General
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UNIT 4:PROPAGATION OF MECHANICAL WAVES
Materials: Two torches of the same intensity, Screen, material with two
small slits and material with big slits.
Procedures:
Arrange the materials as shown in the illustration above following the
procedures to complete the investigation:
a) The first student at position A switched on the torch and light passed
through one slit. What do you think is the nature of image(s) observed on
the screen by second student at position B or C?
b) Explain what causes the nature of the image(s) observed on the screen.
c) Assuming the first Student at position A used two torches giving light of
same intensities torching on two slits simultaneously, would image(s) on
the screen be identical as observed in (a) above. Explain to justify yourobservation.
d) Now, if small slits are replaced with ones of big holes (widened slits).
Explain what this change will have on the images formed on the screen.
e) Explain why do we not ordinarily observe wave behaviour for light, such
as observed in Young’s double slit experiment?f) Explain how this experiment is significant in real life
4.1.1. Coherent sources
Coherent sources are those which emit light waves of the same wavelength or
frequency which are always in phase with each other or have a constant phase
difference. Two coherent and monochromatic sources can together produce
the phenomenon of interference.
When light passes through a slit with a size that is close to the light’s wavelength,the light will diffract, or spread out in waves.
Interference is a phenomenon in which two waves superpose(meet) to form a
resultant wave of greater, lower, or the same amplitude.
Young’s method for producing two coherent light sources involves illuminating
a pair of slits with a single source. Another arrangement for producing aninterference pattern with a single light source is known as Lloyd’s mirror.
A point light source is placed at point S close to a mirror, and a viewing screen
is positioned some distance away and perpendicular to the mirror. Light waves
can reach point P on the screen either directly from S to P or by the path
involving reflection from the mirror.
An interference pattern is produced at point P on the screen as a result of the
combination of the direct ray (blue) and the reflected ray (brown). The reflected
ray undergoes a phase change of 180°.
In order to observe interference in light waves, the following conditions must
be met:
• The sources must be coherent—that is, they must maintain a constant
phase with respect to each other.
• The sources should be monochromatic—that is, of a single wavelength.
• The interfering waves Must Obey the Principal of superposition.
As an example, single-frequency sound waves emitted by two side-by-side
loudspeakers driven by a single amplifier can interfere with each other because
the two speakers are coherent—that is, they respond to the amplifier in the
same way at the same time.
If two light bulbs are placed side by side, no interference effects are observed
because the light waves from one bulb are emitted independently of those
from the other bulb. The emissions from the two light bulbs do not maintain
a constant phase relationship with each other over time. Light waves from an
ordinary source such as a light bulb undergo random phase changes in time
intervals less than a nanosecond. Such light sources are said to be incoherent.
When light passes through two or slits, the waves from one slit will interfere
with the waves from the other:
• Constructive interference occurs when two crests or two troughs meet
forming a wave with a larger crest or lower trough.
• Destructive interference occurs when a crest meets a trough cancelling
each other to produce a smaller wave or no wave at all.
4.1.2. Principle of superposition
The principle of superposition states that when two or more waves meet at a
point, the resultant displacement at that point is the vector sum of the individual
displacement of each wave
4.2.1. Concept of stationary wave
Standing wave also known as a stationary wave, is wave pattern that results
when two waves of the same frequency; wavelength and amplitude travelling in
opposite directions in the same medium interfere or meet.
The point at which the two waves cancel are called node. There is no motion
in the string at the nodes, but midway between two adjacent nodes, the string
vibrates with the largest amplitude. These points are called antinodes. Atpoints between successive nodes the vibrations are in phase.
