---
title: "Spontaneous Raman Spectrometer Interactive Lab | Sandbox Physics"
description: "Do molecular lines keep the same energy shift when the laser changes? Yes. A vibrational transition takes a fixed amount of photon energy, so changing the excitation moves the outgoing wavelength while preserving the Raman shift. Recovering it requires a calibrated spectral axis and enough resolution to separate nearby lines."
canonical_url: "https://sandboxphysics.com/labs/spontaneous-raman-spectrometer/"
language: "en"
page_type: "lab"
alternate_languages:
  zh-CN: "https://sandboxphysics.com/zh/labs/spontaneous-raman-spectrometer/"
last_modified: "2026-10-23"
---

B-X1 · Molecular energy from scattered light

# Spontaneous Raman Spectrometer

**Do molecular lines keep the same energy shift when the laser changes?** Yes. A vibrational transition takes a fixed amount of photon energy, so changing the excitation moves the outgoing wavelength while preserving the Raman shift. Recovering it requires a calibrated spectral axis and enough resolution to separate nearby lines.

Calibrate a spectrometer, acquire weak scattered light and compare two excitation wavelengths. Separate a close pair of synthetic vibrational modes from fluorescence and counting noise.

> Interactive simulation: open this page in a web browser to change the parameters and run the model.

This scalar teaching instrument assumes two nonresonant Stokes modes, a smooth fluorescence background and a Gaussian response. The mode positions and photon rates describe a synthetic material. They are not measured material data. The wavelength axis has an adjustable offset; a separately acquired reference at a known shift estimates that offset. Counts are seeded Poisson samples. The model omits polarization, resonance, damage, self-absorption, spatial mapping and anti-Stokes thermometry. Optical routing and magnification are schematic. Colors identify paths, including invisible infrared light.

Recorded measurements

## From photons to a Raman shift

Record a sample to view its measurement and analysis here.

Physical assumptions and sources

Two synthetic modes are broadened and integrated over physical wavelength-bin edges. A separate known reference estimates the wavelength offset. Background is estimated from edge bins, so broad backgrounds or unresolved overlap limit peak recovery. Intensities and widths are assumed; the model does not identify a real substance. Apparatus sizes and ray angles are schematic. Physical CCD columns show the selected counts with logarithmic brightness; moving light markers are path guides.

[HORIBA · MacroRAM educational laboratory](https://static.horiba.com/fileadmin/Horiba/Products/Scientific/Molecular_and_Microanalysis/MacroRAM_Educational_Labs/Experiment_1_Intro_to_Raman.pdf)
[NIST · spectral intensity calibration](https://www.nist.gov/programs-projects/relative-intensity-correction-standards-fluorescence-and-raman-spectroscopy)

## Read the experimental evidence

The simulation uses the assumptions stated above. Read how the historical apparatus measured the effect, what its records supported, and where the interpretation was limited.

[Raman scattering and molecular spectra](https://sandboxphysics.com/stories/probe-evidence/raman-shift-and-molecular-evidence/) [All six reading paths](https://sandboxphysics.com/stories/probe-evidence/)

Physics tutorial

## Read a molecular energy difference

**Background** A spectrograph records wavelengths. Molecular vibrations are compared by energy differences from the excitation light, so the laser wavelength belongs in the calibration.

**Why it matters** A peak that moves when the laser changes can still represent the same vibration. Compare the stored spectra in wavelength and Raman-shift coordinates.

Start with the essentials

- **Focus question**: Do molecular lines keep the same energy shift when the laser changes?
- **One-sentence intuition**: A fixed vibrational energy gives a fixed reciprocal-wavelength difference, not a fixed wavelength difference.

### Core mathematical model

#### Wavelength to Raman shift

$$
\Delta\widetilde\nu=10^7\left(\frac1{\lambda_0}-\frac1{\lambda_s}\right)\;\mathrm{cm}^{-1}
$$

Both wavelengths are inserted in nanometres. Positive shift means the photon left energy in the sample.

#### Energy transferred to the vibration

$$
\Delta E=hc(100\Delta\widetilde\nu)
$$

The factor of one hundred converts inverse centimetres into inverse metres. The mode energy is held fixed when the excitation changes.

#### Independent photon counts

$$
N_i\sim\operatorname{Poisson}(\mu_i),\qquad\operatorname{Var}N_i=\mu_i
$$

Each detector bin records its own signal and background photons. Subtracting an estimated background does not remove their counting noise.

### Common difficulties

#### A smooth spectrum can hide two lines

**Typical misconception** Every visible maximum is one molecular vibration, and smoothing can always recover a hidden pair.

**Better mental model** Finite response blends nearby lines. The analysis requires two separated maxima with a resolved valley; otherwise it reports the pair as unresolved.

### Run the experiment

1. 01

   #### Calibrate before acquiring the sample

   Use Resolve the pair and Acquire calibration reference. Acquire the sample, then analyze stored samples.

   **What to observe:** The known reference estimates the wavelength offset. The sample analysis uses the stored counts and that frozen calibration.

2. 02

   #### Change the laser

   Select Change excitation, calibrate the second laser, and acquire another sample. Switch the graph between wavelength and Raman shift.

   **What to observe:** The wavelength peaks move farther from the infrared source, but the recovered energy shifts agree within counting and calibration errors.

3. 03

   #### Test resolution and background

   Acquire the Blurred doublet and Strong fluorescence presets.

   **What to observe:** Broadening can merge the pair. More background increases fluctuations even when its smooth trend is subtracted.

Physical conclusion

Calibrate the axis before assigning a Raman shift, and keep blurred or noisy peaks unresolved.

Waves & optics · Starter path

## From one ray to interference measurement

Start with refraction, find where the ray picture stops working, then use interference to measure tiny distances.

[Start here → **Optical Boundary Explorer** Turn the incident angle and watch reflected and transmitted power trade off. Then switch to the critical-angle scene. Observe · 11 min](https://sandboxphysics.com/labs/optical-boundary-explorer/)

1. [1 Optical Boundary Explorer](https://sandboxphysics.com/labs/optical-boundary-explorer/)
2. [2 Rainbow Caustic Observatory](https://sandboxphysics.com/labs/rainbow-laboratory/)
3. [3 Camera Obscura](https://sandboxphysics.com/labs/camera-obscura/)
4. [4 Coherent Wave Composer](https://sandboxphysics.com/labs/coherent-wave-composer/)
5. [5 Young Double-Slit Bench](https://sandboxphysics.com/labs/young-double-slit-bench/)
6. [6 Michelson Precision Lab](https://sandboxphysics.com/labs/michelson-precision-lab/)

[View the full path →](https://sandboxphysics.com/topics/waves-and-optics/#starter-path)