Mechanics
Lab report
Date:25-4-2022
Group A4
Group members:
(i) Muhammad Zeeshan Ayyub khan
(ii) Mian Muhammad Bilal
(iii) Muhammad Asad Saeed
Experiment no.1
Reflection of waves in ripple tank
Abstract:
The purpose of this activity is to study the reflection of a plane wave from
different shaped barriers, a long straight barrier, and a curved barrier.
Theory:
A ray is a line that indicates the direction of
motion of a plane wave. Wave fronts are
perpendicular to the ray. When a wave
reflects from a surface, the angle of
incidence is the angle between the
incoming (or incident) ray and the normal
(a line perpendicular to the surface). The
angle of reflection is the angle between the
outgoing (reflected) ray and the normal.
A. Reflection using straight barrier:
Procedure:
I. Arrange the long barrier in the middle of the tank so the barrier is at an
angle to the plane wave dipper (see Figure 1.2).
II. Turn on the ripple
generator and the
light source. Set the
light source to
‘STROBE’. Set the
ripple generator
frequency to 20 Hz.
Set the amplitude to
slightly less than half
of maximum.
III. On the paper below
the tank, place the
ruler parallel to the plane waves that are incoming to the barrier. Make
a line to show the incoming wave front.
IV. Place the ruler parallel with a reflected wave and again make a to show
outgoing (reflected) wave front.
V. Trace the position of the straight barrier.
VI. Turn off the ripple generator and light source.
Observations and calculations:
I. Draw a line that is perpendicular to the incoming wave front and
extend the line to the outline of the straight barrier. This represents
the incoming ray, so draw an arrow on it pointing to the barrier.
II. Draw a line from the point where the incoming ray intersects the
straight barrier, so it crosses the reflected wave front at a right angle.
This represents the reflected ray, so draw an arrow on it pointing away
from the barrier.
III. Draw the normal (perpendicular) line at the point of reflection on the
outline of the straight barrier.
IV. Measure the angle of incidence and the angle of reflection and record
the measurements in the table.
V. Repeat the procedure with the barrier at a different angle.
For such reflection, θi=θr
From experiment, θi=41
θr =43
θi−θr
Percentage error¿ ×100
θi
41−43
¿ × 100
41
¿ 4.87 %
Questions:
Q1: What is the relationship of the angle of incidence and the angle of reflection?
Answer: Angle of incidence is equal to the angle of reflection
B. Reflection using a
curved barrier:
Procedure:
I. Replace the straight
barrier with the curved
barrier and position the
curved barrier so it is aligned ‘parallel’ to the plane wave dipper as
shown in Figure 1.3.
II. Turn on the light source. Trace the position of the curved barrier on the
paper below the ripple tank.
III. Turn on the ripple generator.
Observations and calculations:
I. Use a drawing compass to complete the traced circular shape of the
curved barrier. Mark the center of the circle and measure the radius.
For such reflection, Radius of curvature = R = 2(focal point)
From experiment, f = 7.2 cm
Rthoeratical =2 ×7.2=14.4 cm
Rexperimental =14.1 cm
R thoeratical – R experimental
Percentage error¿ ×100
Rthoeratical
14.4−14.1
¿ ×100
14.4
¿ 2.08 %
Questions:
Q1: What is the shape of the wave fronts that
reflect from the curved barrier when you dropped
the droplet of water into the ripple tank?
Answer: Upon reflection off the curved barrier,
the water waves will change direction and head
towards a point. It is as though all the energy
being carried by the water waves is converged at a
single point - the point is known as the focal point.
Q2: How is the radius of the circle related to the distance between the curved barrier and the
point where the reflected plane waves from the plane wave dipper appeared to converge?
Answer: The point where waves appear to converge is called focal point. Focal point and radius
of curvature are related as
R = 2f
It means that radius of curvature is 2 times the focal point of the reflected waves.
______________________
Experiment no.2
Refraction of waves in ripple tank
Abstract:
The purpose of this activity is to show how waves change direction as they pass from one region
to another where the wave speed is different.
Theory:
As a wave travels from one medium to another where the
wave speed is different, the wave bends to a new direction.
If the wave slows down, the wave will bend toward the
normal of the interface between one medium and the other
as shown in Figure 2.1. This bending is called refraction.
Procedure:
I. Arrange the trapezoidal refractor in the water in the middle of the tank so the rectangular
end of the refractor is parallel to the plane wave dipper and about 5cm from the dipper
(see Figure 2.2).
II. Add just enough water to the tank so that the refractor is evenly covered by less than
1mm of water.
III. Turn on the ripple generator and the light source. Set the light source to ‘STROBE’. Set
the ripple generator frequency to 15 Hz or less. Set the amplitude to slightly less than half
of maximum and adjust it as necessary to make a clear pattern of plane waves.
IV. On the paper below the tank, trace the outline of the trapezoidal refractor
V. Place the ruler parallel to
the plane waves that are
incoming to the refractor.
Sketch lines to show the
incoming wavefronts.
VI. On the outline of the
refractor, trace the shapes
of the refracted waves to show the bending of the refracted waves as they travel over the
refractor.
VII. After sketching the waves, reverse the trapezoidal refractor so that the triangular end of
the refractor points toward the plane wave dipper and repeat the procedure.
VIII. Turn off the ripple generator and light source.
Observations and calculations:
I. Draw a line that is perpendicular to the incoming wave front and extend the line to the
outline of the trapezoidal refractor. This represents the incoming ray, so draw an arrow on
it pointing to the refractor.
II. At the point where the line representing the incoming ray meets the outline of the
refractor, draw a new line that is perpendicular to the wave fronts of the refracted waves
as they pass over the trapezoidal refractor.
sin θi
Refractive index¿ η=
sin θ r
sin 56
¿ =1.31
sin 39
Questions:
Q1: What happens to the direction of the wave fronts as they move over the
trapezoidal refractor?
Answer: The direction of the wave fronts changes as they move over the trapezoidal refractor,
and this is due to the change in speed.
Q2: As the plane wave from the deep water moves through the shallower water over the
refractor, does the plane wave speed up or slow down?
Answer: The waves get slower after the refraction.
_____________________
