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Learn about light, sources of light, luminous vs. non-luminous objects, how light is produced, and different types of light sources like incandescence, fluorescence, and LEDs.
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SNC2P Light and Geometric Optics
Optics is the science dealing with light and vision. • Optical means ‘relating to the eye’
Light • Light is radiant energy that travels (radiates) outward in all directions from the object giving off the light. • Light energy is the form of radiant energy that can be detected by the unaided human eye.
Sources of Light • All objects that we see are sources of light, even if they do not produce light themselves. • Objects that produce light are said to be luminous. • Sun, burning candle, light bulb, flash of lightning.
Non-Luminous Objects • Most objects do not produce light on their own. Objects are able to be seen because light reflects (bounces off) them to our eyes. • Coloured liquids and stained glass windows allow light to pass through them and this allows us to see the objects. • Objects that we see because light reflects from them or passes through them are called non-luminous sources of light.
Non-luminous objects are sometimes referred to as illuminated objects.
Luminous Sources of Light • Luminous Sources of Light can be both natural and artificial. • Natural sources of light are those that produce light without human intervention. • Sun, Northern Lights, glow of red-hot lava
Artificial sources of light are those that are produced and controlled by humans. • Car headlights, neon signs, flashlights, televisions • Some sources of light, such as burning wood, may be classified as both natural and artificial.
How Light is Produced • Light is produced when other forms of energy (heat or chemical) are converted into light energy. • The four most common ways in which luminous objects produce light are: • Incandescence • Passing an electric current through a gas • Fluorescence and Phosphorescence • Light Emitting Diode (LED)
Incandescence • When steel is heated to temperatures over 2000 degrees Celsius, it gives off white light. • A substance that gives off light because it has been heated to a high temperature is called incandescent.
Some substances are heated to incandescence when they are rubbed (friction).
Substances can be heated to incandescence by electricity. • The light from an old household light bulb is produced by heating a fine coil of tungsten wire (filament) until the temperature rises and gives off a bright light. • These old bulbs are called incandescent bulbs. • Toasters and ovens also become incandescent when heated.
Did you know that… • The space inside an incandescent light bulb is filled with nitrogen or argon. • If the space was filled with oxygen, the gas would react with the hot filament.
Substances may also be heated to incandescence using chemical energy. • When wood, wax, kerosene, or oil is burned, energy is given off in the form of light and heat because of a chemical reaction.
Passing an Electric Current through a Gas • When an electric current is passed through a gas, light is sometimes produced. • A natural occurrence of this is lightning produced during a thunderstorm. • Another natural example of this is the Aurora Borealis (Northern Lights).
An artificial way of producing light by passing an electric current through a gas is Neon Lights. Other gases are used to produce different colours.
Sodium vapour lamps produce an intense light and is often used to illuminate streets and highways. Mercury vapour lamps are very bright and are used to light hockey rinks and sports stadiums.
Fluorescence • Some objects give off light when invisible ultraviolet light is shone on them. • Substances that produce visible light when exposed to ultraviolet light are called fluorescent.
Stamp collectors shine UV light onto stamps to check for special fluorescent markings.
A Fluorescent Light Bulb • A fluorescent light tube consists of a long, cylindrical tube that filled with mercury vapour. • The inside of the tube is coated with a fluorescent powder. Electrodes are placed at either end of the tube. • When an electric current is passed through the vapour, UV light is produced. The UV light hits the powder which then produces visible light.
A fluorescent tube is much more efficient than a regular incandescent light bulb. • About 20% of the electrical energy used in a fluorescent light is converted to visible light, but only 5% of the electrical energy used in an incandescent bulb is converted to visible light. • If you place you hand near a fluorescent tube, it feels cool. If you place you hand near an incandescent bulb, it feels warm.
When a fluorescent tube is switched off, no more UV light is produced and the coating stops giving off light. • Some fluorescent substances though, continue to give off light for hours after the energy source has been removed. These substances are called phosphorescent. • Luminous dials on watches, glow in the dark objects and similar items are made of phosphorescent materials.
CFL bulbs • Governments around the world recommend that homes and businesses switch to compact fluorescent light bulbs (CFLs). • CFL bulbs last longer than an incandescent bulb and use less energy. • CFLs cost more than an incandescent bulb and the mercury in the CFLs can cause environmental pollution. • All used fluorescent bulbs should be taken to a hazardous waste collection site.
CFLs were devised to reduce the length of regular fluorescent light fixtures.
Light Emitting Diode (LED) • The Light Emitting Diode is a very efficient light producing technology. • An electric current passes through an LED, it emits light. • LEDs save energy, last longer and stay cooler than Incandescent bulbs of CFL bulbs.
Other ways that light is produced • Chemiluminescence • Bioluminescence
Chemiluminescence • Light can be the result from the energy released in chemical reactions. The products of the chemical reaction give off visible light. This process is called chemiluminescence.
Bioluminescence • Some living things can make themselves luminous using a chemical reaction similar to chemiluminescence. This is called bioluminescence. • Many organisms that live deep in the ocean use bioluminescence because so little sunlight reaches far below the surface of the water. • Fireflies, glow worms, some fish, some bacteria
Light that is being transmitted through the air is reflected from water, ice, and rocks.
The Transmission, Reflection, and Absorption of Light • Light can be affected in three different ways when it strikes matter: • Transmitted • Reflected • Absorbed
A substance, such as air, allows light to pass though it as it the air wasn’t even there. • When light passes through a substance, light is transmitted through the substance. • Any substance that transmits light is called a medium
Other substances cause light to bounce or reflect from them. • Mirrors and white paper reflect most of the light that strikes them.
Some substances absorb the light that strikes them. • A black surface absorbs most of the light energy hitting it. This light energy is then turned into heat energy which is why dark surfaces feel warm.
Transparent, Translucent, and Opaque Substances • All substances can be classified according to how they transmit, reflect, or absorb light. • A transparent substance is one that transmits light readily. • Objects can be seen clearly through a transparent substance. • Some examples of transparent substances are: • window glass, air, clean water, and plastic food wrap.
A translucent substance is one that transmits some of the light that hits it. • Objects can be detected, but not clearly seen through a translucent substance. • Frosted glass
An opaque substance is one that does not transmit any light. • When light strikes an opaque object, the light is reflected or absorbed. • Rocks, wood, and metals
