Activity on X-rays and Ultraviolet Light

Activity 11

X-rays, Ultraviolet Light, and Infrared

Introduction

What exactly happens when the doctor "takes an X-ray" of our teeth or a broken bone? What are the X-rays? Can they harm us? Why can they "see" through bones and tissue? Have you ever held a blacklight up to a scorpion? What are "blacklights"? How is infrared (IR) radiation used in analysis of paintings?

Procedure

These activities are more exploration than hands-on.

X-rays

Talk to your dentist or doctor to obtain a radiograph of teeth or bones. Ask what energy of X-rays was used and what film was used. If the doctors are user-friendly, perhaps they will let you put some samples on the film and make an X-ray radiograph.

Objective: Investigate whether the amount of X-ray absorption depends on the number of electrons per unit area exposed to the beam of X-rays (see Reading Visible Light and X-Rays).

Look at a dental X-ray to see whether you can tell the difference between the filling materials used. Can you see cavities?

Ultraviolet (UV) Radiation

This activity does not require the purchase of a UV light source (called "blacklight" and available at Edmund Scientific for about $40). You can hold up objects in front of UV lights in toy stores or museum gift stores. The Arizona Science Center in Phoenix uses blacklight in some of their displays -- your shirt glows brightly.

The object of this activity is to observe visible light emission under UV illumination. Visible light is at lower energies than ultraviolet (UV) light. Fluorescence occurs when light is absorbed at one energy and part of the energy is re-emitted at longer wavelengths. When visible light is re-emitted, ultraviolet light has been absorbed. Fluorescence occurs at other energies; for example, for X-rays it is termed X-ray fluorescence (XRF). In some cases fluorescence occurs after the light source is removed. This slow emission of light is called phosphorescence.

Two rocks fluorescing under blacklight. Note the different colors in the rock signifying different minerals embedded in the rock.

Infrared (IR) Radiation

At the Goldwater Center at Arizona State University, you can explore what lies beneath the surface of paintings by the use of an IR sensitive camera. This camera is similar to those used in art museums such as the Metropolitan Museum of Art in New York and consists of a light-sensitive array of charge-coupled devices (CCDs) and a light filter that blocks off all light with wavelengths less than about 1 micrometer (1000 nanometers) or in terms of energy, light with energies greater than about 1.24 electron volts (eV). The wavelength to energy conversion given in the Electron Readings is

Energy (eV) = 1240 / wavelength (nanometers)

In the experimental setup, the painting is placed about 2 feet below the camera (so that a good portion of the painting is in the field of view of the camera) and the painting is illuminated with an incandescent (tungsten filament) lamp which has a portion of its light output in the infrared (see Sources of Light). The infrared radiation penetrates throught the pigments which are mostly oxides or sulfides such as titanium oxide white, cadmium sulfide red, or mercury sulfide vermillion, and reflects from the white undercoating. Any charcoal black underdrawings or IR-absorbing pigments (many of the Cu-containing green pigments absorb IR) will not reflect IR and these underdrawing will be apparent in the IR display seen in the video monitor.

Figure 1. Visible light view of "Portrait of the Artist as a Landscape", W.S. Taft, 1997.

The figure above is a painting made for Patterns in Nature by W. Stan Taft of Cornell University entitled "Portrait of the Artist as a Landscape". Taft sketched a head in charcoal and then covered it with a oil-based painting of the fall foliage around Cayuga Lake (one of the finger lakes of upper New York state).

Figure 2. Infrared view taken with a Hamamatsu camera with filter to pass infrared light of "Portrait of the Artist as a Landscape".

The infrared view of this portrait shows the charcoal underdrawing with eye, nose, and mouth visible. Note the evidence of this underdrawing in the visible image of Fig. 1.

You can test this IR viewing technique yourself in one of the two Saturday lab sessions. Use some of the paints available in the lab and cover some transparent sheets. Then place them over the black outline of a head (available in the lab) and see which paints are transparent to IR radiation.

Figure 3

The last figure is an X-radiograph taken at the ASU Health Center by Lisa using 40 kV X-rays. The overall details of the underdrawing are not distinct but the round eye orients one in the figure. The artist Taft used lead (Pb) white for ground layer on the canvas. Note that you can "see" the canvas weave (above the diagonal line) which is an important application in identification of canvases in old paintings (click on the image to see a more detailed view). Another important feature is the type of brush stroke. Taft used vermillion (mercury sulfide) and his strokes leap out of the X-radiograph.

Go to the Readings on Visible Light and X-Rays

or Go to the Readings on Infrared and Ultraviolet Light

or Go to the Readings on X-Radiography