Sound and Light

1. Sound and Light Physical Science Chapter 16

2. Sound • Produced by the vibration of objects. • The energy from the vibrations is carried through a medium. • Sound wave spread out in all directions from the source of the sound.

3. Sound waves • Longitudinal waves. • Need a medium to travel through.

4. Sound waves • Travel faster through media where the particles are closer together. • Particles can transmit the motion faster when they are closer together • Travel faster when the temperature is higher. • Particles move faster when they are warmer

5. Intensity • Depends on the amount of energy in each wave. • Depends on the amplitude of the wave

6. Loudness • The human perception of the intensity of sound waves • Measured in decibels (dB) • 0 dB is the softest sound most people can hear. • 120 dB is the pain threshold – can quickly damage your hearing

7. Pitch • How high or low a sound sounds • Depends on frequency • Higher pitches have higher frequencies • Most people can hear sounds from 20 Hz to 20 000 Hz. • Ultrasound – above 20 000 Hz • Infrasound – below 20 Hz

8. Discuss Which two properties of a sound wave change when the pitch gets higher? Which two properties of a sound wave change when the sound gets louder What are two factors that affect the speed of sound?

9. Natural frequency • Most objects have one. • The frequency it naturally vibrates at. • When you pluck a guitar string, it produces a standing wave at its natural frequency.

10. Resonance • When an object is subjected to a sound vibrating at its natural frequency it begins to vibrate. • This amplifies the sound.

11. Hearing Sound waves make the eardrum vibrate. Vibrations pass through three small bones – hammer, anvil, and stirrup. Stirrup sends waves to cochlea and basilar membrane. Hairs near the membrane send an impulse through nerves to the brain.

12. Ultrasound imaging • Ultrasound waves can travel through most materials, but some are reflected at a boundary between different materials. • Reflected waves (echoes) can be made into a computer image called a sonogram. • In order to see details, the wavelength must be smaller than the smallest parts of the object being viewed.

13. Sonar • Uses reflected ultrasound waves to determine distance. • Measures the time it takes for the waves to come back. • Used to calculate ocean depth and detect fish or submarines • Used by bats to “see” their surroundings and find food.

14. Discuss To create sonograms, why are ultrasound waves used instead of audible sound waves?

15. Is light a wave or a particle? • Newton -- particles. • In the early 19th century, Young, Fresnel, and others -- wave. • In 1860 Maxwell -- electromagnetic wave.

16. Photoelectric effect • 19th century -- Hertz -- shining light on a metal plate would make it emit electrons – producing an electric current. • Kinetic energy of the emitted electrons was independent of the intensity of the light. • Didn’t fit with wave theory

17. Photons • In 1905 Einstein proposed that light comes in small bundles called photons. • The energy of a photon depends on the frequency of the light, not the intensity.

18. Wave Particle Duality • In some situations, light behaves like a wave. • In other situations, it behaves like a particle. • Bottom line – there probably is a single model that works all the time, but no one has figured it out yet.

19. Speed of light • In a vacuum (or in air) 3 x 108 m/s • Nothing in the universe goes faster than this • Slower in media where the particles are closer together

20. Intensity of light • Determines brightness • Brighter light has higher intensity

21. Electromagnetic waves • All types travel the same speed in a given medium. • Wavelengths and frequencies vary. • Short wavelengths have high frequencies. • Long wavelengths have low frequencies.

22. Radio waves • Long wavelength and low frequency • Low energy

23. Microwaves • The highest energy (highest frequency) radio waves.

24. Infrared radiation • Wavelength slightly longer than visible light. • Just outside the red of visible light. • Felt as heat.

25. Visible radiation • The light we see. • White light is a combination of all the colors

26. Ultraviolet radiation • Higher frequency than visible light • More energy • More penetrating • Just outside the violet of the visible spectrum

27. X Rays • Higher frequency than UV rays • Greater penetrating power • Can go through skin and muscles, but not bone • Harmful in large doses

28. Gamma rays • Emitted from radioactive atoms • Come to Earth from space • Highest frequency (and energy) • Most penetrating

29. Reflection • When a wave strikes an object and bounces off. • See diagram on page 561 • To study reflection, we draw a normal to the surface. • Normal means perpendicular

30. Incident and reflected waves • Incident wave – before it hits the surface • Reflected wave – after it reflects off the surface

31. Law of Reflection • The angle of reflection equals the angle of incidence. • Angles are measured from the normal, not the surface.

32. Plane mirrors • One flat surface • Image is behind the mirror and upright • Image is same size as object • Image is same distance from mirror as object • Image is virtual • No light rays pass through it

33. Concave mirrors • Surface is curved inward • Form images differently • Depends on how far in front of the mirror the object is

34. Focal point • Light rays parallel to the optical axis are all reflected through the focal point. • The distance from the center of the mirror to the focal point is the focal length.

35. Concave mirrors • If the object is farther from the mirror than the focal point, the image is • larger • Inverted • Farther in front of the mirror than the object

36. Concave mirrors • If the object is closer to the mirror than the focal point, the image is • larger • virtual • upright • Behind the mirror

37. Concave mirror • If the object is at the focal point, no image is formed.

38. Convex mirrors • Surface curves outward • Light rays parallel to axis always reflect as if they came from the focal point • Image is always • Virtual • Upright • Smaller than object • Behind mirror

39. Colors • Objects that appear blue reflect blue light and absorb all other colors • Objects that appear white reflect all colors • Objects that appear black absorb all colors

40. Discuss How is reflection from objects that appear blue different than reflection from objects that appear yellow?

41. Refraction • The bending of light waves caused by a change in their speed. • Entering a slower medium • bends towards the normal. • Entering a faster medium • bends away from the normal

42. Refraction

43. Mirages

44. Mirages

45. Convex lenses • Thicker in the middle • Parallel light rays are refracted towards the center • Images can be • Real or virtual • Upright or inverted • Larger or smaller

46. Concave lenses • Thicker at the edges • Bend light towards edges • Image is • Virtual • Smaller • Upright • In front of lens

47. Color • Red (longer wavelength) is refracted less than violet (shorter wavelength).

48. Rainbows • Rainbows are reflected light from water droplets in the air. • Different colors are refracted at different angles by the water, so they are separated.

49. Discuss Describe how a mirage is formed. If light traveled at the same speed in raindrops as it does in air, could rainbows exist? Explain your reasoning.