Interference fringes

I was in Kota Kinabalu last week for a week long conference of 785 school principals from all over Malaysia. The highlight of the conference for me was the talk by Datuk Wira Ameer Mydin ( the Mydin shops CEO) and a most wonderful bench marking visit to SM La Salle, Kota Kinabalu.

 I was fortunate my hotel room had a window view of the sea and I watched several beautiful  sunsets through my window.

The view while being beautiful reminded me of a SPM exam question.
"What is the colour of the sky above you at sunset? 
What is the colour of the sky in front of you at sunset? 
Why do the colours of the sky differ at different positions and at different times. 
Give your explanation with the help of labelled diagrams.

Then when sunset was almost over, I drew my lace curtains and caught this view.

I wonder if this picture causes you to think of anything in Physics. Can you see wavy bright and dark lines in the sections where the curtain folds and pleats are? What do you think these wavy lines are?

They are actually interference fringes, the bright lines being constructive interference and the dark lines being the destructive interference. For interference to occur,there must be 2 coherent sources. These two coherent sources are in the top fold of the curtain and the bottom fold of the curtains. Notice you only see the light fringes where the two different curtains pleats or folds are located - one big fold near the center and one smaller fold near the side.

I remember the first time I visited Pulau Redang almost 23 years ago in 1990. During that trip, there were 2 Physics teachers among many English teachers. Suddenly we two Physics teachers saw the interference fringes on the seabed!! The white sandy sea bed was a perfect screen for interference fringes to show up. Notice the wavy fringes below the sea turtle. I wonder if you can identify the very bright points and very dark points of constructive interference and  the semi bright dark lines of destructive interference.

Suddenly there,  we two Physics teachers realized the ripple tank experiments are actually simulations of the water waves at the sea. We got so excited and jumped up and down in the sea and laughed for joy in the hot sun with the English teachers thinking we two had gone crazy.  You see, it was only then  we realized the lamp at 50 cm height above the ripple tank represent the distant sun which give parallel rays. The waves on the sea are represented by the waves we create in the ripple tank. The white screen is inspired by the white seabed.

Recently while I was in Putrajaya for the hot air balloon fiesta, I bought the giant Stinson Soap Bubble Kit and we played giant soap bubbles both in the Form 5 and Form 3 motivational slot in school. I first saw these giant soap bubbles in San Francisco . Children are always fascinated by soap bubbles whether they be big or small and in this pic, you see children running to play with the soap bubble. But do you see the colorful wavy interference fringes in the soap bubble? Interference occur because the two coherent sources are the top of the soap film and the bottom of the soap film in the bubble.

Just like the curtains, the two coherent sources are very near to each other.  This is why the distance between the two slits in the Young double slits are so close. We had fun doing the Young double slit experiment with the green laser and red laser lamps ( RMB 30/- ) I bought from Shanghai but sorry we forgot to take photos. Next time ....!

Enjoy Physics - it add beauty and wonder to life.