Les Vagues

Question 1

What is wavelength?

Answer 1

distance btw 2 successive pts (eg consecutive crests/troughs) in a wave which are in phase

Question 2

Consider a wave of amplitude A, period T, what are 2 possible displacement-time eqn?

Answer 2

y = Asin(2πt/T)
OR
y = Acos(2πt/T)

Question 3

From a graph, how to know motion of particle?

Answer 3

Draw same graph slightly to the right; next waveform (to show particle’s position at next instant)
eg at amplitude particle is instantaneously at rest

Question 4

What are the meanings of:
- in phase
- in antiphase
- out of phase

Answer 4

1. In phase
   when 2 oscillating particles hv φ of 0 or any integer multiple of 2π
2. In antiphase
   When 2 oscillating particles hv φ of π or any odd integer multiple of π
3. Out of phase
   when 2 oscillating particles hv φ other than 0 or any integer multiple of π (any other angle; NOT to confuse w antiphase)

*NOTE: don’t forget that φ is in rad

Question 5

What is the eqn of phase diff, φ, for distance and time?

Answer 5

1. distance:
   φ = 2πx/λ
2. time:
   φ = 2πt/T

Question 6

How to derive v=fλ?

Answer 6

1. Consider wave of wavelength λ, period T
2. From def n of wavelength, in 1 cycle, wave energy moves distance λ, & time taken for 1 cycle is T
3. Speed v is distance moved per unit time
   v= λ/T
4. Since f is related to T by f=1/T, thus
   v=f λ

Question 7

Define transverse and longitudinal wave

Answer 7

• Transverse wave is one in which direct n of oscillat n of particles is perpendicular to direct n of energy transfer of wave
• Longitudinal wave is one in which direct n of oscillat n of particles is parallel to direct n of energy transfer of wave

Question 8

What is relationship between energy and amplitude of wave?

Answer 8

energy is directly proportional to square of amplitude

Question 9

Define intensity

Answer 9

power per unit area perpendicular to direct n of wave propagat n

eqn: Intensity = Power/Area

Question 10

intensity … amplitude² ?

Answer 10

is directly proportional to

Question 11

Intensity of wave at distance, r, fr pt source is given by what eqn?

What can you tell abt this eqn?

Answer 11

Intensity, I = Power/4πr²
*NOTE: surface area of sphere, not volume!

eqn shows inverse square law, where

Intensity is inversely proportional to r²

Question 12

For a spherical wave emitted fr pt source of constant power,
amplitude … r?

Answer 12

is inversely proportional to

Question 13

Explain what is polarisation

Answer 13

Said to occur when particles in wave oscillate oni in single direct n perpendicular to direct n of energy transfer of wave

Question 14

What is Malus’ Law

Answer 14

New Intensity = Icos²θ,

where polarising filter is at an angle θ to polarising axis, and I is og intensity

Question 15

How does amplitude and intensity of unpolarised light change when it is polarised?

Answer 15

Amplitude same, intensity is halved

Question 16

What are the wavelengths of electromagnetic spectrum? (hint: Germaine do not…)

Answer 16

C (Cosmic rays) - 10^-12 m
G
X - 10^-9 m
U
V
I - 10^-6 m
M
radar - 10^-3
R - 1m
broadcast band - 10^3m / 1km

*All travel at 3.00 x 10^8 m s^-1 in vacuum

Question 17

What are the wavelengths of visible light?

Answer 17

• 700nm
  R
  O
  Y
• 600nm
  G
  B - 500nm
  I
  V - 400nm

Question 18

Define progressive wave

Answer 18

progressive waves move energy IN DIRECTION OF TRAVEL OF WAVE from place to place without transfer of matter