Speed of light

2. Speed of light In a vacuum, light travels at a speed of 3 x 108 m/s. In any other medium, such as air, water, glass, etc., light travels slower.

3. Model of a car If a car travels from pavement to gravel, it slows down. Suppose that it is incident on the pavement/gravel interface at an angle. One tire slows down before the other tire hits the gravel. As a result, the axle rotates.

4. Plane wave incident on glass

5. Bending of light at an interface of two mediums If slowing down (v2 < v1), the ray bends toward the normal.

6. Bending of light at an interface of two mediums If speeding up (v2 > v1), the ray bends away from the normal.

7. Wavelength The medium affects the wavelength of the wave, but not the frequency. The slower the medium, the shorter wavelength.

8. Refraction Refraction is the bending of light toward the normal or away from the normal as a result of speeding up or slowing down as the light travels from one medium to another medium. The ratio of the speed of light in vacuum to the speed of light in the medium is called the index of refraction, n.

9. Poll In which material does light travel slowest? vacuum air (at STP) water glass

10. Poll Suppose that light of wavelength 600 nm in vacuum is incident on the following materials. In which material will the wavelength be shortest? vacuum air (at STP) water glass

11. Snell’s Law Angle of refraction is given by Snell’s Law. The larger the difference in n, the larger the difference in angles.

12. Low index to high index If light travels from a medium of low index tohigh index, it bends toward the normal. If light travels from a medium of high index tolow index, it bends toward the normal.

13. Example Laser light in air strikes a flat surface of glass at the angle shown. On the other side of the glass is water. At what angle does it travel through the water? 30

14. If a ray now strikes the sapphire from outside in the direction shown, it will ____________. A. go in at an angle q2 > 62.3° B. go in at an angle q2 between 62.3° and 30° C. go in at an angle q2 = 30° D. go in at an angle q2 < 30° E. be reflected back into the air 62.3° If a ray strikes the surface of a cut sapphire from within at an incident angle of 30°, it passes into the air at an angle q2 = 62.3°. All angles are measured from the normal. 62.3° 30° Show in Slide Show view to have question appear in two parts.

15. Above are four rays of light from the same monochromatic laser source. Each ray passes from one medium to another. Water is always one of the two media; the other media (A, B, C, and D) may be different. Which of the following correctly compares the index of refraction of the media? D.nA > nC > nB = nD E. nB = nC = nD > nA A.nB = nD > nC > nA B.nC > nD = nB > nA C.nD > nB = nC > nA

16. qr = 60 qi = 30 The diagram shows a ray passing from the blue medium into the pink medium. If the incident angle qi is doubled, the refracted angle qr will be ______________. A. doubled B. less than doubled C. more than doubled D. none of the above

17. Total internal reflection n1 > n2 increase n1 until the refracted angle=90

18. Critical Angle the incident angle is called the critical angle

19. Example

20. Total Internal Reflection (Example) Can light passing from crown glass into air be totally internally reflected? If so, what is the critical angle? n1=1.52 n2=1 YES!!! since n1>n2 Crown Glass Air

21. Poll If light from air is incident on glass, it can not refract at the critical angle. it may refract at the critical angle and not be transmitted through the glass.

22. Dispersion The index of refraction n depends on the frequency (i.e. color) of the light. As a result, different colors bend different amounts as they pass through glass or water, for example. If white light is incident on glass or water, then the different frequencies bend different amounts and the white light separates into its component colors. this effect is called dispersion.

23. Poll Which will have a greater angle of refraction, red or violet light? red violet neither, they bend the same amount

24. Converging Lens All rays parallel to principal axis pass through focal point F. Double Convex F P.A. nlens > noutside

25. Converging Lens Image F P.A. Object F 1) Rays parallel to principal axis pass through focal point. 2) Rays through center of lens are not refracted. 3) Rays through F emerge parallel to principal axis. Image is:real, invertedandenlarged(in this case).

26. 3 Cases Image Object Image Image Object Object Past 2F Inverted Reduced Real This could be used in a camera. Big object on small film Between F & 2F Inverted Enlarged Real This could be used as a projector. Small slide on big screen Inside F Upright Enlarged Virtual This is a magnifying glass

27. Lens Equation f In Front of Lens Behind Lens do F Image P.A. Object F di • so = distance object is from lens: • Positive: object in front of lens (this is usually the case) • Negative: object behind lens • si = distance image is from lens: • Positive: real image (behind lens) • Negative: virtual image (in front of lens) • f = focal length lens: • Positive: converging lens • Negative: diverging lens

28. Example A 2.0 cm high object is placed 5 cm in front of a +10 cm focal length lens. Where is the object located? Is it real or virtual? Find the height of the image. Image F P.A. Object F The image is upright, virtual, and enlarged!

29. Example A 4.0 cm high object is placed 18 cm in front of a +8 cm focal length lens. Where is the object located? Is it real or virtual? Find the height of the image. F Object F