Uses of Visible Light and Ultraviolet

Two neighbouring parts of the electromagnetic spectrum

Visible light and ultraviolet are both electromagnetic waves. Visible light is the limited range of electromagnetic radiation that our eyes can detect. Ultraviolet, usually shortened to UV, lies just beyond the violet end of the visible spectrum, so human eyes cannot see it.

A use depends on how radiation interacts with matter. Visible light can be emitted, reflected and detected to provide information about an object. UV can be absorbed by particular substances and produce effects such as fluorescence or damage to microorganisms. These different interactions make the two radiations useful in different ways.

Visible light and vision

We see an object when visible light from it reaches our eyes. A luminous object, such as a lamp, emits light. A non-luminous object can be seen when light from another source strikes it and some of that light is reflected towards the observer. The detected light allows the brain to construct an image.

This explains why an object cannot normally be seen in complete darkness: there is no visible light travelling from the object to the eye.

Visible light for illumination

Illumination means providing visible light so that people, cameras or other detectors can see objects and their surroundings. Lamps illuminate a dark room by emitting visible light. This light travels to surfaces and is then partly reflected in many directions, including towards an observer.

Illumination is therefore closely connected to vision: the lamp does not need to shine directly into the eye. Its useful role is to provide light that can reflect from the scene.

Visible light in photography

Photography records the pattern of visible light arriving from a scene. A camera lens collects and focuses the light to form an image on a light-sensitive surface.

In a digital camera, light-sensitive cells produce electrical signals. These signals are processed and stored as an image. In a film camera, the light instead causes chemical changes in light-sensitive film. In either case, the recorded differences in the light arriving from different parts of the scene produce the photograph.

The aperture and the time for which the shutter is open control how much light reaches the detector. Too little light may produce a dark image; too much may lose detail in bright areas.

Why ultraviolet can reveal hidden markings

Some materials absorb UV and then emit visible light. This process is called fluorescence. The material appears to glow while it is being illuminated with ultraviolet, even though the UV itself remains invisible to the eye.

Security pens use ink that is difficult or impossible to see under ordinary visible light. Under a UV lamp, the ink fluoresces, revealing a mark that can help identify property.

Genuine banknotes can also contain specially designed fluorescent markings. When a note is placed under UV, its revealed features can be compared with the expected pattern. Missing or incorrect features may indicate a forged banknote.

Ultraviolet in fluorescent lamps

Inside a fluorescent lamp, an electrical discharge in a low-pressure gas produces ultraviolet radiation. A fluorescent coating on the inside of the tube absorbs this UV and emits visible light. It is the emitted visible light, rather than the UV, that illuminates the room.

The same central process links fluorescent lamps, security marking and banknote checking: a fluorescent substance absorbs ultraviolet and gives out visible light.

Ultraviolet for disinfecting water

Ultraviolet can also be used to disinfect water. Water is exposed to UV radiation, which damages or kills harmful microorganisms such as bacteria. This reduces the number of living microorganisms in the water without relying on fluorescence.

The water must be exposed effectively to the radiation, so UV treatment works best when the radiation can reach the microorganisms. This application uses UV's biological effect, whereas security marking and fluorescent lamps use its ability to cause fluorescence.

Visible light

• Vision: emitted or reflected visible light enters the eyes and allows an image of the scene to be perceived.
• Photography: a lens focuses visible light onto light-sensitive film or an electronic sensor to record an image.
• Illumination: lamps provide visible light which reflects from objects, allowing them to be seen.

Ultraviolet

• Security marking: normally hidden ink fluoresces under UV and becomes visible.
• Detecting forged banknotes: UV reveals fluorescent security features for comparison with those expected on a genuine note.
• Fluorescent lamps: a gas discharge produces UV; a coating absorbs it and emits visible light.
• Disinfecting water: UV damages or kills harmful microorganisms.

Essential distinction

Fluorescence: UV is absorbed $\rightarrow$ visible light is emitted. The glow seen by the observer is visible light, not ultraviolet.