Waves

Question 1

What is waves?

Answer 1

Wave is a transfer of energy from one point to another due to oscillation without physical transfer of matter

Question 2

What is the difference between mechanical and EM waves?

Answer 2

• mechanical waves require medium while electromagnetic waves do not require medium
• mechanical waves propagate due to compression and rarefaction of molecules while EM waves consist of mutually perpendicular time-varying electric and magnetic fields
• mechanical waves’ speed depends on medium while EM waves travel at speed of light in vacuum

Question 3

What are some examples of mechanical waves?

Answer 3

Sound, waves on string, water waves

Question 4

What are some examples of EM waves?

Answer 4

Microwaves, visible light, gamma ray, X ray

Question 5

What are transverse waves?

Answer 5

Waves where the plane of oscillations are perpendicular to the direction of energy propogation

Question 6

What are longitudinal waves?

Answer 6

Waves where the plane of oscillations of the particles are parallel to the direction of energy propagation

Question 7

What is the difference between transverse and longitudinal waves besides the direction of oscillations of the particles?

Answer 7

Transverse waves can be polarised while longitudinal waves cannot be polarised

Question 8

What is a progressive transverse mechanical wave?

Answer 8

Energy and wave profile are propagated from one point to another due to vibrations. For transverse mechanical waves, there is no net transfer of matter

Question 9

What is displacement?

Answer 9

Distance of a particle in a given direction from its equilibrium position

Question 10

What is amplitude?

Answer 10

Maximum distance of a particle from equilibrium point

Question 11

What is a wavelength?

Answer 11

• shortest distance between two particles that are in phase
• shortest distance between two crests

Question 12

What is a period?

Answer 12

• time for a particle in the wave to complete one complete oscillation
• time taken for the wave to travel one wavelength

Question 13

What is frequency?

Answer 13

The number of oscillations per unit time

Question 14

What is speed of wave?

Answer 14

Distance travelled by the wave profile per unit time

Question 15

What is a wavefront?

Answer 15

An imaginary line that joins all the points of the wave that are in phase

Question 16

What is phase angle?

Answer 16

An angle in degrees or radians which measures fraction of a cycle that has been completed by an oscillating particle

Question 17

What is phase difference?

Answer 17

Phase difference is the difference in oscillation stages as a fraction of a cycle between two points and two waves at a specific time, expressed in radians or degrees

Question 18

What is polarisation?

Answer 18

Polarisation is where oscillations are restricted to one plane only, perpendicular to the direction of wave propagation

Question 19

What is Malus law?

Answer 19

intensity of a light passing through an analyser and polariser is proportional to cos²θ, where θ is the angle through which the analyser has been rotated with respect to the polariser

Question 20

How does a polarised transverse wave differ from an unpolarised transverse wave?

Answer 20

• For a polarised transverse wave, the oscillations are in a single plane, normal to the direction of energy transfer
• For an unpolarised transverse wave, the oscillations are in multiple planes and directions

Question 21

Why can’t longitudinal waves be polarised?

Answer 21

In a longitudinal wave, the direction of vibration of particles is the same is the direction of wave propagation, hence there is no plane to restrict the movement of the vibrating particles

Question 22

Why only transverse waves can be polarised by the polariser?

Answer 22

• For transverse mechanical wave, the vibration of the particles is perpendicular to the direction of wave motion
• Hence, the movement of the particles will be restricted in one plane if the plane of polarisation is parallel to the direction of wave motion
• For electromagnetic waves, the electric field component parallel to the axis of polarisation will be allowed to pass through, while its perpendicular component will be absorbed by the polariser

Question 23

What is the principle behind the usage of Polaroid glasses used in 3D movies?

Answer 23

• The polarisation axis of the polaroid filters are set at right angles to each other - one horizontal and one vertical
• Each eye sees a separately polarised picture and the brain then interprets the two pictures as a single picture with depth, hence 3D effect is achieved

Question 24

How does a polaroid polarise light?

Answer 24

• Electrons absorbed the energy of electric field components in the plane of the direction of travel of wave propagation
• Only vibrations of electric field components in the perpendicular plane can pass through

Question 25

What is intensity?

Answer 25

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

Question 26

What is progressive wave?

Answer 26

A wave in which energy is carried from one point to another by means of vibrations or oscillations within the wave, particles within the wave are not transported along the wave