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Which is the odd one out?. What are infrared waves?. We experience infrared waves everyday in the form of heat. Infrared radiation is a form of electromagnetic radiation that lies between microwaves and visible light on the electromagnetic spectrum. visible light. microwaves. infrared.
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What are infrared waves? We experience infrared waves everyday in the form of heat. Infrared radiation is a form of electromagnetic radiation that lies between microwaves and visible light on the electromagnetic spectrum. visible light microwaves infrared 10mm 0.1 mm 0.001 mm 0.00001 mm wavelength of electromagnetic waves Infrared waves have wavelengths between 1 mm (about the size of a pin head) and about 0.0001 mm (about the size of a cell).
What produces infrared waves? Infrared radiation is emitted by all objects but is not visible to humans. The hotter an object is, the more infrared radiation it emits. As an object gets hotter, visible light is also emitted. This is what can be seen when an object glows ‘red hot’. Infrared radiation can be detected by special cameras. A thermogram shows how objects with different temperatures emit different amounts of infrared radiation. Here, the white areas are warmestand the blue areas are coldest.
How are infrared waves used? Infrared waves have various uses. How many can you spot?
How is infrared used for heating and cooking? The infrared radiation emitted by the Sun, radiators, solid fuel fires and electric fires is used for heating. The heating effect of infrared radiation is detected by temperature sensitive nerve-endings in skin. Grills, cookers, toasters, campfires and barbecues use infrared radiation to cook food. Infrared waves are emitted by the heating element or lit fuel and absorbed by the surface of the food. Heat is transferred through the food by conduction.
How is infrared used in remote controls? Not all infrared radiation is thermal. Infrared radiation with short wavelengths are not hot and cannot be detected by skin. This type of infrared radiation is used by remote controls to send information through the air over short distances. The pulses of infrared radiation from the remote control can only travel in a straight line to the device being controlled. Infrared waves can also be used to transmit information through optical fibres. In this way, infrared radiation can senddata over long distances and around corners.
How do infrared thermometers work? An infrared thermometer enables the remote sensing of temperature. It does this by detecting the infrared radiation emitted by an object and converting it into a temperature. The eardrum is an accurate point for measuring the body’s temperature, because it is deep within the head. A digital ear thermometer can do this without touching the eardrum, which is very fragile. A probe is inserted into ear canal and then measures the infrared radiation emitted from the eardrum and surrounding tissues.
How can infrared waves fight crime? Devices containing infrared detectors are used for various security purposes. Sensors in security lights and burglar alarms can detect the infrared radiation given off by an approaching body. Thermal imaging equipment, as used by the military and police, detects infrared radiation and converts it into an image. Objects such as people and cars, that are hotter than the background can be easily tracked even at night. The emergency services use thermal imaging cameras to help locate people in situations where visibility is low.
What is light? Every time someone opens their eyes, they are making use of ‘visible’ light. Light is the only type of electromagnetic radiation that is visible to eye. It lies between infrared and ultraviolet radiation on the electromagnetic spectrum. visible light ultra- violet infrared 0.1mm 0.001 mm 0.00001 mm 1x10-7mm wavelength of electromagnetic waves Light waves have wavelengths between 390 and 780 nm. The different wavelengths of light are seen as the differentcolours of the spectrum.
What is light used for? • Human sight Human eyes are sensitive to visible light. The different wavelengths of light are detected as different colours. • Imaging Photography uses light-sensitive chemicals to produce images. Digital cameras contain sensors that detect light and use this to produce electrical signals. • Navigation Navigation lights are used at sea to mark hazards and safe areas. Lights are also displayed by boats and aircraft, at night, to show their position and direction of travel.
How is light produced? Visible light is emitted by very hot objects. The filament of this light bulb reaches a temperature of 2500°C and glows ‘white hot’, making it a source of light. The Sun, our nearest star, is a natural source of light. This light is part of the energy produced by the nuclear fusion reactions that take place in the Sun. Neon lights contain gases that emit visible light when an electrical current is passed through them. These emit light of certain wavelengths, that isseen as different colours.
How else can light be produced? Certain chemicals can emit visible light when struck by ultraviolet radiation. This process is called fluorescence. Fluorescent tubes contains mercury vapour, which generates ultraviolet radiation an electrical current is passed through it. This UV radiation hits the fluorescent coating on the inside of the tube and creates visible light . Chemical reactions can also emit energy as visible light. This is called chemiluminescence. Some biological organisms have the ability to produce light. This is bioluminescence.
What is total internal reflection? When a light ray hits the boundary between two materials of different densities (e.g. glass and air), the ray is normally bent or refracted. This occurs because the speed of light changes in different materials. If the angle of incidence of the light ray is greater than a specific value, called the critical angle, then the light ray is actually reflected. This is called total internal reflection. It makes the inner surface of glass act like a perfect mirror.
Why is angle of incidence important? If the angle of incidence is smaller than the critical angle, then the light ray is refracted. If the angle of incidence equals the critical angle, then the light ray is refracted along the boundary. If the angle of incidence is greater than the critical angle, then the light ray is not refracted and total internalreflection occurs.
Is the critical angle always the same? The critical angle is the smallest angle of incidence at which total internal reflection occurs. Different materials have a specific value for the critical angle: Material Critical angle water 49° acrylic plastic 42° glass 41° diamond 24° Diamond has the lowest critical angle at 24°. This means that diamond reflects more light than the other materials and accounts for its characteristic sparkle.
Is communicating with light a new idea? Light has been used to send messages throughout history. For example, the Egyptians used sunlight to communicate. A visual signalling device called an Aldis lamp was widely used by the British Navy to send messages over short distances. The messages were translated into Morse code. The encoded messages were transmitted using a flashing signal lamp. Short flashes to represent ‘dots’ and long flashes for ‘dashes’. Today, computers encode information as digital signals, which can be sent along optical fibres using light.
What are optical fibres? Optical fibres are thin strands of solid glass, about the size of a human hair. They are widely used in communication, medicine, lighting and as sensors. The first transatlantic telephone cable to use optical fibres went into operation in 1988. Optical fibres can transmit light signals at high speed over long distances and are used in phone and internet connections.
How do optical fibres carry light? Optical fibres do not have to be straight to carry light and can even carry light around corners. This curved Perspex rod shows how light travels in an optical fibre. Light travels through the Perspex rod, and optical fibres, by aprocess called total internal reflection.
What is total internal reflection? A ray of light enters the optical fibre. As the light enters the optical fibre, it is refracted. This means that the direction the light is travelling in changes. light enters optical fibre The ray of light hits the wall of the fibre and is totally internally reflected when the angle of incidence is greater than the critical angle. light leaves optical fibre The ray of light passes down the optical fibre by repeated total internal reflection.
How are optical fibres used in medicine? Optical fibres can be used by doctors to look inside a patient’s body without having to cut them open to check that they are healthy. The device used for this purpose is called an endoscope. It contains a bundle of optical fibres that are held together and can be inserted into the body. Light is sent down some of the fibres and the image is reflected back through the other fibres.
Glossary • critical angle – If light hits a boundary between two materials at an angle greater than this value, then total internal reflection occurs. • infrared radiation –Electromagnetic radiation that lies between microwaves and visible light. It is involved in heating. • optical fibres –Long, thin strands of glass or plastic used to carry digital signals as pulses of infrared or visible light. • total internal reflection –An optical effect that occurs when light hits a boundary between two materials at an angle greater than the critical angle, so that all the light is reflected. • visible light –The only type of electromagnetic waves that can be detected by the human eye.
