Waves: Topic 4.3 Wave Characteristics

Question 1

What are wavefronts

Answer 1

Wavefronts - lines joining all the points that oscillate in phase and are perpendicular to the direction of motion (and energy transfer)

The distance between successive wavefronts is equal to the wavelength of the waves

Question 2

What are rays

Answer 2

Rays - lines showing the direction of motion (and energy transfer) of the wave that are perpendicular to the wavefront

Question 3

Define intensity

Answer 3

The intensity of a wave is defined as follows:
Power per unit area or the rate of energy transfer per unit area

Intensity is measured in W m–2

Question 4

What kind of relationship is observed with intensity of a spherical wave

Answer 4

For spherical waves being emitted by a point source equally in all directions, the intensity follows an inverse square law with distance from the point source

Question 5

What is the relationship between intensity and amplitude?

Answer 5

By definition, the intensity of a wave (its power per unit area) is proportional to the energy transferred by the wave
The intensity of a wave at a particular point is related to the amplitude of the wave at that point
The energy transferred by a wave is proportional to the square of the amplitude
Therefore, the intensity of a wave is proportional to the square of the amplitude

Question 6

The principle of superposition states that:

Answer 6

When two or more waves meet, the resultant displacement is the vector sum of the displacements of the individual waves

Question 7

What is interference and what are the conditions for it?

Answer 7

Interference occurs whenever two or more waves superpose

For a clear stationary interference pattern, the waves must be of the same:
Type
Amplitude
Frequency
They must also have a constant phase difference

Question 8

What is constructive interference

Answer 8

Constructive interference occurs when the waves superpose and have displacements in the same direction (both positive or both negative)

Question 9

What is Destructive interference

Answer 9

Destructive interference occurs when the waves superimpose and have displacements in opposite directions (one positive and one negative)

Question 10

How does constructive interference take place?

Answer 10

When two waves with the same amplitude meet at a point, they can:

Be in phase and interfere constructively, so that the displacement of the resultant wave is double the displacement of each individual wave

Question 11

How does destructive interference take place?

Answer 11

When two waves with the same amplitude meet at a point, they can:

Be in anti-phase and interfere destructively, so that the displacement of the resultant wave is equal to zero

Question 12

Outline how superposition can result in a mix of constructive and destructive interference

Answer 12

Superposition occurs for any two waves or pulses that overlap, and can result in a mix of constructive and destructive interference
For example, the peak of one wave superposes with the peak of another wave with a smaller displacement
The resultant peak will have a displacement that is in the middle of the displacement of both waves

Question 13

Explain superposition in terms of pulses

Answer 13

Superposition can also be demonstrated with two pulses
When the pulses meet, the resultant displacement is the algebraic sum of the displacement of the individual pulses
After the pulses have interacted, they then carry on as normal

Question 14

What is polarisation

Answer 14

When a transverse wave is polarised, its electric field is only allowed to oscillate in one fixed plane perpendicular to the direction of motion of the wave
A transverse wave can be vertically polarised, horizontally polarised, or polarised in any direction in between

Question 15

Why can’t longitudinal waves be polarised

Answer 15

Since longitudinal waves oscillate in the same direction as the direction of motion of the wave, polarisation of longitudinal waves cannot occur

Question 16

How can unpolarised light become polarised?

Answer 16

Light waves can be polarised by making them pass through a polarising filter called a polariser
The filter imposes its plane of polarisation on the incident light wave
A polariser with a vertical transmission axis only allows vertical oscillations to be transmitted through the filter (A)
After polarisation, If vertically polarised light is incident on a filter with a horizontal transmission axis, no transmission occurs (B), and the wave is blocked completely

Question 17

What is the intensity of polarised light

Answer 17

The intensity of unpolarised light is reduced as a result of polarisation
If unpolarised light of intensity I0 passes through a polariser, the intensity of the transmitted polarised light falls by a half

Question 18

What is the result if the analyser has same orientation as polariser

Answer 18

If the analyser has the same orientation as the polariser, the light transmitted by the analyser has the same intensity as the light incident on it

Question 19

What is Malus’s Law

Answer 19

Malus’s Law states that if the analyser is rotated by an angle θ with respect to the polariser, the intensity of the light transmitted by the analyser is:

I = I0 cos2 θ

Question 20

Which orientation of polariser and analyser gives maximum intensity

Answer 20

If an unpolarised light source is placed in front of two identical polarising filters, A and B, with their transmission axes parallel:
Filter A will polarise the light in a certain axis
All of the polarised light will pass through filter B unaffected
In this case, the maximum intensity of light is transmitted

Question 21

Which orientation of polariser and analyser gives minimum intensity

Answer 21

As the polarising filter B is rotated anticlockwise, the intensity of the light observed changes periodically depending on the angle B is rotated through
When A and B have their transmission axes perpendicular to each other:
Filter A will polarise the light in a certain axis
This time none of the polarised light will pass through filter B
In this case, the minimum intensity of light is transmitted