16.1 Light Interference

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 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
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
16.3 Laser
LASER stands for
Amplification by
Emission of
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
• 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.flv
How lasers work (in theory) - YouTube
Class work – today’s date
The diagram shows straight wave fronts passing through an
opening in a barrier. This wave phenomenon is called
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
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 if
a. 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?
11.Which diagram best represents light emitted
from a coherent light source?
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
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
d. amplitude of the incident wave and the size of the

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