.

P.S. Later a google search revealed that this is a well known phenomena,must we say and I am not the one to first observe it.