Waves

Question 1

What are the 2 main features of waves?

Answer 1

Waves are travelling disturbances
Waves carry energy from place to place.

Question 2

Define progressive wave.

Answer 2

Transfer energy from one point to another; without the material medium travelling from one place to another.

Question 3

Define transverse wave.

Answer 3

A transverse wave’s disturbance is perpendicular to the direction of travel of the wave.

Question 4

Define longitudinal wave.

Answer 4

A longitudinal wave’s disturbance is parallel to the direction of travel of the wave.

Question 5

Define wavelength.

Answer 5

The distance between 2 successive crests/troughs

Question 6

Define amplitude.

Answer 6

Maximum distance of a vibrating particle from its equilibrium position - scalar quantity.

Question 7

Define compression.

Answer 7

Region on a longitudinal wave where particles are closer together. Pressure is the highest.

Question 8

Define rarefaction.

Answer 8

Region on a longitudinal wave where particles are further apart. Pressure is the lowest.

Question 9

Define period (T).

Answer 9

The time for a vibrating particle to complete a cycle.

Question 10

Define frequency (f).

Answer 10

The number of complete cycles per unit time. Unit is Hertz (Hz)

Question 11

What is the relationship between f and T?

Answer 11

f = 1/T
f is inversely proportional to T

Question 12

Define phase difference.

Answer 12

Physical quantity that measures the relative positions of particles on a wave. Unit is radians.

Equation:

Phase difference = distance between crest and trough (x) / wavelength (lambda) x 360

OR

Phase difference = distance between crest and trough (x) / wavelength (lambda) x 2 pi

1. From a displacement - distance graph

x / lambda = phase difference / 360

2. From a displacement - time graph

time / T = phase difference / 360

Phase difference can have values from 0 to infinity.

Question 13

Define in phase.

Answer 13

If phase difference is equal to 0 or 360 or 720 or 360n when n = 1,2,3… degrees then we say that the particles / points are in phase.

Question 14

Define completely out of phase.

Answer 14

If phase difference is equal to 180 or 540 or 900 degrees… the particles or points are said to be completely out of phase.

Question 15

Define intensity.

Answer 15

The rate of energy transmitted per unit Area by the wave, in a direction parallel to the velocity of the wave.

Intensity = Power / Area
Unit is Wm-2
Intensity is inversely proportional to the Area
Scalar quantity

Question 16

What is the relationship between intensity and amplitude?

Answer 16

Intensity is directly proportional to the amplitude squared

I = k (A squared)

Question 17

Define CRO

Answer 17

Cathode Ray Oscilloscope.
Voltmeter that measures changes in voltage with time.

Question 18

How is period calculated on a CRO?

Answer 18

Record time-base setting
Count number of divisions for the amount of T

Equation:
Number of periods = div x time base

Question 19

What is the formula for wave speed?

Answer 19

Velocity = frequency x wavelength

OR

Velocity = wavelength / T

Question 20

Define Doppler Effect.

Answer 20

The change in observed frequency due to the relative motion between the source of the wave and the observer.

Equation:

f0 = fs V / V +- Vs

f0 - observed frequency
fs - frequency of source of wave
V - speed of the wave
Vs - speed of the source of the wave

for low frequencies:

f0 = fs V / V + Vs

for high frequencies:

f0 = fs V / V - Vs

Question 21

Describe electromagnetic waves.

Answer 21

Electromagnetic waves have a vibrating electric and magnetic field at 90 degrees to the direction of travel of the wave.

This means that all electromagnetic waves are all transverse waves.

All electromagnetic waves travel at a speed of 3.0x10 8 ms-1 in a vacuum.

All electromagnetic waves can be polarised.

Question 22

Describe the electromagnetic spectrum.

Answer 22

Example ——– Wavelength/m ——– Frequency/Hz

Radio waves ——– >10 6 to 10 -1 ——– 300 to 3x10 9
Micro waves ——– 10 -1 to 10 -3 ——– 3x10 9 to 3x10 11
Infra-red ——– 10 -3 to 7x10 -7 ——– 3x10 11 to 4.3x10 14
Visible light ——– 7x10 -7 to 4x10 -7 ——– 4.3x10 14 to 7.5x10 14
Ultraviolet ——– 4x10 -7 to 10 -8 ——– 7.5x10 14 to 3x10 16
X-rays ——– 10 -8 to 10 -13 ——– 3x10 16 to 3x10 21
Gamma rays ——– 10 -10 to 10 -16 ——– 3x10 18 to 3x10 24

Question 23

Define plane polarised light.

Answer 23

Light that has passed through a polaroid.

Question 24

What is the Malus Law equation?

Answer 24

I = I0 cos squared theta

If I/I0 = 1, I = I0, plane polarised light
If I/I0 = 1/2, I = I0/2, half of the intensity is able to pass through.
If I/I0 = 1/4, I = I0/4, 3/4 of the intensity is able to pass through.
If I/I0 = 0, no light is able to pass through.

I - Transmitted Wave Intensity
I0 - Initial Incident Wave Intensity