Intensity and Color

1. Intensity and Color

2. Intensity • The brightness of a light source is called its luminous intensity (IL). • It is measured in candelas (cd).

3. Intensity • defined: the luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency 540 × 1012 Hz and that has a radiant intensity in that direction of 1/683 watt per steradian

4. Intensity • Steradian: a conical solid angle with its vertex at the center of a sphere that cuts off a circular area on the surface of the sphere equal to the square of the sphere’s radius

5. Intensity • A high-wattage light bulb uses its power more efficiently than a low-wattage bulb.

6. Intensity • Luminance: rate of flow of light energy reaching a surface in a given direction from the source • measured at a point on an illuminated surface • units: cd/m²

7. Luminous Flux • Luminous flux (Φ) is the total amount of light that a source gives off • measured in lumens (lm), the product of candelas and steradians (cd·sr)

8. Luminous Flux • 1 lumen is the luminous flux of a 1 cd light source in 1 sr. • A 1 cd point light source has a total luminous flux of 4π ≈ 12.57 lm.

9. Luminous Flux • Luminous flux and luminous intensity are both measures of power.

10. Illuminance • the amount of light an object receives from a light source • abbreviated: E • Illuminance is directly proportional to luminous flux

11. Φ E = r² Illuminance • Illuminance is inversely proportional to the square of the distance from the source • units: lm/m² = lux (lx)

12. d²unk IL unk = IL std d²std Comparing Light Sources • Photometers are used to measure the intensity of a light source. • For one type of photometer:

13. Transmittance • defined: the ratio of the transmitted luminous flux to the incident flux • Transparent materials allow most light through, allowing us to see clearly through them.

14. Transmittance • Translucent materials transmit light but distort it so that we cannot see clearly through them. • Opaque materials do not transmit visible light.

15. Color • What you “see” as color is actually different frequencies of light. • directly from a source • reflection from a surface • transmission through a material

16. Color • The light’s frequency is related to the energy change of the electrons by the equation E = hf • f is the frequency • h is Planck’s constant • 6.626 × 10-34 J·s

17. Color • How color appears to us is not only affected by the wavelength, but also by: • Hue • Saturation • Brightness

18. Color • There are many ways to model the spectrum of colors seen by our eyes. • An object’s color also depends on the colors of surrounding objects.

19. Additive Color Mixing • The additive primary colors are red, green, and blue. • Combinations of these three colored lights can produce any color in the spectrum.

20. Additive Color Mixing • No combination of additive primary colors can produce black. • Only the total absence of light is perceived as black.

21. Colored Objects Why does a red object appear red? • The light coming from it to your eyes is red. • Three reasons why the light might be red:

22. Colored Objects • It may be illuminated only by red light. • It may reflect only red light (absorbs all other colors). • If not opaque, it may transmit only red and absorb all other colors.

23. Colored Objects • Pigment: the substance in an object that absorbs certain colors and reflects others

24. Subtractive Color Mixing • The subtractive primary colors are cyan, magenta, and yellow. • This governs the color of reflected light, since some colors will be absorbed and others will not.

25. Subtractive Color Mixing • No mixture of the subtractive primary colors can produce white light.

26. Optical Instruments

27. The Microscope • magnifies minute objects • Anton van Leeuwenhoek • double convex lens with a short focal length mounted in a frame • simple, yet capable

28. The Microscope • Compound microscope • two short-focal-length converging lenses • objective lens • ocular (eyepiece)

29. The Microscope • Binocular microscope • Stereo microscope • Some have film, cameras, or projection capabilities.

30. The Telescope • Telescopes make distant objects appear nearer. • Hans Lipperhey • Galileo • objective lens and an eyepiece • very narrow field of view

31. The Telescope • Refracting telescopes today use two double convex converging lenses. • These telescopes are described by the diameter of their objective lens.

32. θI fo M = = θO fe The Telescope • Magnifying power:

33. The Telescope • Refracting telescopes have a practical limit to the size of their lenses. • Most of the largest refracting telescopes have lenses about 1 m in diameter.

34. The Telescope • Reflector telescopes can use enormous mirrors because they can be supported. • can be several meters in diameter • NiccolòZucchi • James Gregory

35. The Telescope • Isaac Newton’s reflector telescope solved the problem of chromatic aberration. • Light also has to be reflected “out of the tube.” • Newtonian reflector

36. The Telescope • Cassegrainian telescopes have an eyepiece behind the main mirror, which has a hole in it. • Schmidt-Cassegrainian telescopes removed the spherical aberration problem.

37. The Telescope

38. The Telescope • Why do astronomers prefer larger telescopes? • A larger telescope gathers more light and produces a brighter image. • A larger telescope has better resolution.

39. The Telescope • Why do astronomers prefer larger telescopes? • A larger telescope can magnify finer details better than a smaller telescope.