Witness a classic experiment using a laser pointer to create a fascinating pattern of light and dark.
BY IKBAL AHMED
To read the second part of this experiment, go to page 2.
Materials needed:
(1) Laser pointer, (2) pet comb (or an eyelash comb with closely spaced teeth), (3) black tape, (4) white screen or an index card (5 x 7 inches), (5) two large binder clips (1 inch wide, 2 inches long), (6) two medium binder clips (1/2 inch wide, 1 inch long), and (7) single-edge razor blade or straight-edge knife.
Note on Laser: Be careful when using the laser source, it should not directly hit your eyes.
Assembly:
1. Cover all teeth of the lice comb with black tape, leaving only two slits between adjacent teeth exposed.
2. Insert the comb’s handle into a large binder clip, placing it on a flat surface so the teeth are vertical.
3. Attach the two medium binder clips to the barrel of the laser pointer, positioning them for the laser to rest horizontally.
4. Use the remaining large binder clip to create a stand for the index card.
5. Align the laser pointer and comb so the beam passes through the two slits onto the white screen, ensuring the screen is at least 4 feet away.
Instructions:
1. Observe the pattern on the screen as light passes through the two slits. Notice alternating dark and light regions.
2. Block one of the slits with the razor blade and observe changes in the pattern. Try blocking the other slit.
3. Remove the razor blade and notice the reappearance of dark bands when light passes through both slits.
Explanation
Imagine you’re standing by the shore, watching waves roll in from two different directions. When the waves meet, sometimes they combine to make bigger waves, and sometimes they cancel each other out. This is like what happens when light waves from two slits overlap on a screen.
When light passes through a single slit, it spreads out like ripples in a pond, creating a pattern of bright and dark bands on the screen. But when light passes through two slits, something magical happens. The waves from each slit overlap and interact. Where the waves line up perfectly, they add up to make brighter spots on the screen. But where the peaks of one wave meet the troughs of another, they cancel out, creating dark spots.
This phenomenon, called interference, is a key piece of evidence that supports the idea that light behaves like a wave. Just like waves in water, light waves can interfere with each other, creating patterns of light and dark.
Now, let’s talk about the particle nature of light. Imagine playing with a handful of tiny balls, like marbles. You can bounce them off walls and see them travel in straight lines. Light can sometimes behave like these tiny balls too. When you shine a light on a mirror, it bounces off just like a ball bouncing off a wall.
So, depending on the situation, light can act like waves and particles. This idea might seem strange, but it’s one of the amazing things about physics!
In our experiment, when light passes through a single slit, it diffracts and spreads, creating a predictable pattern on the screen. Passing light through two slits produces new dark regions due to interference. Interference occurs when light waves from different slits overlap on the screen. Where wave crests align, brightness increases, forming light regions. Where a crest meets a trough, they cancel out, forming dark regions. The appearance of dark bands supports the wave theory of light, which was demonstrated by Thomas Young in 1801.