This demonstration requires no material other than your own fingers. Hold your index and middle fingers close to each other, leaving a small slit between them about 1 mm in width.
Look through the slit into a source of light such as the window or a lamp. You will need to look with one eye up close to the slit. Warning: do not look directly at the sun.
You will be able to see a number of vertical dark lines between the fingers.
So where do these vertical lines come from? They are dark fringes caused by destructive interference of light when it diffracts through your finger tips.
This phenomenon can be explained using Huygens' principle. Huygens pictures every point on a primary wavefront as a source of secondary wavelets and the sum of these secondary waves determines the form of the wave at any subsequent time. Hence, each of these secondary wavelets can interference with one another.
Constructive interference takes place when the difference in path lengths between two coherent waves is an integer multiple of the wavelength. This is when the resultant wave is the brightest. Destructive interference occurs when that difference in path length is a half-integer of the wavelength (e.g. , , , etc.) and gives a dark fringe.
The alternating bright and dark fringes is a diffraction pattern, which becomes observable by the eye looking through the slit.