Chp 9: Waves

Question 1

define Progressive Wave:

Answer 1

A wave is a disturbance that travels through a medium from one location to another location. A progressive wave transfers energy from one point to another without the transfer of matter.

Question 2

define displacement of a wave:

Answer 2

distance is travelled in a specific direction from its equilibrium position.

Question 3

define amplitude of a wave:

Answer 3

magnitude of the maximum displacement of a particle in the wave from its equilibrium position

Question 4

define period of one wave:

Answer 4

time taken for a particle of a wave to complete one oscillation.

(For a progressive wave, the period is also the time taken for the waveform to travel a distance of one wavelength)

Question 5

define frequency of a wave:

Answer 5

number of oscillations per unit time made by a particle of a wave.

Question 6

define wavelength of a wave:

Answer 6

distance between two consecutive points which are in phase. (2 successive crests or 2 successive troughs)

Question 7

define speed of a wave:

Answer 7

distance travelled by the wave over the time taken.

Question 8

define phase of a particle in a wave:

Answer 8

stage of motion of the particle, usually expressed as an angle in radians

Question 9

define phase diff between 2 particles in a wave of 2 waves:

Answer 9

fraction of a cycle by which one is ahead of the other, usually expressed as an angle in radians

Question 10

define wavefront of a wave:

Answer 10

a line or surface joining points on a wave that are in phase.

wave travels in a direction perpendicular to the wavefront.

Question 11

define wave intentsity

Answer 11

rate of transfer of energy per unit area normal to the direction of wave propagation.

Question 12

define point source of a wave:

Answer 12

waves are emitted radially in all directions.

Question 13

define transverse wave:

Answer 13

particles oscillate in a direction perpendicular to the direction of energy transfer.

Question 14

define longituidinal wave:

Answer 14

particles oscillate in a direction parallel to the direction of energy transfer

Question 15

define polarization:

Answer 15

oscillations of a transvers wave are restricted to a single plane.

Question 16

define malus’ law:

Answer 16

intensity of a beam of plane-polarized light after passing through a polarizer varies with the square of the cosine of the angle through which the polarizer is rotated from the position that gives the maximum intensity.

Question 17

For sound wave in the gas, state the origin of:

• Energy of the wave:
• Restoring force on a molecule as it vibrates:

Answer 17

Energy of the wave: The oscillator that sets the air molecules oscillating like a tuning fork.

Restoring force on a molecule as it vibrates: The inter molecular forces between the molecules.

Question 18

how a polarized transverse wave differs from an unpolarized transverse wave.

Answer 18

In a polarised wave, the vibrations are in only one plane but in an unpolarised wave, there are infinite planes of vibration.

Question 19

Distinguish between longitudinal and transverse waves

Answer 19

For longitudinal waves, the particles vibrate in a direction parallel to the direction of propagation of the wave.

For transverse waves, the particles vibrate perpendicularly to the direction of propagation of the wave.

Transverse waves can be polarized, but longitudinal waves cannot

Question 20

Two progressive sound waves, each of amplitude A and wavelength 𝝀, meet at a point such that their phase difference is 2π.

i) Show that the ratio of the intensity of the resultant wave to the sum of the intensities of the individual wave is 2.

ii) Explain how the ratio in (i) is consistent with the principle of conservation of energy.

Answer 20

i) 𝐼 ∝ 𝐴² where A and I are the amplitude and intensity of each wave 𝑖𝑛𝑡𝑒𝑛𝑠𝑖𝑡𝑦 𝑜𝑓 𝑡ℎ𝑒 𝑟𝑒𝑠𝑢𝑙𝑡𝑎𝑛𝑡 𝑤𝑎𝑣𝑒 𝑠𝑢𝑚 𝑜𝑓 𝑡ℎ𝑒 𝑖𝑛𝑡𝑒𝑛𝑠𝑖𝑡𝑖𝑒𝑠 𝑜𝑓 𝑡ℎ𝑒 𝑖𝑛𝑑𝑖𝑣 𝑤𝑎𝑣𝑒𝑠 = (𝐴 + 𝐴)²/(A² + 𝐴²) = 2

ii) The total energy of the resultant wave is redistributed such that the intensity at points of constructive interference is 4 and zero at points of destructive interference. However, the average intensity is still 2

Question 21

Describe how the representation in the figure below illustrates a longitudinal wave. (water underneath, above water is a wave in tube. at this instant, wave touch water at a single point and away from tube, wave splits into 2 curve lines)

Answer 21

The representation shows the amplitude of air particles along the axis of the tube.

At the open end, it is an anti-node and the particles vibrate with the highest amplitude.

At the closed end (water surface), it is a node and the particles vibrate with zero amplitude.