LIGHT

Key Points for today What are E-M Waves? Where does light fit in to all of this? Comparing λ and f for different E-M waves Reading quiz next day (16.1)

Just another wave……. Albeit a special kind of wave! Light is an ________________ wave. This means that no medium (water, air, your desk) is needed for the wave to travel.

Albert Einstein

LIGHT: What Is It? Light Energy –Atoms As atoms absorb energy, electrons jump out to a higher energy level. Electrons release light when falling down to the lower energy level. –Photons - bundles/packets of energy released when the electrons fall. Light: Stream of Photons © 2000 Microsoft Clip Gallery

Transverse Waves Energy is perpendicular to direction of motion Moving photon creates electric & magnetic field –Light has BOTH Electric & Magnetic fields at right angles! © 2000 Microsoft Clip Gallery

Electromagnetic (E-M) Waves The light we see is only a small part of the E-M spectrum

Electromagnetic Spectrum © 2000 Microsoft Clip Gallery

Electromagnetic Spectrum Visible Spectrum – Light we can see –Roy G. Biv – Acronym for Red, Orange, Yellow, Green, Blue, Indigo, & Violet. –Largest to Smallest Wavelength.

Questions How long are Radio waves? (give a range) –This distance is approximately equal to _____ –These E-M waves have more / less energy than X-Rays

Diffraction Gratings: The Bending of Light A wave is bent when it hits a rough surface or passes through a small opening, or aperture. This is called diffraction. Diffraction happens to all types of waves, including light. The white light we see from the sun and light bulbs is not really white. It is made up of many different colors. These colors that make up the light are known as a spectrum. If you have ever seen a rainbow, then you have seen the visible spectrum. Diffraction gratings separate light into the different colors of light that it is made of.

Electromagnetic Spectrum Invisible Spectrum –Radio Waves Def. – Longest wavelength & lowest frequency. Uses – Radio & T.V. broadcasting. © 2000 Microsoft Clip Gallery

Modulating Radio Waves Modulation - variation of amplitude or frequency when waves are broadcast –AM – amplitude modulation Carries audio for T.V. Broadcasts Longer wavelength so can bend around hills –FM – frequency modulation Carries video for T.V. Broadcasts © 2000 Microsoft Clip Gallery

Electromagnetic Spectrum Invisible Spectrum (Cont.) –X-Rays Def. - EM waves that are shorter than UV rays. Uses: Medicine – Bones absorb x-rays; soft tissue does not. Lead absorbs X-rays.

Electromagnetic Spectrum Invisible spectrum (cont.) –Gamma rays Def. Highest frequency EM waves; Shortest wavelength. They come from outer space. Uses: cancer treatment.

LIGHT: Particles or Waves? Wave Model of Light –Explains most properties of light Particle Theory of Light –Photoelectric Effect – Photons of light produce free electrons © 2000 Microsoft Clip Gallery

Intro to Velocity To find out how fast an object is moving, we need two things: 1.Distance 2.Time velocity = distance/time v = d/t

Man….those things move fast (Pg. 331) How far is it across Earth’s orbit? How much time does it take light to travel across Earth’s orbit?

TO DO….. Read P. 330 – 336 Do “Notes on Light” – Page 1 Do: Practice Problems (1 - 10) in Chapter 16

Phosphorescence: A Big Word for Glow-In-the-Dark Certain compounds contain things called phosphors. Phosphors emit visible light when exposed to energy, like from ultraviolet light. They do this because of electrons jumping between energy states. In order to give the electrons enough energy, you have to use light with a short wavelength. Some phosphors, like the ones in fluorescent lights, do not glow after the energy source is taken away. Those that continue to glow after the energy is taken away are called phosphorescent. Phosphorescence in art!

Light Can Carry a Lot of Information Holograms and fiber optics are two methods of information transfer that use light. Holograms are made using lasers to record a 3 dimensional object on a 2 dimensional surface, like a piece of paper. Our eyes see the object because of controlled use of diffraction. Fiber optics is a method of sending information as light through a cable. This light signal can be changed to music, video and more!

Fun Facts A hologram the size of a sugar cube could hold enough information for about 30 DVD movies! CD’s, like holograms, use diffraction patterns to store information. Light from the sun can be transformed into usable energy to power homes, cars and more. Light at the far ends of the spectrum, like microwaves (far into the infrared) and gamma rays (far into the ultraviolet) have some very interesting properties. You could read about them in the library, or on the internet!

Key Points – Ch. 16 Light is merely a small part of a large E-M spectrum The range of wavelengths of visibile light (and their corresponding colours The speed of light The difference b/w intensity, flux & illuminance The difference b/w transparent, translucent, and opaque (using vocab: transmitted, reflected, aborb) How do diffraction gratings work (see LIGHT.ppt on website) What primary and secondary colours are The meaning of polarized light Questions: p. 343 RC (2, 7, 8, 12) Problems (1, 3, 5, 6, 7)