Waves

Question 1

Describe what is meant by wave motion as

illustrated by vibration in ropes and springs

Answer 1

Ropes:
- Waves on ropes can be produced by moving one end of the rope from side to side repeatedly. This action sends waves along => each part of the rope vibrates from side to side
Spring:
- Waves on a spring can be produced by fixing one end of the spring and moving the other end repeatedly to and fro. Each part of the spring vibrates and makes the next part vibrate

Question 2

Describe what is meant by wave motion as

illustrated by experiments using water waves

Answer 2

Make straight waves by moving ruler up and down on the water surface. The wavefronts travel away from ruler. This is wave motion. Straight wavefronts are plane waves

Question 3

Wave equation

Answer 3

velocity = frequency x wavelength
period = 1/ freq.
frequency= 1/period

Question 4

Definition of frequency

Answer 4

the number of complete waves passing a point. Measured in Hertz

Question 5

Definition of wavelength

Answer 5

is the distance of one wave crest to the next

Question 6

Definition of amplitude

Answer 6

is the height of the wave crest/the depth of the wave trough from the middle.

Question 7

Distinguish between transverse and longitudinal

waves and give suitable examples

Answer 7

Transverse:
- The vibrations of the waves are perpendicular to the direction the waves are travelling in
- Can be polarised or unpolarised
e.g. light
Longitudinal
- The vibrations of the waves are parallel to the direction of the waves are travelling in
- Cannot be polarised
e.g. sound

Question 8

Describe the use of water waves to show:
– reflection at a plane surface
– refraction due to a change of speed
– diffraction produced by wide and narrow gaps

Answer 8

Reflection (fig.7.3.1 pg108 to support)
- Plane waves reflect from a straight barrier at the same angle to the barrier as the same incident waves
Refraction:
- Incident waves in deep water is refracted when entering shallow water due to change of speed
Diffraction:
- The narrower the gap, the more the waves spread out

Question 9

Relationships between angle of incidence and reflection

Answer 9

equal

Question 10

Describe the formation of an optical image by a

plane mirror, and give its characteristics

Answer 10

The image in the mirror is the same size as the object

The image is behind the mirror

Question 11

From shallow to deep water/ From glass to air

Answer 11

away from normal

Question 12

Definition of refractive index in terms of speed and its equation

Answer 12

Def: is a measure of the change of direction of a light ray at non-normal incidence when it passes from air into substance, therefore changes speed at boundary
Eq: speed of light in air/speed of light in substance = sin i/ sin r

Question 13

Use the terminology for the angle of incidence i and
angle of refraction r and describe the passage of
light through parallel-sided transparent material

Answer 13

Angle of incidence > Angle of Refraction when light travels from a less dense material to a more dense material.
Likewise, the opposite occurs when light travels from a dense medium to a less dense medium.

Question 14

Definition of critical angle

Answer 14

The angle of incidence at which maximum refraction occurs is called the critical angle

Question 15

As the angle of incidence increases

Answer 15

• There is less refraction

- There is more reflection

Question 16

Angle of incidence < critical angle
Angle of incidence = critical angle
Angle of incidence > critical angle

Answer 16

• Refraction away from normal
• Angle of refraction is 90 degrees/ Refracts along the surface
• Total Internal Refelection

Question 17

Describe internal and total internal reflection

Answer 17

Total: because all energy is reflected
Internal: all energy stays within the material
Reflection: light is reflected

Question 18

Describe the action of optical fibres particularly in medicine and communications technology

Answer 18

Optical fibers are essentially cables made from high-quality glass.
When light enters one end of an optical fiber cable, it undergoes total internal reflection until it reaches the end of the cable.
The advantages of using optical fiber is
- signals carried by the optical fiber do not weaken over long distances
- optical fiber cables can carry a larger quantity of information
- can be transmitted at high speeds since digital information can be converted to visible light or infrared signal and be sent across optical fiber cables.

Question 19

Describe the action of a thin converging lens on a

beam of light (use the term principal focus and focal length)

Answer 19

Parallel rays of light when entering a converging lens will be focused at one point. This point is the principal focus. The focal length is the distance from the lens to the principal focus

Question 20

Applications of the converging lens

Answer 20

• beyond 2F: real, inverted, diminished (for camera)
• at 2F: real, inverted, same size
• between F and 2F: real, inverted, magnified (for projector)
• at F: no image/ image appears at infinity
• between F and the lens: virtual, upright, magnified (for magnifying glass)

Question 21

Definition of monochromatic light

Answer 21

one-coloured light

Question 22

Definition of dispersion and give examples

Answer 22

the splitting of monochromatic light into separate colours.
An example would be the splitting of white light using glass prism, the effect occurs because the refractive index of glass varies with the colour of the light

Question 23

State the approximate value of the speed

of electromagnetic waves

Answer 23

300 000 km/s through space
All electromagnetic (e.m) waves travel through space at the same speed

Question 24

List the e.m spectrum in order of decreasing wavelengths

Answer 24

radio waves -> microwaves -> infra-red -> light -> ultraviolet -> x-rays -> gamma rays

Question 25

Role of each e.m spectrum

Answer 25

• Radio waves for radio, TV, mobile phone communications
• Microwaves for satellite, telephone, heating food
• Infra-red for electrical appliances, remote controllers, optical fibre
• Light for photography and optical instruments
• Ultraviolet for security markers, sunbeds
• X-rays for airport security and kill cells
• Gamma rays for radiotherapy and killing cancer cells

Question 26

Demonstrate an awareness of safety issues

regarding the use of microwaves and X-rays

Answer 26

• Microwaves can penetrate into the body and have a heating effect on body tissues. Brain damage and cancer
• X-rays can cause cell damage and cancers

Question 27

State the approximate range of audible frequencies

Answer 27

20Hz to 20kHz

Question 28

State the order of magnitude of the speed

of sound in air, liquids and solids

Answer 28

air (340 m/s) -> liquid (1400 m/s) -> solid (3000 m/s)

Question 29

How is sound produced?

Answer 29

By vibrations

Question 30

Describe the longitudinal nature of sound waves

Answer 30

When waves pass through air, the air molecules move forwards and backwards along the direction in which the waves are moving. Therefore, sound waves are longitudinal because the vibrations are along/parallel to the direction they travel in

Question 31

Where can sound waves travel

Answer 31

in medium (solids, liquids and gases)
cannot travel in vaccuum

Question 32

Relate the loudness and pitch of sound waves to

amplitude and frequency

Answer 32

Higher the amplitude -> louder

Higher the freq. -> higher the pitch

Question 33

How is echo produced? How to prevent echo?

Answer 33

Echo happens when sound waves bounced off a smooth surface. This is a reflection of sound. To prevent echo:

• cover it with soft fabric to absorb sound waves
• make the surface uneven so reflected sound waves are scattered