1. Chap 16-1 Light Fundamentals

2. What is Light? A transverse electromagnetic wave

3. What is Light? A small part of the electromagnetic spectrum The part which stimulates the retina of the human eye Visible light has wavelengths 400-700 nm RED Longer λ Lower f Lower E BLUE Shorter λ Higher f Higher E

4. How does light interact with matter? Light does not require matter (no medium) for transmission BUT if light does pass through matter Light travels in a straight line through a –Vacuum or a –Uniform medium

5. Ray Model A ray is a straight line that represents the path of a narrow beam of light ignores the wave nature of light a useful model in analyzing reflection and refraction (ray diagrams)

6. Speed of Light Before 17th century: light thought to be instantaneous Galileo: thought speed of light was finite but could not measure it Ole Roemer (~1700) –through measurements of the period of Io, a moon of Jupiter, –Calculated that light took 22 min to cross the diameter of Earth’s orbit –Calculated speed of light at 2.2 x 10 8 m/s (75% of true value)

7. Roemer’s Method for calculating the speed of light:

8. Albert Michelson Michelson (1926) made a more precise measurement for speed of light, c with the Michelson-Morely Experiment:

9. Michelson Won the Nobel Prize Successfully measured the speed of light c = 3.00 x 10 8 m/sin a vacuum this is a “defined value” for light Now objects lengths are determined by how long it takes for light to travel from one end to the other – Examples: Definition of the meter Light year

10. Sources of Light A luminous body emits light waves. An illuminated body reflects light waves produced by an outside source. Incandescent: light produced by a hot body.

11. Luminous Flux The rate at which visible light is emitted by a source Represented by the letter P The unit is the lumen, abbreviated lm A typical 100 W light bulb emits 1750 lm

12. Illuminance The rate at which light falls on a surface Represented by the letter E The unit is lumens per square meter, lm/m 2 = lux, abbreviated lx

13. Consider a 100-W light bulb in the middle of a sphere. What is the illumination (illuminance) of the sphere?

14. Note that illumination is proportional to 1/r 2 Inverse-Square Relationship: Light from a point source spreads out over an area proportional to the square of the distance from the source:

16. Luminous Intensity the luminous flux that falls on 1 m 2 of a sphere 1m in radius. Unit: candela, cd which is the SI unit of light intensity. Luminous Intensity = P/4 π

17. 6.A lamp is moved from 30 cm to 90 cm above the pages of a book. Compare the illumination on the book before and after the lamp is moved.

18. 7. What is the illumination on a surface 3.0 m below a 150 Watt incandescent lamp that emits a luminous flux of 225 lm?

19. 9.A 64 cd point souce of light is 3.0 m above the surface of a desk. Wht is the illumination on the desk’s surface in lux?

20. 10.The illumination on a tabletop is 20 lx. The lamp providing the illumination is 4.0 m above the table. What is the intensity of the lamp?

21. 11.A public school law requires a minimum illumination of 160 lx on the surface of each student’s desk. An architect’s specifications call for classroom lights to be located 2.0 m above the desks. What is the minimum luminous flux the lights must deliver?

22. End 16-1