Finish spectrum, visible light, energy of EM waves

1. Finish spectrum, visible light, energy of EM waves

3. Radios: How do they work? • AM radio: AM stands for amplitude modulation. • Set frequency but radio station varies the amplitude • Problems • Lots of natural and man-made noise. • Weak signals are quieter than strong ones.

4. Radios: How do they work? • FM radio: FM stands for frequency modulation. • Set amplitude but radio station varies the frequency

5. Microwave ovens • Water. There are trillions of water molecules in a single drop. The temperature of the water depends on the motion of the molecules. They move with respect to each other and vibrate. • The hotter the water, the more activity.

6. Microwaves continued • Water molecules absorb microwaves at a certain frequency. This extra energy causes an increased amount of vibrations in the molecules. • As the water molecules vibrate, they bump into other molecules, transferring energy or heat.

7. Visible Light • A few things you need to know: • For Violet light, λ= 400 nm • For red light, λ= 700 nm • Example: Compare the frequency of red light compared to violet light.

8. Visible Light • c=fλ Plug in the numbers • Red λ= 700 nm = 700*10-9 m • Violet λ= 400 nm = 400*10-9 m • C=3.00*108 m/s • Frequency of Red light=4.3*1014 Hz • Frequency of Violet light=7.5*1014 Hz • Notice the trend, Violet has a higher frequency than Red

9. Light Particle • Photon—A discrete unit of light energy • A photon is “localized energy”

10. Energy of a Photon • E=hf • Energy = constant * frequency • h=Planck’s constant=6.626*10-34 J * Sec

11. Example • What is the energy of a photon of green light? Wavelength = 550nm • The equation is E=hf. First, find the frequency of green light. • λ=550 nm = 550*10-9 m • C=f λ, f=5.45*1014Hz • E=hf, h=6.62*10-34Jsec • E=3.60*10-19 J

12. Example 2 • If the energy of a photon is 2.5*10-18 J, what is the frequency of that photon? What is the wavelength? • Answer: Frequency =3.77*1015 Hz • Wavelength = 7.944*10-8 m