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Sources and Nature of Light

Sources and Nature of Light . Unit: Light and Geometric Optics. Types of Light Emissions. Some light sources release heat, such as light bulbs. These sources emit many colours of light combined together and are called white light .

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Sources and Nature of Light

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  1. Sources and Nature of Light Unit: Light and Geometric Optics

  2. Types of Light Emissions • Some light sources release heat, such as light bulbs. These sources emit many coloursof light combined together and are called white light. • Other light sources do not release heat, such as some microorganisms. These sources usually emit one main colour of light.

  3. What Causes the Light Emissions? • Atoms within the light sources absorb a form of energy and become “excited”. However, these atoms almost instantaneously release this extra energy, some of it in the form of light.

  4. Light From the Sun • The Sun releases a huge amount of light and heat. • This large amount of energy is caused by nuclear fusion reactions primarily between the collision of fast moving hydrogen atoms. This results in larger atoms being formed, such as helium.

  5. Nuclear Fusion in the Sun

  6. Light From Incandescence • In an incandescent light source, some form of energy is changed into heat energy and light energy. • Light is emitted because of a very high temperature. • For example, steel emits light when heated to temperatures of around 800oC.

  7. Incandescent Light Bulb • Consists of a glass bulb, from which the air has been removed, and a small tungsten wire (melting point 3 400oC) attached to an electrical connection. • As the bulb is turned on, electrons flow through the wire, providing the energy for the wire to heat up and emit light.

  8. The Light Bulb

  9. Incandescent Light Bulb • 95% of the energy is heat energy. • 5% of the energy is light energy. • Thus, the incandescent light bulb is very inefficient as a light source!

  10. Light From Electric Discharge • Light is often produced when an electric current passes through a heated gas or vapour ( ex. Lightning). • An electric discharge bulb has an electrode at each end connected to anenergysource. • In a sodium vapourbulb- a drop of Na and Hgare placed in the bulb, most of the air is removed, and the Na and Hg produce a vapour within the bulb.

  11. Light From Electric Discharge • As electric current is passed through the vapour, atoms become excited and then release the energy in the form of heat and light.

  12. Fluorescence Fluorescence – Visible light that is emitted when a substance gets exposed to ultraviolet light. • A fluorescent light bulb is an electric discharge tube that contains mercury gas and the gas of an inert element, such as argon. • A fluorescent light bulb is coated with phosphor, a powdery substance. • When the electrodes are charged they emit electrons that travel through the gas.

  13. Fluorescence • As the electrons travel from one electrode to the other, they collide with mercury atoms and excite them. • The mercury atoms release extra energy as ultraviolet light. • The phosphor absorbs the UV light and converts it to visible light.

  14. Fluorescence • The efficiency of fluorescent light bulbs for light energy is 20% and they last much longer than incandescent light bulbs.

  15. Types of Luminescence 1) Luminescence = light that is generated without heating the object. • The energy used to excite the atoms can come from a variety of sources. 2) Phosphorescence = light that is emitted due to exposure of the source to ultraviolet light, and that continues to be emitted for some time in the absence of the ultraviolet light (ex. Glow-in-the-dark objects)

  16. Types of Luminescence – contn’ 3) Chemiluminescence = light that is produced by a chemical reaction without a rise in temperature (glow sticks) 4) Bioluminescence = light that is produced by a biochemical reaction in a living organism

  17. The Nature of Light • Light travels like a wave through empty space and through some materials - called an electromagnetic wave.

  18. Wavelength crest trough - Wavelength = the distance from one crest (or trough) of a wave to the next crest (or trough) • Electromagnetic waves are invisible and can travel through a vacuum such as space, at the speed of light (3.00 × 108 m/s)

  19. Electromagnetic Spectrum • The electromagnetic spectrum covers a wide range of wavelengths and photon energies. • Light used to "see" an object must have a wavelength about the same size as or smaller than the object.

  20. Visible Light • Visible light makes up a small portion of the electromagnetic spectrum • Red - longest wavelength = 700 nm • Violet – shortest wavelength = 400 nm • where nm is nanometres or 10-9

  21. Answer these questions: • Look at the picture of the electromagnetic spectrum and answer these questions: 1. What kind of electromagnetic radiation has the shortest wavelength? The longest? 2. What kind of electromagnetic radiation could be used to "see" molecules? A cold virus? 3. Why can't you use visible light to "see" molecules? 4. Some insects, like bees, can see light of shorter wavelengths than humans can see. What kind of radiation do you think a bee sees?

  22. Homework: • Read pp. 403-410 and answer questions 1-8 on page 410.

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