Waves Archives

Analysing Diffraction of Waves

December 18, 2020December 18, 2020 by Veerendra

Analysing Diffraction of Waves

Figure shows a scenery of a fishing village. It is noticed that when the waves passed through the narrow opening between the two concrete barriers, their shape changed. This observation can be explained by the diffraction of waves.
$Analysing Diffraction of Waves$
Diffraction of waves is a phenomenon that refers to the spreading out of waves when they move through a gap or round an obstacle.

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Characteristics of Diffraction of Waves

Wavelength: The wavelength of the diffracted waves is the same as that of the incident waves.
Frequency: The frequency of the diffracted waves is the same as that of the incident waves.
Speed: The speed of the diffracted waves is the same as that of the incident waves.
Direction: The direction of propagation of the diffracted waves depends on the width of the gaps or obstacles. For smaller gaps and obstacles, the change of direction is more – the spread is bigger.
Amplitude: The amplitude of the diffracted waves is smaller than that of the incident waves.

Diffraction of Plane Waves in a Ripple Tank Experiment

Aim: To study the characteristics of diffraction of plane waves in a ripple tank.
Material: White paper as screen
Apparatus: Ripple tank with its accessories, stroboscope
Method:

$Diffraction of Plane Waves$

The ripple tank is arranged as shown in Figure.
The legs of the ripple tank are adjusted so that the base of the tank is horizontal. The tank is filled with water.
A horizontal vibrating beam is used to create plane waves.
The speed of the motor is adjusted to produce a train of waves that can be clearly observed on the screen with the help of a stroboscope.
The barriers are adjusted for different widths of the gap.
The wavefronts that emerge from the gap are observed and recorded.
The width of the gap is fixed but the frequency of the motor is adjusted.
The wavefronts that emerge from the gap are observed and recorded.
The barriers are replaced with obstacles of different widths and the ‘shadow’ of each obstacle is observed and recorded.

Observations:
$Diffraction of Plane Waves Experiment$
$Diffraction of Plane Waves Experiment 1$ The above figures shows the diffracted wavefronts.
Discussion:

It is observed that the effect of diffraction is greater when the width of the gap or obstacle is smaller.
Waves passing through a narrow gap spread out wider than waves passing through a wide gap.
An obstacle placed in front of incoming incident waves will cause a ‘shadow’ behind the obstacle. For a narrow obstacle, the wavefronts meet and rejoin at a place nearer to the obstacle as compared with a wider obstacle.

Diffraction of Light Experiment

Aim: To study the characteristics of diffraction of light.
Material: White paper as screen
Apparatus: Laser pen, two retort stands with clamps, adjustable single-slit
Method:

$Diffraction of Light Experiment 1$ $Diffraction of Light Experiment 2$

The apparatus is set up as shown in Figure (a).
A ray of light from the laser pen is directed through the adjustable single-slit onto the screen.
The distance and the positions of the laser pen and the single-slit are adjusted until a clear diffraction pattern of light from the laser beam falls on the screen.
The width of the single-slit is adjusted and the pattern on the screen is observed for different widths of the slit.
The single-slit is replaced with a piece of black paper with a tiny pinhole in it.
A ray of light from the laser pen is directed through the hole and the pattern formed on the screen is observed.
The hole is enlarged and step 6 is repeated.

Observations:

From the observation in step 4, the patterns formed by the slits are as shown in Figure.
$Diffraction of Light Experiment 3$
From the observation in steps 6 and 7, the patterns formed by the holes are as shown in Figure.
$Diffraction of Light Experiment 4$

Discussion:

Light undergoes diffraction when passing through a narrow slit and a small hole.
The diffraction pattern becomes less distinct when the slit or hole becomes wider.

Diffraction of Sound Waves Experiment

Aim: To study the characteristics of diffraction of sound waves
Apparatus: A cassette or CD player
Method:

$Diffraction of Sound Waves Experiment 1$

Carry out the outdoor activity as shown in Figure.
Listen to the music from the cassette or CD player at a position that is blocked by a corner of a building as shown in the diagram.

Observation:

The music from the cassette or CD player can be heard although it is blocked by the corner of the building. Discussion:
The music from the cassette or CD player can be heard even though it was blocked by the building. This shows that diffraction of sound waves has occurred.
$Diffraction of Sound Waves Experiment 2$

Conclusions:

Diffraction of waves occurs when there is a spreading out of waves when the waves move through a gap or round an obstacle.
The wavelength, frequency and speed of the diffracted waves are the same as that of the incident waves.
However, due to the spreading of the waves, the energy per unit area of the diffracted waves is less than the incident waves. Hence, the amplitude of the diffracted waves is smaller than the incident waves.

Analysing Reflection of Waves

November 27, 2020November 27, 2020 by Veerendra

Analysing Reflection of Waves

Figure shows the reflection of sea waves by a retaining wall built near the beach.
Reflection of waves is a phenomenon that occurs when all or a part of the waves return after they encounter an obstacle. The obstacle is known as a reflector.
A ripple tank kit is a very useful apparatus for studying waves in the laboratory. Figure shows a ripple tank in use.
(a) When a steady train of waves move across the water surface, each wave crest acts like a convex lens and concentrates the light on the screen as shown in Figure. Flence, a bright pattern on the screen corresponds to a crest of the waves.
(b) A stroboscope is normally used to observe wave patterns on the screen. By rotating the stroboscope at the same frequency as the motor of the ripple tank kit, the motion of the waves can be frozen for more detailed observation.

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Characteristics of Reflected Waves

Figure shows incident waves being reflected by a reflector. The angle of incidence, i is the same as that of the angle of reflection, r.

Direction: The direction of the propagation of the waves changed after reflection.
Angle of reflection: The angle of reflection, r is equal to reflection the angle of incidence, i.
Wavelength: The wavelength of the reflected waves is the same as that of the incident waves.
Frequency: The frequency of the reflected waves is the same as that of the incident waves.
Speed: The speed of the reflected waves is the same as that of the incident waves.

Analysing Reflection of Waves 4

Reflection of Plane Waves Experiment

Aim: To study the characteristics of reflection of plane waves in a ripple tank, light and sound waves.
Material: White paper as screen
Apparatus: Ripple tank and its accessories, stroboscope, protractor
Method:

Analysing Reflection of Waves 5

The ripple tank is arranged as shown in Figure.
The legs of the ripple tank are adjusted so that the base of the tank is horizontal.
A horizontal beam vibrator is lowered until it touches the surface of the water to create plane (straight) waves.
The speed of the motor is adjusted to produce a train of waves that can be clearly observed on the screen with the help of a stroboscope.
A straight reflecting barrier (reflector) is placed on the tank so that the angle of incidence of the waves, i = 30°.
The angle of the reflected waves, r is measured and recorded.
The wavelengths of the incident and reflected waves are observed and compared.
Steps 5 to 7 are repeated for i = 45° and 60°.

Results:

Analysing Reflection of Waves 7

Observation:
The wavelength of the reflected waves is the same as the incident waves.
Discussion:

From the results, the angle of reflection of the waves is equal to the angle of incidence of the waves.
The wavelength of the waves does not change after reflection.
The frequency of the incident and reflected waves are the same as that of the source of the waves.
From 2 and 3, and by using the formula v = fλ, the speed of the reflected waves is the same as that of the incident waves.

Reflection of Light Experiment

Aim: To study the characteristics of reflection of light.
Materials: A piece of white paper, plasticine, pencil Apparatus: Plane mirror, ray box, power supply, protractor
Method:

Analysing Reflection of Waves 8

The apparatus is set up as shown in Figure.
The activity is carried out to determine the angle of reflection, r corresponding to the angle of incidence, i = 30°, 45° and 60°.

Results:Discussion:
From the results, the angle of reflection of the light, r is equal to the angle of incidence of the light, i.

Reflection of Sound Waves Experiment

Aim: To study the characteristics of reflection of sound waves.
Materials: Round cardboard tubes, a piece of soft board, a piece of plywood, a piece of white paper
Apparatus: Protractor, mechanical stopwatch as source of sound
Method:

Analysing Reflection of Waves 10

The apparatus is set up as shown in Figure.
A stopwatch is placed in a cardboard tube with one end open and the other end closed.
The tube is arranged at an incident angle, i = 30°.
Another cardboard tube with both ends open is adjusted so that one ear is placed close to one end of the tube as to hear the loudest possible ticking sound of the stopwatch.
The angle of reflection, r of the sound wave is measured and recorded.
Steps 3 to 5 are repeated for i = 45° and 60°.

Results:
Discussion:
From the results, the angle of reflection, r is equal to the angle of incidence, i of the sound waves.
Conclusions:

The direction of propagation of the reflected waves is different from that of the incident waves.
The angle of reflection, r is equal to the angle of incidence, i of the waves.

Analysing Electromagnetic Waves

November 27, 2020November 27, 2020 by Veerendra

Analysing Electromagnetic Waves

The electromagnetic spectrum consists of a group of waves of similar nature. The members of the electromagnetic spectrum arranged in increasing frequencies (decreasing wavelengths) are radio waves, microwaves, infrared rays, visible light, ultraviolet rays, X-rays and gamma rays.
Electromagnetic waves are joint electric and magnetic fields which can travel through space with no need of a medium to carry them. Figure shows the representation of electromagnetic waves.
All the members of the electromagnetic spectrum
(a) transfer energy from one place to another
(b) are transverse waves
(c) can travel through a vacuum
(d) travel with a speed of 3 x 10⁸ m s^-1 in a vacuum

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The Electromagnetic Spectrum

Table lists out the sources, characteristics and applications of the electromagnetic spectrum.

Type and wavelengths	Sources	Characteristics	Applications
Radio waves λ : 10³ — 10^-1 m	Radio Television transmitter	Carry audio and visual information	Broadcasting and wireless communication UHF (ultra high frequency) radio waves – TV and mobile phones VHF (very high frequency) radio waves – local radio and wireless communication used by policemen
Microwaves λ : 10^-1 – 10^-3m	Radar transmitter Microwave ovens	Can penetrate the atmosphere Suitable for satellite communication Can excite water molecules, therefore suitable for cooking	Communication with satellites Used in radar systems Global Positioning System (GPS) For cooking – microwave ovens
Infrared rays λ : 10^-3– 10^-6m	Warm or hot objects The Sun	Also known as infrared radiation When an object absorbs infrared rays, it becomes hotter	For cooking food – ovens, grills and toasters Remote controls for televisions and video players Intruder alarm systems Night vision
Visible light λ : 8 x 10^-7– 4 x 10^-7 m	The Sun Hot objects Electric bulbs Fire LED	Consists of seven components (red, orange, yellow, green, blue, indigo and violet)	Photography Photosynthesis by plants Human and animal sight
Ultraviolet rays λ : 10^-7– 10^-9 m	The Sun Mercury lamps Sparks Very hot objects	Can be absorbed by glass and the ozone layer in the atmosphere of the Earth A small amount is good for producing vitamin D in our skin while a large amount is bad for eyes and can cause skin cancer	Fluorescent lamp Detection of security markings in currency notes Sterilisation of surgical tools and plant seedlings
X-rays λ : 10^-9 – 10^-11 m	X-ray tubes Outer space bodies	High energy High penetrating power Very dangerous	Helps doctors to check bones and teeth Helps engineers to check welds and metal joints Kills cancerous cells X-ray diffraction helps scientists to study the arrangement of atoms in various substances To detect whether an art piece is genuine or not
Gamma rays λ : 10^-11 m or less	Radioactive substances Cosmic rays	High energy High penetrating power Very dangerous	Kills cancerous cells Sterilisation of surgical tools and food Helps engineers to check welds and metal joints

Detrimental Effects of Electromagnetic Waves

The human eye cannot detect ultraviolet rays but an overexposure to these rays can cause blindness. Overexposure to ultraviolet rays can also cause sunburn and skin cancer.
Due to the high energy associated with short wavelength radiations, ultraviolet radiations, X-rays and gamma rays can damage living tissues. These radiations ionise atoms and molecules in living cells. These cells may die or become cancerous.
X-rays have very high penetrating power. These rays have adverse effects on living cells. Cancer and genetic defects can be induced by exposure to X-rays.
Gamma rays have very high penetrating power. Exposure to gamma rays can lead to genetic defects and the harming of living cells.

Analysing Sound Waves

November 27, 2020November 27, 2020 by Veerendra

Analysing Sound Waves

Sounds are mechanical waves. They are caused by vibrating objects. The string of a guitar, the skin of a drum and a tuning fork, all of these vibrate to produce sound.
The vibrating cone of a loudspeaker in Figure produces sound. Its vibrating diaphragm is continually compressing and stretching the air next to it. This makes a series of compressions and rarefactions which travel through the air away from the loudspeaker.
A compression is a region of increased pressure and a rarefaction is a region of decreased pressure. The resulting succession of compressions and rarefactions constitutes the sound waves.
A sound wave is longitudinal in nature because the air molecules vibrate in the direction which is parallel to its direction of propagation.
A sound wave needs a medium for its propagation because its propagation is essentially due to the vibration of the molecules of its medium.
Compressions and rarefactions need a material which can be compressed and stretched. This explains why we do not hear any sound in space which mainly consists of a vacuum.

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Amplitude and Frequency of Sound Waves

The amplitude of sound waves is related to its loudness. The louder the sound, the bigger is its amplitude.
The frequency of sound waves is related to its pitch. The higher the pitch of the sound, the higher is its frequency.
Figure summarises the relationship between, amplitude and loudness; frequency and pitch of a sound.

Application of Sound Waves in Daily Life

Sound can be generated in a wide range of frequencies. Sound waves generated between 20 Hz and 20 kHz can be heard by normal human ears and are known as audio waves. Those below 20 Hz are called infrasonic and those above 20 kHz are known as ultrasonic.
A bat can navigate in complete darkness by emitting very high-pitched sound waves in the ultrasonic range.
(a) When these waves hit an object, they are reflected back and received by the bat.
(b) The time between the emission of the sound waves and the reception of the reflected waves enables the bat to estimate the position of the object accurately.
Dolphins use ultrasonic frequencies of about 150 kHz for communication and navigation.
Figure shows the process of detecting flaws inside a piece of metal using the echo sounding method.
(a) An ultrasonic transmitter sends out pulses of ultrasound in the metal.
(b) A receiver picks up the echoes from different parts of the metal and sends the information to a C.R.O.
(c) The C.R.O. displays the information on its screen. Pulse P is the transmitted pulse, pulse Q has been reflected by the flaw and pulse R is the echo from the end of the metal.
An ultrasonic ruler uses ultrasonic echoes to measure distance. Figure shows an ultrasonic ruler being used to determine the depth of the sea.
(a) Pulses of ultrasounds are directed from the ship to the seabed and a receiver attached to the ship detects the reflected pulses.
(b) The time taken by a pulse to travel to the seabed and return to the ruler enables the depth of the sea to be determined. The ruler can be calibrated to give the value of the depth directly.
Opticians and goldsmiths use ultrasound to clean spectacles, jewellery and ornaments. The water used for the cleaning purpose is vibrated by ultrasound. The vibrations shake off dirt attached to these objects.
Dentists use ultrasonic waves to vibrate and shake dirt and plaque off patients’ teeth.
Animals like elephants and rhinoceroses call to one another using infrasonic waves.

Amplitude and Frequency of Sound Waves Experiment

Aim: To study the effect of amplitude on the loudness and frequency on the pitch of sound.
Apparatus: Cathode ray oscilloscope (C.R.O.), audio signal generator, loudspeaker
Method:

Amplitude and Frequency of Sound Waves Experiment

The apparatus is arranged as shown in Figure.
The audio signal generator and C.R.O. are adjusted so that a sound is produced by the loudspeaker and a sine wave is observed on the screen of the C.R.O.
By fixing the frequency, the volume*of the audio signal generator is increased gradually. The observation is recorded.
By fixing the volume, the frequency of the audio signal generator is increased gradually. The observation is recorded.

Observation:

Adjustment done on the audio signal generator		Change in the sine wave on the screen of the C.R.O.		Change in the sound produced by the loudspeaker
Volume	Frequency	Amplitude	Frequency	Change in the sound produced by the loudspeaker
increased	Fixed	Increased	Unchanged	Louder
Fixed	Increased	Unchanged	Increased	FHigher pitch

Conclusion:
Based on Table, it can be concluded that
(a) the bigger the amplitude of the sound, the louder is the sound.
(b) the higher the frequency of the sound, the higher is the pitch of the sound.

Analysing Refraction of Waves

November 27, 2020November 27, 2020 by Veerendra

Analysing Refraction of Waves

Figure shows a surfer approaching the beach. As the waves enter the shallow water near the beach, the wavelength of the waves decreases. Hence, the speed of the waves decreases as the water gets shallower.
$Analysing Refraction of Waves 1$
Refraction of waves is a phenomenon that occurs when there is a change of direction in the propagation of waves travelling from one medium to another due to a change of speed.
The refraction of water waves occurs when water waves travel from one area to another area of different depths. Hence, the different depth of water is equivalent to a different medium.

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Characteristics of Refraction of Waves

Figure shows the refraction of water waves travelling from a deep area to a shallow area.
$Analysing Refraction of Waves 2$

Direction: The direction of the propagation of waves changed after refraction.
Angle of refraction: When waves propagate from a less dense medium to a denser medium, the angle of refraction is lesser than the angle of incidence.
Wavelength: The wavelength of the refracted waves in the denser medium is shorter (as in the case of water, shallower depth is equivalent to denser medium).
Frequency: The frequency of the waves remains unchanged.
Speed: The speed of the waves in the denser medium is less than that in the less dense medium.

Refraction of Plane Waves Experiment

Aim: To study the characteristics of refraction of plane waves in a ripple tank.
Material: White paper as screen
Apparatus: Ripple tank with its accessories, stroboscope, trapezium-shaped perspex plate, convex-shaped perspex plate, concave-shaped perspex plate
Method:

$Refraction of Plane Waves Experiment 1$ $Refraction of Plane Waves Experiment 2$

The ripple tank is arranged as shown in Figure.
The legs of the ripple tank are adjusted so that the base of the tank is horizontal. The tank is filled with water.
A trapezium-shaped perspex plate is placed in the tank to create a shallow area as shown in Figure.
$Refraction of Plane Waves Experiment 3$
A horizontal vibrating beam is used to create plane (straight) waves.
The speed of the motor is adjusted to produce a train of waves that can be clearly observed on the screen with the help of a stroboscope.
The position of the perspex plate is adjusted with its edge parallel and then at an angle with the vibrating beam. For each case, the wavefronts at the deep and shallow areas of the tank are observed.
The trapezium-shaped perspex plate is replaced with the convex-shaped perspex plate and then with the concave-shaped perspex plate. For each case, the wavefronts are observed at the deep and shallow areas of the tank.

Observations:
$Refraction of Plane Waves Experiment 4$
$Refraction of Plane Waves Experiment 5$
$Refraction of Plane Waves Experiment 6$
$Refraction of Plane Waves Experiment 7$ The above Figures show the observed wavefronts for the respective perspex plates.
Discussion:

When water waves travel from a deep area to a shallow area as in Figure, the direction of the waves is refracted towards the normal.
$Refraction of Plane Waves Experiment 8$
Therefore, the angle of incidence, i of the water waves is greater than the angle of refraction, r.
When water waves travel from an area into another of different depths, their speeds vary. The frequency of the waves which is equal to the frequency of the motor remains unchanged.
Figure shows plane water waves travelling from a deep area to a shallow area. Using the formula v = fλ, where v_d = fλ_d and v_s = fλ_s, f remains unchanged. Since λ_d > λ_s, therefore, v_d > v_s The wave speed at the deep area, v_d is greater than the wave speed at the shallow area, v_s.

$Refraction of Plane Waves Experiment 9$

Refraction of Light Experiment

Aim: To study the characteristics of refraction of light.
Material: White paper
Apparatus: Glass block, ray box, power supply, protractor
Method:

$Refraction of Plane Waves Experiment 10$

The apparatus is set up as shown in Figure.
A ray of light is directed at an angle of incidence, i = 30⁰ to the glass block.
The ray that enters the glass block is observed and the angle of refraction, r is measured.
Steps 2 and 3 are repeated for angles of incidence, i = 45⁰ 60⁰ and 75⁰.

Observation:
It is observed that when a light ray travels from air into the g ass block, it is refracted towards the normal.
$Refraction of Plane Waves Experiment 11$ Discussion:

Glass is a denser medium than air. A light ray travels at a slower speed in glass compared with in air.
When a light ray travels from a less dense medium to a denser medium and the ang e of incidence, i is not zero, then the angle of incidence, i is greater than the angle of refraction, r.

Refraction of Sound Waves Experiment

Aim: To study the characteristics of refraction of sound waves.
Material: Balloon filled with carbon dioxide
Apparatus: Audio signal generator, loudspeaker, microphone, cathode ray oscilloscope (C.R.O.)
Method:

$Refraction of Plane Waves Experiment 12$

The apparatus is set up as shown in Figure.
The C.R.O. settings are adjusted to detect the sound waves emitted from the loudspeaker.
The position of the microphone is adjusted by moving it nearer to the balloon or further away from it until the biggest amplitude of the waves is displayed on the screen.
The balloon is removed as shown in Figure and the amplitude of the trace displayed on the screen is observed and compared with that observed in step 3.
$Refraction of Plane Waves Experiment 13$

Observation:

In step 3, the amplitude of the trace displayed on the screen is bigger than that in step 4.
This shows that a louder sound is detected when a balloon is placed between the loudspeaker and the microphone.

Discussion:

When the screen of the C.R.O. displays a trace of bigger amplitude for a sound wave, it indicates a louder sound is detected. This shows that the balloon which is filled with carbon dioxide has focused the sound waves emitted from the loudspeaker to the microphone.
From the observation, the sound waves that travel through ordinary atmospheric air (less dense medium) to carbon dioxide (denser medium) undergo refraction.