Infrared waves

Today, infrared radiation is perhaps most famous for enabling people to see at night via military night-vision goggles. These effectively transform the infrared radiation into visible wavelengths that we can see.

Most magnifying glasses are double-convex lenses and are used to make objects appear larger. This is accomplished by placing the lens close to the object to be viewed. In this way the light rays are bent toward the center of the lens. When these bent rays reach the eye they make the object appear much larger than it actually is. However, if the object is far enough away from the lens, the image will flip, appearing smaller and upside down. The distance at which this flip occurs is twice the focal length (the distance from the optical center of a lens to the point where the light rays converge) of the lens. The focal length of any lens is determined by the amount of curve on the lens’ face. The magnified image is called a virtual image while the smaller, inverted image is called the real image.

Infrared wavelength

The magnifying glass was the forerunner of the compound microscope (in which a series of lenses are used to focus, magnify, and refocus an image), one of the basic tools used in medicine.

Many people have used a magnifying glass and sunlight to ignite a piece of paper. When the lens is held at exactly two focal lengths from the paper, all of the light will be concentrated into a tiny point, generating enough heat to start a fire.

Near-infrared light comes from relatively cool (750 – 3000 Kelvin) objects in our Universe, such as red giants and cool red stars. Mid-infrared radiation can comes from (140-750 Kelvin) dust which has been warmed by stars, proto-planetary discs, planets and comets. Due to the longer wavelength, and thus lower frequency (f = c/ where c is the speed of light and is the wavelength) and energy (E = h x f where h is Planck’s constant) of far-infrared radiation, it traces even colder objects (12 – 140 Kelvin), such as cold molecular gas clouds and dust clouds.

Infrared frequency

Infrared light

Infrared uses

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Around 1800, the German-born British-astronomer William Herschel discovered infrared radiation. He did so with a simple experiment in which he dispersed sunlight through a prism and placed a thermometer at the location of each colour. He noticed that the thermometer temperature increased when he did this, which was not really unexpected since sunlight carries warmth. However when he placed the thermometer past the red end of the spectrum – where there was no visible sunlight – the thermometer’s temperature still increased! Herschel had discovered infrared radiation – radiation beyond the red end of the visible spectrum.

Infrared radiation is a form of electromagnetic radiation with wavelengths longer than those at the red-end of the visible portion of the electromagnetic spectrum but shorter than microwave radiation. This wavelength range spans roughly 1 to several hundred microns, and is loosely subdivided – no standard definition exists – into near-infrared (1-5 microns), mid-infrared (5-40 microns) and the far-infrared (40 to 350 microns).

The magnifying glass is one of the most ancient optical devices known to science. Thousands of years ago Egyptians used chips of crystal or obsidian (a type of shiny stone) to better view small objects. In Rome Emperor Nero (a.d. 37-68) was known to have peered through gemstones at actors on a distant stage. The first magnifier constructed for scientific purposes is believed to have been designed by the English philosopher Roger Bacon (circa 1220-1292) sometime during the thirteenth century.