EM spectrum: ultraviolet, X-rays and gamma rays
The short-wavelength end holds ultraviolet, X-rays and gamma rays. They carry high energy, so they are useful in medicine but harmful to living tissue.
Ultraviolet to gamma rays
| Wave | Wavelength | Produced by | Detected by | Main uses |
|---|---|---|---|---|
| Ultraviolet (UV) | 400 nm to 1 nm | The Sun, very hot bodies and special lamps; inner-shell electrons of atoms changing levels | Photocells, photographic film | Killing germs in water purifiers, LASIK eye surgery |
| X-rays | 1 nm to 10⁻³ nm | X-ray tubes, where fast electrons hit a metal target; inner-shell electrons changing levels | Photographic film, Geiger tubes, ionisation chambers | Finding bone fractures and other medical diagnosis, treating some cancers |
| Gamma rays | Less than 10⁻³ nm | Nuclear reactions and radioactive nuclei | Photographic film, Geiger tubes, ionisation chambers | Destroying cancer cells (radiotherapy) |
Ultraviolet: elementary facts
- The Sun is an important source of UV.
- Most of the Sun's UV is absorbed by the ozone (O₃) layer high in the atmosphere. Chemicals such as CFCs (chlorofluorocarbons) thin this layer, which is a worldwide concern.
- Large amounts of UV are harmful: they cause sunburn and can damage the eyes.
- A welding arc gives out a lot of UV, so welders shield their eyes with masks or goggles of special glass.
- UV has a short wavelength, so it can be focused into very narrow beams. This makes it useful in precise surgery such as LASIK eye surgery.
- UV lamps kill germs in water purifiers.
X-rays vs gamma rays
Their wavelength ranges overlap, so the name depends more on the source than on the wavelength. Both damage living tissue, so exposure must be kept as low as possible.
How to draw the spectrum chart
- Draw a long horizontal strip and divide it into seven boxes.
- Label the boxes from left to right: radio, microwave, infrared, visible, ultraviolet, X-rays, gamma rays.
- Make the visible box very narrow. Mark red at its left end and violet at its right end.
- Above the strip, draw an arrow pointing right, labelled frequency increases.
- Below the strip, draw an arrow pointing left, labelled wavelength increases.
- Write rough wavelengths at the boundaries: 0.1 m, 1 mm, 700 nm, 400 nm, 1 nm and 10⁻³ nm.
Worked example: frequencies at the short end
Find the frequency of (a) UV of wavelength 300 nm and (b) X-rays of wavelength 0.1 nm. Which of them carries more energy? (c = 3 × 10⁸ m s⁻¹)
- Formula: ν = c/λ.
- (a) λ = 300 nm = 3 × 10⁻⁷ m, so ν = 3 × 10⁸ ÷ (3 × 10⁻⁷) = 10¹⁵ Hz.
- (b) λ = 0.1 nm = 10⁻¹⁰ m, so ν = 3 × 10⁸ ÷ 10⁻¹⁰ = 3 × 10¹⁸ Hz.
- The X-rays have about 3000 times the frequency of the UV. Higher frequency means more energy, so the X-rays carry more energy and penetrate further.
(a) 10¹⁵ Hz; (b) 3 × 10¹⁸ Hz; the X-rays carry more energy
A little UV from the morning sun helps your skin make vitamin D, but too much causes sunburn. Hospitals use X-rays to check for fractures and gamma rays in radiotherapy for cancer.
1 marks: “Which part of the EM spectrum is absorbed by the ozone layer?” Ultraviolet radiation.
2 marks: “How are X-rays produced? Give one medical use and one harmful effect.” Fast electrons strike a metal target in an X-ray tube; used to detect bone fractures; they damage living tissue.
3 marks: “Name the EM waves (a) used to kill germs in water purifiers, (b) used to detect fractures and (c) given out by radioactive nuclei. Arrange them in order of increasing frequency.” (a) UV, (b) X-rays, (c) gamma rays; in increasing frequency: UV, X-rays, gamma rays.
UV burns skin and kills germs, X-rays see bones, and gamma comes from the nucleus and fights cancer.