Close Double Source
Lesson notes
Overview
Sources close together create wide, widely spaced fringes. This topic is rated intermediate and is number 157 of 227 in Physics. Builds on the basics with relationships, methods and trade-offs.
What you are seeing
The panel above shows an interactive animation of the idea. Every slider changes one quantity so you can see its effect on the outcome. The animation is drawn live from the model, so everything you see responds to the sliders. Press Space to pause, use Restart to replay from the start, and change the speed to slow down quick events.
In depth
In two-source interference, the fringe spacing is inversely proportional to the separation of the sources, since maxima occur where d sin θ = mλ. If the sources are close together the angle between maxima is large, so the pattern is spread out into a few wide fringes. This is why in Young’s experiment the slits must be very close, often under a millimetre, to see broad fringes with visible light of tiny wavelength.
Reading the relationship
The expression "d small → wide fringes" is a compact summary of the idea. It uses the symbols d, s, m, a, l, w; name what each one stands for before you start. Then use the sliders to see how changing one quantity changes the others.
A guided exploration
1. Run the animation once with the default settings and describe in words what happens. 2. Change one slider at a time and note which quantity responds most. 3. Push each slider to its extreme and decide whether the result still makes sense.
Try this
Predict the outcome first, then test extreme slider values to find where the pattern changes or breaks.
Common pitfalls
Check units and directions first. Many mistakes come from mixing up a scalar with a vector, or from using a formula outside the conditions where it holds.
How it connects
It builds on “Source at Sound Speed”, the topic before it in Physics. Once you are comfortable, continue with “Wide Double Source”.
Why it matters
Physics explains how the world moves, heats, shines and interacts, and the same principles power technology around you.
Where you might use it
- Revising “Close Double Source” before a class, exam or certification
- Designing vehicles, bridges and devices where motion, forces or waves must be predicted
- Understanding how sensors, lasers and communication links actually work
- Explaining everyday phenomena such as sound, light, heat and orbits
Frequently asked questions
What is Close Double Source?
Sources close together create wide, widely spaced fringes. It is an intermediate topic in Physics.
How do I use the Close Double Source simulation?
The panel at the top shows an interactive animation of the idea. Every slider changes one quantity so you can see its effect on the outcome. Move the sliders one at a time, then press Space to pause the animation and look closer.
What does the formula mean?
The expression d small → wide fringes packs the idea into symbols. Name each symbol first, then change one quantity with the sliders and see which others respond.
Who is this topic for?
Builds on the basics with relationships, methods and trade-offs. If the quiz feels hard, reread the lesson notes and try the previous topic first.
What should I learn before this?
Start with “Source at Sound Speed”, the previous topic in Physics. Each topic in a subject builds on the ones before it.
How do I track my progress?
Press “Mark as learned” on this page. Progress is stored only in this browser and is never sent anywhere. The Browse page can show topics you have learned or still need to learn.