---
title: "Electronic Spectra, Franck-Condon and the Origin of Colour"
description: "An electronic transition happens far faster than the nuclei can move, so the molecule lands on the upper surface at the geometry it already had. That single fact explains band shapes, vibrational stru"
canonical: https://lightmysky.com/learn/science/electronic-spectra-franck-condon-and-the-origin-of-colour-mt_y1hKy9nvAY
source: https://lightmysky.com/learn/science/electronic-spectra-franck-condon-and-the-origin-of-colour-mt_y1hKy9nvAY.md
retrieved: 2026-09-12
---

> **Agent view.** This is the Markdown twin of the page, for tools and assistants.
> When to use this site, and the call that answers each job: https://lightmysky.com/agent-instructions.md
> API description (OpenAPI 3.1): https://lightmysky.com/openapi.json · Authentication: https://lightmysky.com/auth.md
> Pricing: https://lightmysky.com/pricing.md · Catalog: https://lightmysky.com/llms.txt · Full catalog: https://lightmysky.com/llms-full.txt
> Every machine-readable file on this domain: https://lightmysky.com/.well-known/ai-catalog.json
> Ask for Markdown with `Accept: text/markdown`, a `.md` address, or `?mode=agent`.

# Electronic Spectra, Franck-Condon and the Origin of Colour

An electronic transition happens far faster than the nuclei can move, so the molecule lands on the upper surface at the geometry it already had. That single fact explains band shapes, vibrational structure and why colour is a molecular property.

Subject: Science · Area: Chemistry · Ages 19 to 20
Page: https://lightmysky.com/learn/science/electronic-spectra-franck-condon-and-the-origin-of-colour-mt_y1hKy9nvAY

## Ready when they can

- States the Franck-Condon principle and draws the vertical transition on two potential energy curves
- Explains why an electronic band is broad and structured rather than a single line
- Links extent of conjugation to absorption wavelength and predicts the direction of a shift
- Names the transition types available to a carbonyl compound and ranks their intensities

## Lesson: Why molecules wear colour

An electronic jump happens far faster than the nuclei can move. The electrons rearrange thousands of times faster, so the nuclei stand still while the electron leaps. Draw two potential curves and drop a vertical arrow between them. That vertical arrow is the Franck Condon principle, landing on whichever vibration overlaps best.

Atoms give sharp lines, but molecules give broad structured bands. Each electronic jump carries an entourage of vibrational and rotational sub jumps at slightly different energies. What would be one line for an atom spreads into a band system for a molecule. The brightest peak is the landing with the biggest vibrational overlap.

**Example.** Take two dyes that differ by one extra double bond in a row. The more conjugated dye absorbs at longer wavelength. Longer conjugation narrows the gap, so the photon costs less. That is why most dyes are highly conjugated. Conjugation squeezes the gap into the visible range, so the molecule drinks in visible colours instead of only ultraviolet.

Sort carbonyl transitions by intensity. The pi to pi* jump is strong and allowed, sitting deeper in the ultraviolet. The n to pi* jump is weak because symmetry blocks it, sitting nearer the visible. Strong field ligands widen the split in metal complexes and push absorption toward the blue, while weak field ligands shrink it toward the red.

**Recap.** Electrons leap vertically while nuclei stand still, and the gap they jump sets the colour you see.

## Practice

8 questions on this page, each with its working shown.

## Needs first

- [Reading Bond Lengths and Force Constants from Spectra](https://lightmysky.com/learn/science/reading-bond-lengths-and-force-constants-from-spectra-mt_5m7J1mFw68)

## Opens up

- [What an Excited Molecule Does Next: Fluorescence and Phosphorescence](https://lightmysky.com/learn/science/what-an-excited-molecule-does-next-fluorescence-and-phosphorescence-mt_5AVF6uWed_)
- [Excited States by Calculation and Simulated Electronic Spectra](https://lightmysky.com/learn/science/excited-states-by-calculation-and-simulated-electronic-spectra-mt_DsZ3wfCRkX)
- [Choosing Between Vibrational Methods: Infrared, Raman and Resonance Raman](https://lightmysky.com/learn/science/choosing-between-vibrational-methods-infrared-raman-and-resonance-raman-mt_YxLAvBIBAR)