Waves

Question 1

What is a wave?

Answer 1

A wave is a disturbance of particles that transfers energy through a medium.

Question 2

Define Mechanical wave

Answer 2

A mechanical wave is a wave that propagates through a medium.

Question 3

Compare transverse and longitudinal waves

Answer 3

Transverse waves:

• Oscillation of particles is perpendicular to the propagation of energy
• Crests and troughs
• Amplitude is measured from the equilibrium
• Wavelength is the distance between consecutive crests/troughs
• Example: water and light

Longitudinal waves:

• Oscillation of particles is parallel to the propagation of energy.
• Amplitude is measured from the equilibrium
• Wavelength is the distance between the middle of wave compressions
• Example: Sound

Question 4

Compression

Answer 4

a region in the longitudinal wave where the particles are closest

Question 5

Rarefaction

Answer 5

A region in a longitudinal wave in which the particles are furthest apart

Question 6

Crest

Answer 6

the point on a transverse wave of maximum positive displacement

Question 7

Trough

Answer 7

the point on a transverse wave of maximum negative displacement

Question 8

Amplitude

Answer 8

The maximum distance from an equilibrium

Question 9

Period

Answer 9

The amount of time taken for one cycle to complete

P = 1/F

Question 10

Frequency

Answer 10

The number of cycles per second

F = 1/P

Question 11

Wavelength

Answer 11

the minimum distance between two consecutive points that are in phase.

Question 12

The velocity of a wave

Answer 12

V = d/t

Question 13

Reflection of a wave

Answer 13

A process whereby an incident wave changes direction back into the same medium according to the law of reflection.

Question 14

Refraction of a wave

Answer 14

A process whereby an incident wave changes direction and speed at a boundary when passing into another medium.

Question 15

Diffraction of a wave

Answer 15

The process by which an incident wave bends around a barrier or the wavefront is broken into smaller wave sources.

Question 16

Describe the superposition of a wave

Answer 16

The process whereby two waves overlap in space whose resultant wave is the algebraic sum of the individual waves.

Question 17

Using the wave model of light, explain phenomena relating to reflection and refraction

Answer 17

Due to the fact that light is both a wave and a particle, it can be reflected at a boundary according to the law of reflection and can refract through different media by changing speed and direction. This explains why the speed of light varies when moving from different media.

Question 18

Describe what happens when an incident wave of a certain medium comes in contact with a boundary of the same medium, but of different density?

What happens at fixed and free ends?

Answer 18

Lighter to heavier density

• The incident wave is reflected and inverted
• The transmitted wave is in phase
• If the density is negligible, the transmitted wave will be larger than the reflected wave. However, if the density is large, then the transmitted wave will be smaller

Heavier to a lighter density

• The incident wave is reflected and phase
• The transmitted wave is larger and in phase

Fixed ends
- The reflected wave is inverted

Free Ends
- The reflected wave is in phase

Question 19

Explain Constructive and Destructive interference of two waves

Answer 19

Constructive interference occurs when two waves of the same frequency superimpose to create a super crest with a larger amplitude. Destructive interference occurs when two waves of the same frequency but 180 degrees out of phase overlap to create a node.

Question 20

Explain the process by which standing waves are created.

Answer 20

A standing wave occurs when two waves of similar frequency and amplitude continuously constructively and destructively superimpose on one another, creating set nodes and antinodes of minimum and maximum displacement. They first meet and constructively interfere, then destructively interfere and create a node, then meet again to produce a super-crest.

Question 21

Define the concept of resonance in a mechanical system. Is energy transferred efficiently in resonating systems?

Answer 21

Resonance is the process by which the energy of one vibrating system causes another to vibrate at its natural frequency.

Energy is transferred efficiently in resonating systems.

Question 22

Define Natural Frequency

Answer 22

The frequency at which an object naturally oscillates in resonance.

Question 23

Is light a mechanical or electromagnetic wave?

Answer 23

It is an electromagnetic transverse wave that can travel in a vacuum.

Question 24

Explain Total internal reflection

Answer 24

An incident ray of light that encounters a boundary of media reflects according to the law of refraction and refracts away from the normal if the medium is denser and towards it if it is less dense. if a ray of light hits a boundary at the critical angle, the light reflects back into the medium but does not refract because the angle is such that the refracted ray becomes the boundary.

Question 25

Why is the wave model of light useful?

Answer 25

It helps us explain the reason for why light can reflect, refract, interfere, diffract and disperse, which is not possible with the particle model.

Question 26

Describe the process of Polarisation

Answer 26

Light is an electromagnetic wave, propagated by the changing magnetic and electric fields that exist in two planes. Polarisation is the process whereby only the direction of the electromagnetic field that is parallel to the polarization material is ‘let through’ a material. Therefore, a plane wave is created.

Question 27

Snell’s Law

Answer 27

When light travels from one medium to another, the ratio of the sin of the angle of incidence to the sine of the angle of refraction is equal to the refractive index of the material

Question 28

Recall the speed of light in a vacuum. Contrast the speed of light and the speed of mechanical waves

Answer 28

3 * 10^8 m/s. Light is much faster than any other mechanical wave becasue it is an electromagnetic wave. Unlike sound, which requires particles for its waves to propogate, light can travel without a medium, with less resistance, and therefore it can travel faster.

Question 29

Define intensity

Answer 29

Intensity is the average rate of flow of energy per uni area.

I = P/A