Wave - Particle Duality of Light

Transcription

1 Properties of Light Objectives Explain wave-particle duality State the speed of light Describe electromagnetic waves and the electromagnetic spectrum Explain how light interacts with transparent and opaque materials

2 Wave - Particle Duality of Light Particle - Einstein light interacts with matter, not in waves but in tiny packets of energy called photons. Wave Huygens- light exhibits certain behaviors that are characteristic of any wave.

3 Speed of Light In a vacuum = c = 3 x 10 8 m/s Light from the Sun takes 8 minutes to reach the earth Light Year = distance light travels in one year. 9.4 x m (5.9 x miles) Closest star to earth is 4.22 light years away. Light will take over 4 years to get to us from there.

4 Electromagnetic Waves Originates from the accelerated motion of electrons Made up of vibrating electric and magnetic fields Two transverse waves that act perpendicularly Light is the only thing we see

5 Electromagnetic Wave Equation All radiation travels at the speed of light (or c) In a vacuum c = 3 x 10 8 m/s c = λƒ λ = wavelength in meters ƒ = frequency in hertz

6 Calculate the wavelength of the yellow light emitted by a sodium lamp if the frequency of the radiation is 5.10 x Hz. (5.10 x s -1 ) Data c= 3 x 10 8 m/s ƒ = 5.10 x Hz λ =? Equation λ = c /ƒ Math λ = (3 x 10 8 m/s)/(5.10 x Hz) Answer 5.88 x 10-7 m

7 Electromagnetic Spectrum Classification of electromagnetic waves according to frequency Lowest frequency of light we can see is red. Highest frequency of light we can see is violet. Higher frequency of light is ultraviolet more energetic and causes sunburns. Beyond are X-ray and gamma ray. No sharp boundary between regions

8 Electromagnetic Spectrum The electromagnetic spectrum spans waves ranging from lowest to highest frequencies. The smallest portion of the electromagnetic spectrum is that of A. radio waves. B. microwaves. C. visible light. D. gamma rays.

9 Electromagnetic Spectrum CHECK YOUR ANSWER The electromagnetic spectrum spans waves ranging from lowest to highest frequencies. The smallest portion of the electromagnetic spectrum is that of A. radio waves. B. microwaves. C. visible light. D. gamma rays.

10 Electromagnetic Spectrum Which of these is fundamentally different from the others? A. Sound waves B. Light waves C. Radio waves D. X-rays

11 Electromagnetic Spectrum CHECK YOUR ANSWER Which of these is fundamentally different from the others? A. Sound waves B. Light waves C. Radio waves D. X-rays Explanation: All are electromagnetic waves except sound, which is a mechanical wave.

12 Transparent Materials How light penetrates transparent material Electrons or molecules in the glass are forced into vibration. Energy is momentarily absorbed and vibrates the electrons in the glass. This vibrating electron either emits a photon or transfers the energy as heat. Time delay between absorption and re-emission of energy of vibrating electrons results in a lower average speed of light through a transparent material.

13 How light penetrates transparent material such as glass

14 Transparent Materials In glass, infrared waves cause entire atoms or molecules to vibrate, increasing the temperature of the structure. Glass is transparent to visible light, but not to ultraviolet and infrared light.

15 Speed of Light Changes in Different Transparent Materials Vacuum = c (300,000,000 m/s) Atmosphere = slightly less than c (but rounded off to c) Water = 0.75c Glass = 0.67c, depending on material Diamond = 0.41c

16 Transparent Materials Strictly speaking, the photons of light incident on glass are A. also the ones that travel through and exit the other side. B. not the ones that travel through and exit the other side. C. absorbed and transformed to thermal energy. D. diffracted.

17 Transparent Materials CHECK YOUR ANSWER Strictly speaking, the photons of light incident on glass are A. also the ones that travel through and exit the other side. B. not the ones that travel through and exit the other side. C. absorbed and transformed to thermal energy. D. diffracted. Explanation:

18 Transparent Materials Compared with the frequency of illuminating light on a sheet of transparent plastic, the frequency of light that is transmitted A. is slightly less. B. is the same. C. is slightly higher. D. depends on the type of plastic.

19 Transparent Materials CHECK YOUR ANSWER Compared with the frequency of illuminating light on a sheet of transparent plastic, the frequency of light that is transmitted A. is slightly less. B. is the same. C. is slightly higher. D. depends on the type of plastic Explanation: Speed of light in plastic may vary, but the frequency transmitted doesn t.

20 Transparent Materials The average speed of light is less in A. air before entering glass. B. glass. C. air after emerging from glass. D. None of the above.

21 Transparent and Opaque Materials CHECK YOUR ANSWER The average speed of light is less in A. air before entering glass. B. glass. C. air after emerging from glass. D. None of the above.

22 Opaque Materials Opaque - absorb light without re-emission Most things are opaque (people, dogs, swim fins)

23 Opaque Materials Vibrations given by light to their atoms and molecules are turned into random kinetic energy (internal energy). These materials become slightly warmer Light shining on metal forces free electrons in the metal into vibrations that emit their own light as reflection.

24 Opaque Materials When light is incident on dry surfaces bounces directly to your eye. wet surfaces bounces inside the transparent wet region, absorbing energy with each bounce, and reaches your eye darker than from a dry surface.

25 Shadows Opaque Materials ray - thin beam of light. Opaque objects cast a shadow a region where light rays do not reach.

26 Opaque Materials Either a large, far-away light source or a small, nearby light source will produce a sharp shadow. A large, nearby light source produces a somewhat blurry shadow.

27 Opaque Materials There is usually a dark part on the inside and a lighter part around the edges of a shadow. A total shadow is called an umbra and A partial shadow is called a penumbra. A penumbra appears where some of the light is blocked but where other light fills it in. A penumbra also occurs where light from a broad source is only partially blocked.

28 Opaque Materials In a solar eclipse, because of the large size of the Sun, the rays taper to provide an umbra (total eclipse) and a surrounding penumbra (partial eclipse). In a lunar eclipse, the Moon passes completely into the shadow of Earth.