Lights and Its Properties: Part Two

Recall – What do we know about light? Form of energy Can be luminous or non-luminous Travels in a straight line (linear propagation) Invisible unless it is reflected by translucent or opaque objects Can be transmitted, reflected or absorbed by different types of material

Electromagnetic Spectrum Electromagnetic wave: a wave that has both electric and magnetic parts, is radiated and travels at the speed of light (299,792,458 m/s) The speed of light is always constant for every wave but waves can have different energy. The higher the energy (eg. Gamma rays), the shorter the wavelength (increased frequency). The lower the energy (eg. Radiowaves), the longer the wavelength (decreased frequency). The electromagnetic spectrum is the classification of electromagnetic waves by its energy level.

Visible Light Electromagnetic waves that the human eye can detect. White light is composed of a spectrum of colours (ROYGBIV) When you shine white light such as sunlight through a prism, dispersion occurs in which the white light “breaks apart” to show the individual wavelengths of the colours that make up white light. Why does violet light bend more than red light?

Types of Light Sources Incandescence Production of light as a result of high temperatures Light bulb has a wire filament (tungsten) that is heated to high temperatures by an electric current passing through it.

Types of Light Sources Electric Discharge Process of producing light by passing an electric current through a gas Fluorescent light bulb – filled with an inert gas (eg. Argon, krypton, neon, xenon) as well as mercury vapor. When an electric current is passed through, it causes the mercury atoms to emit UV light. The UV light is absorbed by the fluorescent materials that coat the tube and excites them, which makes them re-emit visible light. Neon light has glass tubes filled with a gas like neon, argon or krypton at low pressure. At both ends of the tube there are metal electrodes. When you apply an electric current, the gas ionizes and electrons flow through the gas. The electrons excite the gas’ atoms and cause them to emit light. Different gases emit different lights (neon = orange, argon = blue, krypton = pink, xenon = purple).

Types of Light Sources Phosphorescence Process of producing light by the absorption of ultraviolet light resulting in the emission of visible light over an extended period of time. Phosphor chemicals are used and there are many different kinds with various brightness, colour and glowing time lengths. These chemicals store energy when exposed to light and they have to be exposed for a while. When they are exposed to light, they get energized and then radiate visible light after being energized by slowly releasing their stored energy over time. As they release the energy, they emit small amounts of light, making them appear to glow in the dark. Examples of common phosphors are zinc sulphide and strontium aluminate.

Types of Light Sources Fluorescence Immediate emission of visible light as a result of the absorption of ultraviolet light. Fluorescent light bulb – filled with an inert gas (eg. Argon, krypton, neon, xenon) as well as mercury vapor. When an electric current is passed through, it causes the mercury atoms to emit UV light. The UV light is absorbed by the fluorescent materials that coat the tube and excites them, which makes them re-emit visible light.

Types of Light Sources Chemiluminescence Direct production of light as the result of a chemical reaction with little or no heat produced. Chemiluminescence makes objects glow in the dark because of a chemical energy. When certain chemicals are mixed in a chemical reaction, they produce energy that is released thereafter, creating a glow, as seen in fireflies and glow sticks. Luciferase in abdomen reacts with oxygen gas to produce flashes of light for courtship

Types of Light Sources Bioluminescence Production of light in living organisms as the result of a chemical reaction with little or no heat produced. A naturally occurring light source. Creatures like fireflies, anglerfishes and jellyfishes contain special chemicals within them that cause them to glow for biological reasons (eg. Attracting mates, courtship, attracting prey, protection, etc). In the anglerfish, the light is used to attract prey. The light is created by masses of bacteria living in its “fishing rod” in which the bacteria themselves are bioluminescent!

Types of Light Sources Tribioluminescence Production of light from friction as a result of scratching, crushing or rubbing certain crystals. When certain materials are broken apart, rubbed, scratched or crushed, chemical bonds within them are broken. This separates the electric charges between, giving off sparks of light. It is not fully understood however.

Types of Light Sources Light Emitting Diode (LED) Light produced as a result of an electric current flowing in semiconductors. A semiconductor is a substance (usually a solid chemical element or compound) that can conduct electricity under some conditions but not others – making it a good medium for control of electric currents. Examples of elements are silicon, germanium and silicon carbide. When an electric current is applied, electrons located in the semiconductor material constantly move and their movement illuminates the light bulbs. Different from other light bulbs because it produces way less heat and is more energy efficient. This is because the semiconductor chosen can limit the amount of energy that flows through them.