PhysicsChapter 9: Waves
Interferometer
Explanation level
Meaning and purpose
An interferometer is an instrument that uses the interference of two or more light waves to create a measurable interference pattern. The pattern contains information about the object or phenomenon being studied.
Interferometers are used in science and engineering because they can measure changes that are too small for ordinary measuring instruments. This sensitivity makes them suitable for gravitational-wave detection.
Main parts of a laser interferometer
| Part | Function |
|---|---|
| Laser | Produces a stable, coherent beam of light. |
| Beam splitter | Divides the incoming beam and sends light along two perpendicular arms. |
| Perpendicular arms | Provide the two paths whose lengths are compared. |
| End mirrors | Reflect the light back toward the beam splitter. |
| Photodetector | Measures the intensity of the recombined light and records changes in the interference signal. |
Working principle
- A coherent laser beam reaches the beam splitter.
- The beam splitter sends one part along each perpendicular arm.
- Mirrors at the ends reflect both beams back.
- The returning beams meet again at the beam splitter.
- Their phase relationship determines whether they combine constructively, destructively or partially.
- A detector monitors the resulting intensity or fringe pattern.
If both optical paths remain unchanged, the interference signal remains stable. A tiny change in one path changes the relative phase of the returning waves and produces a measurable signal change.
- \Delta\phi
- phase difference between the recombined beams (rad)
- \delta
- optical path difference (m)
- \lambda
- wavelength of the laser light (m)
When to use: Use this relation to connect a change in optical path with the resulting phase change.
LIGO interferometers
LIGO means Laser Interferometer Gravitational-Wave Observatory. It is a large-scale physics experiment and observatory built to detect gravitational waves and use them as an astronomical tool.
LIGO uses two instruments called interferometers. Each instrument has two perpendicular arms, each 4 km long, arranged in the shape of the letter L.
- Lx, Ly
- lengths of the two perpendicular arms (m)
Effect of a gravitational wave
A gravitational wave changes space itself. As it passes through an interferometer, space stretches along one arm and simultaneously compresses along the perpendicular arm.
A moment later the effect reverses: the first arm becomes shorter and the second becomes longer. This alternating difference continues while the gravitational wave passes.
- h
- gravitational-wave strain (dimensionless)
- \Delta L
- change in arm length (m)
- L
- original arm length (m)
The changing arm lengths alter the travel distances of the laser beams. When the beams recombine, their changing phase difference changes the detected interference intensity. Analysis of this signal reveals the gravitational wave.
Detection sequence
Observation: The recombined laser intensity changes in a characteristic alternating pattern.
Experiment: Send laser beams through two perpendicular 4 km arms and continuously compare their returned phases.
Result: One arm behaves as if it is slightly longer while the other behaves as if it is slightly shorter, followed by the reverse change.
Conclusion: A passing gravitational wave has produced differential stretching and compression of space.
Extraordinary sensitivity
The textbook states that LIGO is designed to measure a distance as small as one ten-thousandth of the width of a proton.
- \Delta L
- small measurable distance change (m)
- d_p
- width of a proton (m)
This comparison shows why laser interference is essential: the required length change is vastly smaller than anything that can be read with an ordinary scale.
Interference and gravitational-wave detection
| Stage | Physical change | Measured result |
|---|---|---|
| No gravitational wave | Arm lengths and optical paths are stable | Stable interference signal |
| Wave begins to pass | One arm stretches and the other compresses | Relative phase changes |
| Wave continues | Stretching and compression reverse | Interference intensity changes back and forth |
| Data analysis | Recorded pattern is compared across detectors | A gravitational-wave signal can be identified |
Opposite arm changes
Situation: The x-arm becomes slightly longer while the y-arm becomes slightly shorter.
Explanation: The two laser beams no longer return with exactly the same relative phase. Their recombination changes the detector intensity. When the arm changes reverse, the intensity changes correspondingly.
Exam-ready explanation
- Define an interferometer as a device that uses light interference for precise measurements.
- Name the laser, beam splitter, perpendicular arms, mirrors and detector.
- Explain splitting, reflection and recombination of the laser beams.
- State that each LIGO arm is 4 km long and that the arms form an L shape.
- Explain that a gravitational wave lengthens one arm and shortens the other, then reverses the changes.
- Connect the changing path difference to a changing interference signal.
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Exam preparation
Exam Focus
Important points
- Definition and interference principle.
- Laser, beam splitter, arms, mirrors and detector.
- Two perpendicular 4 km LIGO arms.
- Opposite stretching and compression caused by a gravitational wave.
- Changing phase and interference intensity as the detected signal.
Common mistakes
- Saying an interferometer produces gravitational waves instead of detecting their effects.
- Forgetting that the two LIGO arms are perpendicular and form an L shape.
- Saying both arms always change length in the same direction.
- Treating the interference pattern as decoration rather than the measurable signal.
- Writing one ten-thousandth as 110,000 in running text instead of using a proper fraction.
Quick revision
Revision Zone
Review the key facts, formulas and checks before you move on.Interferometer
Sticky Notes — Quick Revision
Key Point 1
- An interferometer uses interference between two or more light waves to make precise measurements.
Key Point 2
- A beam splitter sends laser light along two perpendicular paths called arms.
Key Point 3
- Each LIGO interferometer has two perpendicular arms, each 4 km long, arranged in an L shape.
Key Point 4
- A gravitational wave stretches one arm while compressing the perpendicular arm, and then reverses these changes.
Ready to test yourself?
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- 8 concept questions
- Easy → Medium → Hard
- Explanation after each answer
Exam Practice
Important Question
Q1. Explain the use of an interferometer in detecting gravitational waves.
Short Question
Q2. What is an interferometer and why is it useful?
Short Question
Q3. Describe the basic structure of a LIGO interferometer.
Reasoning
Q4. Why does a gravitational wave change the interference signal?
Formula
Q5. Write the strain and phase-difference relations used to describe interferometer measurements.
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