LIGHT LAB / EXPERIMENT 02
Read between
the colors.
A point of light can carry the signature of matter. Choose what emits it, put gas in its path, and watch the bright and missing wavelengths reveal a pattern.
Build a spectrumSOURCE / GAS / SPECTRUM
What reached the detector?
Start with hydrogen absorbing light from an idealized continuum. Then compare an emitting gas, combine fingerprints, or zoom in on sodium’s closely spaced pair.
The starting spectrum and line table are shown. Loading the controls…
Continuum light passes through selected foreground gas. Some wavelengths are reduced.
Optical arrangement is schematic. Colored spread is a wavelength guide, not a brightness prediction.The visible spectrum
380–750 nmThe curve carries intensity; the colored strip makes its distribution visible. Screen colors are schematic. Both plots use the same relative scale, recalculated when settings change.
LOOK CLOSER
One feature can hide another.
Choose a wavelength to inspect its relative intensity and the nearest selected line.
READ THE RESULT
Hydrogen absorbs selected wavelengths from the continuum. Dark lines are reduced arriving light, not black material on the star.
That setting is outside the model’s range. The last valid spectrum is retained.
THE WAVELENGTH REFERENCE
Match a pattern.
Keep the units.
Selected visible lines from NIST, using air wavelengths throughout. Hydrogen and helium features are rounded teaching markers; fine structure and many other lines are omitted. Select a line to center the inspection readout.
In continuous mode, the selected gas lines remain a reference only. Their presence in this table does not put them into the continuum.
| Atom | Feature | Air wavelength |
|---|---|---|
| H I | Hδ | |
| H I | Hγ | |
| H I | Hβ | |
| H I | Hα | |
| He I | He I | |
| He I | He I | |
| He I | He I | |
| He I | He I | |
| He I | He I | |
| Na I | D₂ | |
| Na I | D₁ |
No gases are selected. Choose a gas to show its reference lines.
FOLLOW THE LIGHT
Bright lines.
Missing lines.
Different questions.
The same transitions can leave opposite marks.
An excited, low-density gas can add light at particular wavelengths. Gas in front of a brighter continuum can remove light from selected parts of that continuum. Atomic energy levels set allowed transitions; temperature, density, ionization and viewing geometry determine which features appear and how strong they are.
A fingerprint needs several matches.
One feature rarely identifies a unique substance. Compare several wavelengths, the observing conditions and alternatives. This lab knows which gases you selected; it does not infer an unknown composition from data. Line contrast is an artistic control within the teaching model, not a calibrated abundance scale.
Resolution changes what can be separated.
Sodium’s two D lines are separated by 0.5974 nm. Choose Resolve sodium, then Blend sodium. The close-up shows their profiles merge when the instrumental width grows. The pair still exists even when it looks like one feature.
A continuum is another kind of clue.
The temperature slider changes an idealized Planck continuum. The graph is normalized, so it hides the change in total radiated power. The gas-line strengths do not respond to temperature here. A real stellar atmosphere requires excitation, ionization and radiative-transfer calculations that this experiment does not supply.
MODEL / ASSUMPTIONS
What the calculation includes.
Wavelengths, line shapes and resolving power
The visible teaching window is 380–750 nm in air. Selected hydrogen and helium features are rounded from NIST air-wavelength tables. Sodium uses 588.9950 and 589.5924 nm. Roman numeral I means a neutral atom. Intensities are illustrative and are not copied as physical source ratios from the NIST tables.
Emission lines use area-normalized Gaussian profiles. An illustrative intrinsic standard deviation of 0.08 nm combines in quadrature with a Gaussian instrumental standard deviation λ/(2.35482R). Here R = λ/FWHM for the instrumental profile. Broadening lowers a line’s peak while retaining its integrated emission strength.
Absorption uses a weak-line approximation: Gaussian deficits multiply a smooth continuum, with fixed equivalent-width scales as resolving power changes. It is not a treatment of saturated lines, overlapping optical depths, thermal broadening or a real spectrograph’s full response. The temperature control changes only the continuum; gas excitation is held fixed.
Continuum, color and what remains outside the model
The continuum follows Bλ(T) ∝ λ⁻⁵/[exp(hc/λkT) − 1], with wavelength converted to meters and normalized internally at 550 nm. The plots then share a displayed normalization across the full and detail views. The tiny air–vacuum distinction is ignored for this smooth continuum, so the result is not precision radiometry.
The colored strip uses schematic RGB colors and an enhanced brightness mapping. It does not reproduce calibrated human vision or a telescope image. Molecular bands, infrared and ultraviolet transitions, detector noise, dust, gas populations, Doppler shifts and cosmological redshift are omitted. A spectrum is not a direct distance measurement.
CHECK THE RECORD
From the display to the source.
NASA: Three types of spectra
Compare a continuous source, an excited gas and a continuum filtered through gas. The observing geometry matters alongside the atomic transitions.
NIST: Hydrogen line wavelengths
The selected Balmer features use rounded air wavelengths. Closely spaced fine-structure components are simplified into one teaching feature.
NIST: Helium line wavelengths
Selected neutral-helium features are rounded from the visible air-wavelength entries. This is a selection, not a complete helium spectrum.
NIST: Sodium persistent lines
The neutral-sodium D lines are 588.9950 and 589.5924 nm in air. Their separation is 0.5974 nm.
NIST: Wavelength and intensity conventions
Air and vacuum wavelengths must be distinguished. Relative spectral-line intensities depend on the source and excitation conditions.
NASA: Blackbody radiation
An ideal blackbody produces a continuum whose wavelength distribution depends on temperature. Real stars have atmospheres and depart from this idealized example.
NIST: Temperature conversion
Fahrenheit = 1.8 × kelvin − 459.67. Kelvin uses K without a degree sign. The continuum calculation retains its original kelvin input.
SCIENTIFIC INQUIRY / APPLIED HERE
Explore all twelve principlesA spectral pattern needs a physical model.
Selected atomic wavelengths are reference data; the continuum, line strengths and broadening are teaching assumptions. Compare several features and their observing conditions before assigning a composition. A missing line does not establish an absent element, and moving a control does not repeat a telescope observation.
Ask yourself Which assumptions drive this result, and where would the model stop being useful?