1.2 reflection Fizik SPM

Damping and Resonance

• 1 In an oscillating system such as the oscillation of

a simple pendulum, the oscillation does not

continue with the same amplitudes indefinitely.


• 2 The amplitude of oscillation of the simple

pendulum will gradually decrease and become

zero when the oscillation stops. The decrease in the

amplitude of an oscillating system is called

damping.


• 3 An oscillating system experiences damping

when its energy is drained out as heat energy.

• (a) External damping of the system is the loss of

energy to overcome frictional forces or air

resistance.


• (b) Internal damping is the loss of energy due to the

extension and compression of the molecules in the

system.


• 4 Damping in an oscillating system causes

• (a) the amplitude, and

• (b) the energy of the system to decrease.


• 4 Damping in an oscillating system causes

• (a) the amplitude, and

• (b) the energy of the system to decrease

• (c) the frequency, f does not change.


• 5 To enable an oscillating system to go on

continuously, an external force must be applied to

the system.


• 6 The external force supplies energy to the

system. Such a motion is called a forced oscillation.


• 7 The frequency of

a system which

oscillates freely

without the action

of an external force

is called the natural

frequency. g

lT 2

Tf

1


• 8 Resonance occurs when a system is made to

oscillate at a frequency equivalent to its natural

frequency by an external force. The resonating

system oscillates at its maximum amplitude.


• 9 The characteristics of resonance can be

demonstrated with a Barton's pendulum system as

shown in Figure 1.17.


• (a) When pendulum X oscillates, all the other pendulums are forced to oscillate. It is found that pendulum B oscillates with the largest amplitude, that is, pendulum B resonates.


• (b) The natural frequency of a simple pendulum

depends on the length of the pendulum. Note that

pendulum X and pendulum B are of the same

length. Therefore, pendulum X causes pendulum B

to oscillate at its natural frequency.


10Hz

8 Hz

10 Hz9 Hz

12 Hz

10 Hz8 Hz

12 Hz


• 10 Some effects of resonance observed in daily life:

• (a) The tuner in a radio or television enables you to select the programmes you are interested in. The circuit in the tuner is adjusted until resonance is achieved, at the frequency transmitted by a particular station selected. Hence a strong electrical signal is produced.


• 10 Some effects of resonance observed in daily

life:

• (b) The loudness of music produced by musical

instruments such as the trumpet and flute is the

result of resonance in the air.


• (c) The effects of resonance can also cause

damage. For example, a bridge can collapse when

the amplitude of its vibration increases as a result of

resonance.

THE POWER OF RESONANCE CAN DESTROY A BRIDGE. ON

NOVEMBER 7, 1940, THE ACCLAIMED TACOMA NARROWS BRIDGE

COLLAPSED DUE TO OVERWHELMING RESONANCE.


• (d)Cracking of wine glass

Chapter 1: Waves

1.2 Analysing Reflection of Waves

Ripple Tank

Ripple Tank

Main Parts Functions

Lamp To project the image of the water waves onto the white

paper below the ripple tank

Ripple Tank


Motor The vibrations of electric motor causes the plastic sphere

to produce spherical waves, and the wooden bar to

produce plane water waves

Ripple Tank


Rheostat Controls the frequency of the water waves produced

Ripple Tank


Sponge To line the inside of the transparent tray to prevent

reflection of water waves from the side of the tray.

Ripple Tank


Stroboscope To freeze the image of the water waves

Ripple Tank

Ripple Tank

• 1 A water wave is a type of transverse

wave.

Ripple Tank

• 2. When waves are produced on the surface of

the water, a wave crest will act like a convex lens

while a wave trough will act like a concave lens.

Ripple Tank

• 3. Hence the crest focuses the light to form a

bright fringe on the white screen below the ripple

tank, and the trough diverges the light and forms a

dark fringe on the white screen, as shown in Figure

1.21

Ripple Tank

• 4. Each bright and dark fringe represents the

wavefront of the water wave.

Ripple Tank

• 5. A hand stroboscope can be used to freeze the

motion of the water waves.

Ripple Tank

• 6. When the fringe pattern on the white

screen below the ripple tank is "frozen", the

frequency of the water waves is given by

• f = n x p,

• where

n = number of slits on the stroboscope

p = rate of rotation of the stroboscope

Ripple Tank

• 7. The wavelength, , of the water wave is

related by v = f.

Reflection of Waves

• 1 Reflection of a wave occurs when a wave

strikes an obstacle. The wave undergoes a change

in direction of propagation when it is reflected.

Reflection of Waves

• 2 The incident wave is the wave before it strikes the obstacle, whereas the reflected wave is the wave which has undergone a change in direction of propagation after reflection.

• i = angle of incidence;

• r = angle of reflection

Reflection of Waves

• 3 The phenomenon of reflection of waves obeys

the Laws of reflection where:

• (a) The angle of incidence, i, is equal to the angle

of reflection, r.

Reflection of Waves

• 3 The phenomenon of reflection of waves obeys

the Laws of reflection where:

• (b) The incident wave, the reflected wave and the

normal lie in the same plane which is perpendicular

to the reflecting surface at the point of incidence.

Reflection of Waves

• Experiment 1.1 To investigate the reflection of plane

waves

• Problem statement

• What is the relationship between the angle of

incidence and the angle of reflection of a water

wave?

Reflection of Waves

• Hypothesis

• The angle of reflection is equal to the

angle of incidence.

Reflection of Waves

• Variables

• Manipulated : Angle of incidence of the water

wave

• Responding : Angle of reflection of the water wave

• Fixed : Depth of water, frequency of dipper

Reflection of Waves

• Operational definition

• The angle of incidence is the angle between the direction of propagation of incident wave and the normal. The angle of reflection is the angle between the direction of propagation of reflected wave and the normal.

Reflection of Waves

• Apparatus/Materials

• Ripple tank, plane reflector, a piece of white paper,

wooden bar, lamp, motor, sponge and mechanical

stroboscope.

Reflection of Waves

• Procedure

• 1 A ripple tank is filled with water and is set up as shown in Figure 1.23. The tank is leveled so that the depth of water in the tank is uniform to ensure water waves propagate with uniform speed.

Reflection of Waves

• Procedure

• 2 All the inner surface of the ripple tank is lined with a layer of sponge to prevent reflection of the water waves from the edges.

Reflection of Waves

• Procedure

• 3 The lamp above the tank is switched on and a large piece of white paper is placed below the tank.

Reflection of Waves

• Procedure

• 4 A metallic plane reflector is placed at the centre of the tank. The motor with wooden bar attached is switched on to produce plane waves which propagate towards the reflector.

Reflection of Waves

• Procedure

• 5 The pattern (on the white paper) of the reflected waves produced by the vibrating wooden bar is observed with the help of a mechanical stroboscope. The incident waves and the reflected waves are sketched.

Reflection of Waves

• Procedure

• 6 Steps 4 and 5 are repeated with the reflector

repositioned so that the wave is incident at angles, i

= 20°, 30°, 40°, 50° and 60° on the reflector as shown

in Figure 1.24.

Reflection of Waves

• ResultsPattern of reflected waves Characteristic of waves

(i) Angle of incidence, i = 0

Angle of reflection, r = 0

Wavelength, frequency and speed of

wave do not change after reflection.

Direction of propagation of water

changes.

Reflection of Waves

• Results

Angle of incidence, i, () 20 30 40 50 60

Angle of reflection, r, () 20 30 40 50 60

Angle of incidence, i =Angle of

reflection, r

Wavelength, frequency and speed of

wave do not change after reflection.

Direction of propagation of water

changes.

Reflection of Waves

• Conclusion

• The angle of reflection is equal to the angle of

incidence. The hypothesis is valid.

Reflection of Waves

• Example 6

• A water wave of frequency 20 Hz appears

stationary when observed through a stroboscope

with 4 slits. What is the frequency of rotation of the

stroboscope?

Reflection of Waves

• Example 6

• Solution

• Frequency of wave = Number of slits x Frequency of

stroboscope

• 20 = 4 x f

• f = 5 Hz

Reflection of Waves

• Experiment 1.2 : To investigate the reflection of

sound waves



incidence and the angle of reflection of a sound

wave?

Reflection of Waves

• Hypothesis


incidence.

Reflection of Waves

• Variables

• (a) Manipulated : Angle of incidence of the sound wave

• (b) Responding : Angle of reflection of the sound wave

• (c) Fixed : Distance of the stopwatch from the point of reflection on the wooden board

Reflection of Waves


• The angle of incidence of the sound wave is the angle between the incident sound wave and the normal. The angle of reflection is the angle between the reflected sound wave and the normal.

Reflection of Waves


• Two cardboard tubes, stopwatch, a slab of soft

wood, a wooden board with a smooth surface and

a protractor.

Reflection of Waves

• Procedure

• 1 The apparatus is set up as shown in Figure 1.25.

Reflection of Waves

• Procedure

• 2 The angle of incidence, i = 30° is measured with

a protractor.

• 3 The stopwatch is started.

Reflection of Waves

• Procedure

• 4 The position of the cardboard tube B is adjusted until a loud ticking sound of the stopwatch is heard.

• 5 The angle of reflection, r at this position of the cardboard tube B is measured.

Reflection of Waves

• Procedure

• 6 Steps 2 to 5 are repeated with the angles of

incidence, i = 40°, 50°, 60° and 70°.

• 7 The results are tabulated.

Reflection of Waves

• ResultsAngle of

incidence, i, ()

30 40 50 60 70

Angle of

reflection, r, ()

30 40 50 60 70

Reflection of Waves

• Discussion

• The sound waves from the stopwatch experience a

reflection after striking the wooden board. The slab

of soft wood placed along the normal serves as a

barrier to prevent the sound of the stopwatch from

reaching the observer directly.

Reflection of Waves

• Conclusion

• The angle of incidence, i is equal to the angle of

reflection, r. The laws of reflection are obeyed. The

hypothesis is valid.

Angle of

incidence, i, ()

30 40 50 60 70

Angle of

reflection, r, ()

30 40 50 60 70

Reflection of Waves

• Experiment 1.3: To investigate the reflection of light



incidence and the angle of reflection of a light ray?

Reflection of Waves

• Hypothesis


incidence.

Reflection of Waves

• Variables

• (a) Manipulated : Angle of incidence of light ray

• (b) Responding : Angle of reflection of the light ray

• (c) Fixed: Position of the plane mirror

Reflection of Waves


• The angle of incidence of the light ray is the angle

between the incident ray and the normal. The

angle of reflection is the angle between the

reflected ray and the normal.

Reflection of Waves


• Plane mirror, ray box, plasticine, protractor, white

piece of paper and a sharp pencil.

Reflection of Waves

• Procedure

• 1 A straight line, PQ is drawn on a sheet of white

paper.

Reflection of Waves

• Procedure

• 2 A normal line, ON is drawn from the midpoint of

PQ.

• 3 Using a protractor, lines at angles of incidence

of 20°, 30°, 40°, 50° and 60°, with the normal, ON are

drawn.

Reflection of Waves

• Procedure

• 4 A plane mirror is erected along the line PQ.

Reflection of Waves

• Procedure

• 5 A ray of light from the ray box is directed along the 20° line. The angle between the reflected ray and normal, ON is measured.

Reflection of Waves

• Procedure

• 6 Step 5 is repeated with the angle of incidence, i

of 30°, 40°, 50° and 60°.

• 7 The results are tabulated.

Reflection of Waves

• Results

• The incident ray must be as narrow as possible to obtain a narrow and thin reflected ray. It can be done by adjusting the lens in the ray box (or a laser pen can be used instead).

Angle of

incidence, i, ()

20 30 40 50 60

Angle of

reflection, r, ()

20 30 40 50 60

Reflection of Waves

• Conclusion

• The angle of incidence, i is equal to the angle of

reflection, r. The laws of reflection are obeyed. The

hypothesis is valid.

Applications of Reflection of

waves in Daily Life• Safety

• (a) The rear view mirror and side mirror in a car are used to view cars behind and at the side while overtaking another car, making a left or right turn and parking the car. The mirrors reflect light waves from other cars and objects into the driver's eyes.


waves in Daily Life• Safety

• (b) The lamps of a car emit light waves with minimum dispersion. The light bulb is placed at the focal point of the parabolic reflector of the car lamp so that the reflected light waves are parallel to the principal axis of the reflector. Parallel light waves have a further coverage.


waves in Daily Life• Defence

• A periscope is an optical instrument. It can be constructed

using two plane mirrors for viewing objects beyond obstacles.

The light waves from an object which is incident on a plane

mirror in the periscope are reflected twice before entering the

eyes of the observer.


waves in Daily Life• Medication

• The concept of total internal reflection is used in optical fibres. Light entering one end of an optical fibre experiences multiple total internal reflections as it propagates through the whole length of the fibre before emerging at the other end. Optical fibres are used to examine the internal organs of patients, especially organs with internal cavities such as the colon and stomach, without operating on the patient.


waves in Daily Life

• Telecommunications• (a) Optical fibres have many advantages compared to

conventional cables in the transmission of information. Optical

fibres are lightweight, flexible, electrically non-conducting

(thus are not affected by electromagnetic interference) and

can transmit much more information (information is

transmitted almost at speed of light, 3 x 108 ms -1).


waves in Daily Life• Telecommunications

• (b) Infrared waves from a remote control of

electrical equipment (television or radio) are

reflected by objects in the surroundings and

received by the television set or radio.

Education

1.2 reflection Fizik SPM