1. Light
Physics Mr. Berman

2. Electromagnetic Waves: oscillating electric and magnetic fields.

3. Electromagnetic Waves
Do not require a medium.
Can travel through space (vacuum). Ex.: Light from the sun reaches us.
Transverse

4. Electromagnetic Waves
Type of radiation Wavelengths(m)
Radio TV
Microwave
Infrared radiation
Visible Light 700x x10-9
Ultraviolet
X-rays
Gamma rays shorter than 10-12

5. Visible Light is detected by the human eye.
red 700 x (700nm)
orange
yellow
green 550 x 10-9
blue
indigo
violet 400 x (400nm)

6. Electromagnetic Spectrum

8. Speed of Light, c
All EM waves have a speed through vacuum of 3 x 108 m/sec.
c= f λ

9. Problem
If a light wave’s frequency is 5.45 x10 14 Hz, what is its wavelength?
What color light is it?
Answer: 550x 10-9 m, green

10. Calculating the Speed of Light
Danish Astronomer, Ole Roemer (~1668) found that the period of the moon of Jupiter , Io, varied depending on the location of the Earth. Distances were known.
Estimated speed
of light:
2.2 x108 m/s

11. Albert Michelson, American (1880’s)
Measured time for light to make a round trip in California mountains 35km apart.
Measured speed of light:
x108 m/s
First American to win Nobel prize.

12. Speed of Light (1983)
Seventeenth General Congress on Weights and Measures measured: x 108 m/s We round to 3 x 10 8 m/s

13. Luminous Objects emit light.
Illuminated Objects reflect light rays.

14. Luminous Flux, P Rate at which light is emitted from a source.
Unit: lumen, lm

15. Illuminance, E, measures illumination of a surface.
E = (Luminous flux P) (area around source)
E= P / (4πd2 )
d : distance
from source
Unit: Lumens/ m2=lux=lx

16. Problem
A lamp is moved from 20cm to 40cm from a book. Compare the illumination before and after.

17. Candela (candle power)
Luminous intensity of a point source is the luminous flux that falls on 1m2 of a sphere of 1 m radius.
cd = Luminous flux/π
One of the fundamental SI units