16.1 Light Interference

1. Describe how light waves interfere with each other to produce bright and dark fringes

2. Identify the conditions required for interference to occur

Interference • Interference happens when two waves meet.• Constructive interference happens when two waves are in

phase (0o, or λ) results a bigger amplitude.– Sound: louder– Light: brighter

• Destructive interference happens when two waves are in phase (180o, or ½ λ) results smaller/ no amplitude.– Sound: complete silence– Light: darkness

• The example of sound wave interference is beats• The example of light wave interference is bright and dark

bands.

Monochromatic Interference• Monochromatic means one color• When same color of light meets, they can interfere either

constructively or destructively.

Constructive interference Destructive interference

• Waves must be completely in phase or completely out of phase for interference to be observed.

The bright and dark bands observed on the screen is the result of constructive and destructive interference.

16.2 diffraction• Diffraction is the spreading of waves into a region

behind an obstruction.

All waves diffracts• Water waves• Sound waves• Light waves

How much diffraction?• The amount of diffraction is

determined by the how the wavelength and the size of opening of the barrier compare.

• When the opening is comparable to the wavelength, most diffraction occurs

• When the opening is much larger than the wavelength, diffraction is less.

16.3 Laser

• LASER stands for • Light • Amplification by • Stimulated • Emission of • Radiation • The emitted laser light is monochromatic and

coherent, narrow low diverging beam

Monochromatic and coherent• Monochromatic means one color• Coherent light are light waves that are "in phase" with one

another.• For example, two waves are coherent if the crests of one wave

are aligned with the crests of the other and the troughs of one wave are aligned with the troughs of the other. Otherwise, these light waves are considered incoherent.

stimulated emission• When a photon (light particle) hits an atom that is already

excited, the atom releases a new photon that is completely identical to the incoming photon; same color, going in the same direction. We call this process "stimulated emission".

How lasers work (in theory) - YouTube.flvHow lasers work (in theory) - YouTube.flv

How lasers work (in theory) - YouTube

Class work – today’s date1. The diagram shows straight wave fronts passing through an opening

in a barrier. This wave phenomenon is calleda. reflection b. refraction c. polarization d. diffraction

2. Which wave phenomenon makes it possible for a player to hear the sound from a referee’s whistle in an open field even when standing behind the referee?

a. diffraction b. Doppler effect c. reflection d. refraction

5. The diagram shows a wave phenomenon. The pattern of waves shown behind the barrier is the result of

a. reflection b. refraction c. diffraction d. interference

6. A source of waves and an observer are moving relative to each other. The observer will detect a steadily increasing frequency ifa. he moves toward the source at a constant speed b. the source moves away from him at a constant speed c. he accelerates toward the source d. the source accelerates away from him

7. Which diagram best illustrates wave diffraction? 8. Which diagram best illustrates wave reflection? 9. Which diagram best illustrates wave refraction? 10. Which diagram best illustrates a sound wave?

A B

C D

example11.Which diagram best represents light emitted

from a coherent light source?

A B

DC

12. What is one characteristic of a light beam produced by a monochromatic laser?

a. It consists of coherent waves. b. It can be dispersed into a complete continuous spectrum. c. It cannot be reflected or refracted. d. It does not exhibit any wave properties.

13. Which term best describes the light generated by a laser?a. diffused b. coherent c. dispersive d. longitudinal

14. A laser beam does not disperse as it passes through a prism because the laser beam is

a. monochromatic b. polychromatic c. polarized d. Longitudinal

15. A car radio is tuned to the frequency being emitted from two transmitting towers. As the car moves at constant speed past the towers the sound from the radio repeatedly fades in and out. This phenomenon can best be explained by

a. refraction b. interference c. reflection d. resonance

16. A wave is diffracted as it passes through an opening in a barrier. The amount of diffraction that the wave undergoes depends on both the

a. amplitude and frequency of the incident wave b. wavelength and speed of the incident wave c. wavelength of the incident wave and the size of the

opening d. amplitude of the incident wave and the size of the

opening