2. 27 Light Light is the ONLY thing you see! All visible objects either emit or reflect light.

3. 27 Light Almost everything we see is made visible by the light it reflects. Some materials, such as air, water, or window glass, allow light to pass through. Other materials, such as thin paper or frosted glass, allow the passage of light in diffused directions so that we can’t see objects through them.

4. 27 Light Einstein visualized particles of light as concentrated bundles of electromagnetic energy. Max Planck had proposed that atoms do not emit and absorb light continuously, but do so in little chunks. Each chunk was considered a quantum, or a fundamental unit. 38.2 Light Quanta

5. 27 Light Planck believed that light existed as continuous waves, but that emission and absorption occurred in quantum chunks. Einstein went further and proposed that light itself is composed of quanta. One quantum of light energy is now called a photon.` 38.2 Light Quanta

6. 27 Light The energy of a photon of light is proportional to its vibrational frequency. When the energy E of a photon is divided by its frequency f, the quantity that results is known as Planck’s constant, h. This quantity is always the same, no matter what the frequency. The energy of every photon is therefore E = hf. This equation gives the smallest amount of energy that can be converted to light of frequency f. 38.2 Light Quanta

7. 27 Light Light travels VERY FAST – around 300,000 kilometres per second Or 3x10 8 m/s At this speed it can go around the world 8 times in one second.

8. 27 Light We see things because they reflect light into our eyes: Homework

9. 27 Light Scientists now agree that light has a dual nature, part particle and part wave. 27.1 Early Concepts of Light

10. 27 Light Light has been studied for thousands of years. Some ancient Greek philosophers thought that light consists of tiny particles, which enter the eye to create the sensation of vision. Others thought that vision resulted from streamers or filaments emitted by the eye making contact with an object. 27.1 Early Concepts of Light

11. 27 Light Up until the time of Newton and beyond, most philosophers and scientists thought that light consisted of particles. However, one Greek, Empedocles, thought that light traveled in waves. One of Newton’s contemporaries, the Dutch scientist Christian Huygens, also argued that light was a wave. 27.1 Early Concepts of Light

12. 27 Light The particle theory was supported by the fact that light seemed to move in straight lines instead of spreading out as waves do. Huygens showed that under some circumstances light does spread out and other scientists found evidence to support the wave theory. The wave theory became the accepted theory in the nineteenth century. 27.1 Early Concepts of Light

13. 27 Light In 1905, Einstein published a theory explaining the photoelectric effect. According to this theory, light consists of particles called photons, massless bundles of concentrated electromagnetic energy. Scientists now agree that light has a dual nature, part particle and part wave. 27.1 Early Concepts of Light

14. 27 Light What is the nature of light? 27.1 Early Concepts of Light

15. 27 Light What is light really? Electromagnetic radiation waves The light intensity is from “Amplitude.” The distance from one crest to the next crest is called a “wavelength.” The number of waves passing a given point in one second is called the “frequency.”

16. 27 Light The electromagnetic spectrum consists of radio waves, microwaves, infrared, light, ultraviolet rays, X-rays, and gamma rays. 27.3 Electromagnetic Waves

17. 27 Light Light is energy that is emitted by accelerating electric charges—often electrons in atoms. This energy travels in a wave that is partly electric and partly magnetic. Such a wave is an electromagnetic wave. 27.3 Electromagnetic Waves

18. 27 Light Light is a portion of the family of electromagnetic waves that includes radio waves, microwaves, and X-rays. The range of electromagnetic waves is called the electromagnetic spectrum. 27.3 Electromagnetic Waves

19. 27 Light Remember radio waves are long…and gamma rays are small

20. 27 Light The lowest frequency of light we can see appears red. The highest visible light, violet, has nearly twice the frequency of red light. Electromagnetic waves of frequencies lower than the red of visible light are called infrared. Heat lamps give off infrared waves. Electromagnetic waves of frequencies higher than those of violet are called ultraviolet. They are responsible for sunburns. 27.3 Electromagnetic Waves

21. 27 Light TED TALK – Slow Motion LightSlow Motion Light

22. 27 Light What are the waves of the electromagnetic spectrum? 27.3 Electromagnetic Waves

23. 27 Light Electromagnetic Radiation Electromagnetic radiation can be described in terms of a stream of photons. Each photon is traveling in a wave-like pattern, moving at the speed of light and carrying some amount of energy.lectromagnetic radiationphotons. speed of light The only difference amongst radio waves, visible light, and gamma-rays is the amount of energy of the photons. Radio waves have photons with low energies. Microwaves have a little more energy than radio waves. Infrared has still more energy.RadioMicrowavesInfrared

24. 27 Light Radio (Longest electromagnetic waves) Emitted by –Astronomical Objects –Radio Station Transmitters Detected by –Ground based radio telescopes –*If you turn on a radio, it will convert the radio wave energy into sound energy.

25. 27 Light Television Shorter than radio, also used to carry messages (pictures & sound) to our TV sets. *We can sense the TV waves around us with our televisions.

26. 27 Light Microwave Emitted by: –Gas clouds collapsing into stars –Microwave Ovens –Radar Stations –Cell Phones Detected by –Microwave Telescopes –Food (heated) –Cell phones –Radar (systems)

27. 27 Light Infrared (Heat or Thermal) Are you a source of infrared? YES you are! Emitted by –Sun and stars (Near) –TV Remote Controls –Food Warming Lights (Thermal) –*Everything at room temperature or above,=HEAT Detected by –Infrared Cameras –TVs, VCRs, –Your skin