Physics (Unit 3: Waves)

Question 1

What are waves and what types are there?

Answer 1

Waves are a means of transferring energy without transferring matter. This is described as oscillation (repetitive fluctuations from side to side or as vibration upon a fixed positioon)

Question 2

Explain compression and rarefraction?

Answer 2

• Features of a longitudinal wave affecting matter
• In compression, particles are brought closer together due to higher pressure
• In rarefraction, particles are brought furthur away due to lower pressure

Question 3

Define the following terms:
- Peak/crest
- Dip/trough
- Equilibrium line
- Frequency:
- Time period
- Wavelength
- Amplitude

Answer 3

Crest: maximum height of a wave above the equilibrium line

Trough: minimum height of a wave below the equilibrium line

Equilibrium line: line in the middle of the wave that represent the level if it flattens out

Frequency: number of waves that pass a given point per second

Period: time it takes for a wave to complete one cycle

Wavelength: distance between one entire oscillation of that wave

Amplitude: maximum displacement point of a wave from the equilibrium line

Question 4

Describe and compare electromagnetic and mechanical waves?

Answer 4

Electromagnetic:
- Only transverse
- Doesn’t require matter
- Can travel through a vacuum
- In case of light, higher amplitude means higher intensity

Mechanical:
- Either transverse or longitudinal
- Requires matter
- Can’t travel through vacuum

Question 5

What are the formulae for measuring wave speed?

Answer 5

V=λf
V=Wave speed (m/s)
λ=Wavelength (m)
f=Frequency (Hz)

V=λ/T
T=Time period

Question 6

What is the formula for measuring time period?

Answer 6

T=1/f
f=1/T

Question 7

Explain reflection and its properties

Answer 7

• Refers to the change in direction of wave as a result of contact with something while staying in the same medium
• Angle of incidence is always equal to angle of reflection (i=r)
• Frequency, wavelength, and wave speed remain consistent
• Normal=imaginary line perpendicular to surface the wave comes in contact with
• Rough surfaces reflects the light in all directions

Question 8

Explain diffraction and its properties

Answer 8

• The spreading out of waves as they go around an obstacle or through a gap
• The narrower the gap and the greater the wavelength, the more diffraction there is
• Frequency, wavelength, wave speed unchanged
• Gap width shorter than wavelength = reflection
• Gap width similar size to wavelength = diffraction
• Gap with larger than wavelength = most waves pass through; slight diffraction at edges

Question 9

Explain refraction and its properties

Answer 9

• The redirection of a wave resulting from it entering a substance of different optical density
• Cause by ray of light entering substance at an agnle, causing different parts of the ray to speed/slow at different moments
• As the optical density increase, wave speed and wavelength decreases
• When ray enters more optically dense substance, it bends towards the normal
• In less optically dense substance, it bends away from the normal

Question 10

Explain the refractive index and its relationship with angles of incidence and reflection

Answer 10

• Refractive index is the ratio of speed of light in a vacuum to speed of light in a particular medium
• n=c/v (c is speed of light in vacuum, v is speed of light in substance)
• Snell’s Law: n1xsin θ = n2 x sin θ2
• n = sin i/sin r

Question 11

Briefly explain each type of EM wave

Answer 11

Radio waves:
Sources:
Oscillations in electrical circuits

Uses:
Telecommunications, TV, radio, bluetooth

Dangers:
None

Microwaves:
Sources:
Electronic circuits, cool objects

Uses:
Telecommunications, RADAR, cooking

Dangers:
Burns, cataracts

Infra-red waves:
Sources:
Electronic devices, warm objects

Uses:
Heating, cooking, TV remotes, night vision

Dangers:
Burns

Visible light waves:
Sources:
Sun, hot objects, electronic circuits

Uses:
Seeing, photos, transmitting data

Dangers:
Eye damage from bright lights

Ultraviolet waves:
Sources:
Sun, lamps, very hot objects

Uses:
Tanning salons, counterfeit detections, pollination, discos

Dangers:
Skin cancer

X-rays:
Sources:
Very fast electrons hitting a metal plate

Uses:
Meical imagery, security

Dangers:
Skin cancer

Gamma rays:
Sources:
Changes in atoms and their nuclei which leads to radioactive decay

Uses:
Cancer treatment, sterilizing hospital equipment, observing universe

Dangers:
Cancer

Question 12

Order the following EM waves in ascending order of frequency and descending order of wavelength:

Ultraviolet waves
X-rays
Radio waves
Infra-red waves
Microwaves
Gamma rays
Visible light

Answer 12

Radio waves
Microwaves
Infra-red waves
Visible light waves
Ultraviolet waves
X-rays
Gamma rays

Question 13

Describe the two types of lenses and their properties.

Answer 13

Lenses:
- Due to varying normals since lenses are curved, light refracts in a way that either converges or diverges it

Concave:
- Lens that bends inwards
- Light hits lens and refracts
- Lens is more optically dense so light moves towards the normal
- Causes light ray to diverge
- Images formed can be real or virtual
- Real images are made when the distance of object from centre of the lens is greater than the focal length; therefore they actually converge onto a position and can be projected onto a screen
- Virtual images are made when the distance of the object from the centre of the lens is less than the focal length; therefore they appear to have converged and cannot be projected onto a screen

Convex:
- Lens that bends outwards
- Light hits lens and refracts
- Lens is more optically dense so light moves towards the normal
- Causes light ray to converge towards a focal point
- Distance between middle line and focal point is called the focal length
- Images formed can be real or virtual
- Real images are made when the distance of object from centre of the lens is greater than the focal length; therefore they actually converge onto a position and can be projected onto a screen
- Image is inverted and magnified
- Virtual images are made when the distance of the object from the centre of the lens is less than the focal length; therefore they appear to have converged and cannot be projected onto a screen
- When seen from the right angle, it is upright and magnified

Note: rays that go through optical center continue straight forward

Question 14

Describe the properties of light waves (different types of surfaces, colours, surfaces)

Answer 14

Opaque surfaces: no light transmitted
Transluscent: some light transmitted
Transparent: light transmitted

Colours:
Primary: red, blue, green
Secondary: magenta, cyan, yellow

Black: abscence of light
White: all colouors combined
All other colours: some combinatioon of each primary colour

Filters: only colour of filter passes through

Question 15

Describe transverse and longitudinal waves.

Answer 15

Transverse: vibrations occur perpendicular to the direction energy is transferred.

Longitudinal: vibrations that occur in the same direction as the transfer of energy.