---
title: "Röntgen: what a shadow could establish"
description: "How did a glowing screen become evidence for a new radiation, and what did it leave unexplained?"
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language: "en"
page_type: "story"
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  zh-CN: "https://sandboxphysics.com/zh/stories/probe-evidence/rontgen-rays-and-shadow-evidence/"
last_modified: "2026-10-22"
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[X-rays: discovery, characteristic radiation and precision spectra](https://sandboxphysics.com/stories/probe-evidence/#x-rays)

Apparatus · Observation · Inference

# Röntgen: what a shadow could establish

Sandbox Physics · 2026-10-22

How did a glowing screen become evidence for a new radiation, and what did it leave unexplained?

1. Covered discharge tube
2. Test materials
3. Fluorescent screen or plate
4. Compare transmission and deflection

The diagram names the measurement sequence; it does not reproduce an exposure or specify a modern calibrated X-ray source.

## Start with the control, not the famous hand

Röntgen’s 1895 report describes a discharge tube enclosed in black cardboard. A nearby barium platinocyanide screen still glowed. The covering blocked ordinary light from the tube, so the observation demanded a different path from source to screen. Moving materials into that path turned a surprise into a sequence of comparisons.

The hand shadow was striking because bone and surrounding tissue attenuated the radiation differently. It did not by itself determine a wavelength or explain how the radiation was generated. The experimental discovery preceded both the 1901 prize and the later identification of the rays as short-wavelength electromagnetic radiation.

Sources: [1] [2]

## Keep thickness and composition apart

The report compares stacked foil, several metals and glasses with different lead content. Equal thickness did not give equal transmission. Röntgen also noted that density alone failed to explain all comparisons. These controls matter more than the modern shorthand that dense objects simply look dark.

He checked visual impressions with photographic plates whenever possible. A plate retains an accumulated response, whereas a fluorescent screen permits immediate inspection. Neither response is automatically a linear measure of incident intensity; exposure, detector response and background must be specified before comparing numbers.

Sources: [1]

## The magnetic comparison excluded a simple alternative

Cathode rays inside the tube could be deflected magnetically. The external X-rays showed no comparable deflection in Röntgen’s tests. Deflecting the cathode rays moved their impact point and therefore moved the apparent X-ray source. This linked production to the impact region without identifying the emitted radiation with the incoming electrons.

The original report contains unsuccessful refraction and interference searches and a conjecture about longitudinal ether waves. Preserve those uncertainties when reading it. A historical report is evidence of what was observed and argued at that stage, not a catalogue of conclusions that all survived.

Sources: [1]

## A modern attenuation law adds assumptions

For a narrow monoenergetic beam in a uniform absorber, neglecting photons scattered into the detector, transmitted intensity decreases exponentially. Here thickness is measured along the ray and the linear attenuation coefficient has inverse-length units. This is an explanatory model, not Röntgen’s fitted law.

A broad tube spectrum changes shape as it passes through matter. A single effective coefficient can then depend on thickness, and scattered radiation can fill the shadow. Radiographs integrate information along a path; they are not direct depth maps or chemical-state spectra.

$$
I(t)=I_0\exp(-\mu t)
$$

Sources: [1]

## Follow the next measurement

First read the covered-tube and material comparisons in sections 1–6 of the report, then the magnetic tests in sections 10–12. Ask which competing explanation each comparison restricts. The next article follows the different experiment that used a crystal to constrain wavelength and atomic arrangement.

Sources: [1]

## Explore the related models

Each link states what the existing model can explain and where the historical measurement goes further.

### [Compare emitted X-ray lines](https://sandboxphysics.com/labs/sem-microanalysis/)

Identify photon peaks from electron-excited material.

This is an emission-analysis model, not a reconstruction of the 1895 transmission experiment.

## Sources and reading notes

The original report is read in Barker’s translation. The exponential model below is a modern teaching assumption; no historical plate has been digitized or fitted.

1. [W. C. Röntgen, On a New Kind of Rays (1895), Barker translation (1899), §§1–17](https://web.mit.edu/redingtn/www/netadv/SP20140210.html)

   Primary report read in MIT-hosted transcription; modern commentary and illustrations on that page are separate from Röntgen’s report.

2. [Nobel Foundation, Physics Prize 1901: award motivation](https://www.nobelprize.org/prizes/physics/1901/summary/)

   Indexed official award metadata read for year, recipient and discovery attribution; this is not an experimental report.

[Probes, light and measurement: reading the evidence](https://sandboxphysics.com/stories/probe-evidence/) [Barkla: why the target material changed the radiation](https://sandboxphysics.com/stories/probe-evidence/barkla-characteristic-x-rays/) [Manne Siegbahn: precision changed the atomic diagram](https://sandboxphysics.com/stories/probe-evidence/manne-siegbahn-x-ray-lines/)