Two-Slit Experiment: Understanding Waves
BY IKBAL AHMED
To read the first part of this experiment, go to page 1.
Materials Needed:
(1) Twenty-five 3 x 5 inches index cards (small index card), (2) seventeen 5 x 7 inches index cards (large index card), (3) narrow masking tape, (4) transparent tape, (5) blue and red marker pens, (6) scissors, (7) pencil, (8) optional: string.
Assembly:
Let’s start by creating our sine-wave template and making our cards with the waves drawn on them. Here’s how:
1. Take a small index card and fold it in half lengthwise, like you’re folding a hot dog bun. Then, unfold it.
2. Now, fold the card in half along its width, like you’re folding a hamburger bun. Keep this fold in place.
3. Next, fold the card in half again, but this time, don’t unfold it. You’ll see a long fold mark going down the middle and three shorter fold marks across the width of the card. It’s divided into eight equal parts.
4. On the middle fold, near the left edge of the card, make a mark about halfway down from the top. This is where our wave will start at its highest point.
5. On the bottom edge of the card, on the right-side fold, make another mark about halfway up from the bottom. This is where our wave will end at its lowest point.
6. Draw a wavy line starting from the left side, going up to the top mark, then down through the middle, and ending at the bottom mark on the right side. This is our sine wave.
7. Now, carefully cut along the wave you drew. The top and bottom halves will give you two sine-wave templates. Share one with a friend.
8. Place the template’s bottom along the bottom of a new card. Use a blue marker to draw along the edge of the wave. Do this on all twenty-four cards to make our blue sine wave set.
9. Flip both the template and the smaller cards over so the waves are on the back. Draw another sine wave that starts from the centre-left side and moves downwards. Make sure it matches the front wave.
10. Tape the smaller cards together into two rows of twelve, ensuring all waves move upwards from the left side and don’t overlap.
11. After finishing your two sets of blue sine-wave cards, follow all the steps above again with the larger cards to make two sets of red sine-wave cards with eight cards each.
Now, let’s make our model of two slits:
1. Tear off some long strips of masking tape. Stick them in a line on the floor, leaving two small openings about 25 centimetres apart. These are our slits.
2. Tear off another long strip of tape, about two meters long. Place it parallel to the first strip, about 55 centimetres away. This strip represents our screen.
To Do and Observe:
1. Compare the blue and red sine-wave cards. Take note of the difference in length between the blue and red waves. The red-light waves will typically be longer than the blue ones. You’ve effectively created a model showcasing the differences between red and blue light waves.
2. Examine any sine-wave card and pay attention to three crucial points: the maximum (the highest point), the zero crossings (where the wave intersects the middle line of the card, including at the leading and trailing edges), and the minimum (the lowest point). These points are described by scientists using the term “phase.” When two waves add up “out of phase,” it means the highest point of one wave aligns with the lowest point of the other, resulting in light cancellation. Conversely, when waves add up “in phase,” it means the highest point of one wave aligns with the highest point of the other, enhancing the light.
3. Consider a light source like a laser directed into the two slits. The waves entering the slits oscillate in phase, moving together.
4. Take one strip of blue sine-wave cards and align it with the centre of one slit. Do the same with the other strip of blue sine-wave cards, aligning it with the centre of the other slit. Ensure that both strips start in phase at the slits. Extend both strips towards the “screen” (second tape line), angling them so they intersect the screen simultaneously at a point opposite the midpoint between the two slits. Notice that both waves maintain the same phase on the screen. They combine in phase, resulting in a bright spot where they intersect on the screen. Mark this point on the tape with a blue marker.
5. Explore other points on the screen by extending the strips from each slit to intersect at different locations along the screen. Attempt to find a point where the two strips arrive out of phase. At this point, light cancellation occurs, producing a dark spot. Mark the intersection points on the tape with a black marker and draw a blue circle around it to indicate where the blue light cancels out.
6. Repeat the experiment using the red sine-wave cards. Observe and mark the locations of both the bright and dark regions on the screen.
7. Once you’ve investigated both the blue and red sine-wave cards, adjust the distance between the slits and repeat the experiment using fresh pieces of tape.
Instructions:
1. Compare the blue and red sine-wave cards, noticing wavelength differences.
2. Identify the maximum (highest point), zero crossings (where the wave crosses the middle line), and minimum (lowest point) on the cards.
3. Think of these points as phases. When waves add up “out of phase,” they cancel each other out. When they add up “in phase,” they strengthen each other.
Experiment:
1. Align one strip of blue sine-wave cards with each slit, ensuring they start in phase.
2. Extend the strips to the screen, observing where they overlap.
3. Find points where waves are out of phase. These spots will be dark.
4. Repeat with red sine-wave cards, noting differences due to their longer wavelength.
Explanation:
– Bright spots occur when waves add up in phase, creating a stronger light.
– Dark spots form when waves add out of phase, cancelling each other.
– The spacing between bright and dark regions depends on the wavelengths of light.
Going Further:
– Explore how changing the distance between slits affects the interference pattern.
– Understand that the spacing between bright spots is related to the wavelength and distance between slits.