Waves Part 1

Question 1

what is a wave

Answer 1

• transfers energy from one place to another with no overall transfer of matter
• to make a wave something needs to vibrate and this can be a collection of particles or an electromagnetic field - the thing that vibrates is called a medium

Question 2

what evidence is there that waves travel and matter does not

Answer 2

• particles do not move in sound waves or all the particles would accumulate at the end with an absence of particles in its path
• in the sea, water particles do not move with the wave otherwise there would be an absence of particles in the sea

particles vibrate around a fixed position

Question 3

what is a transverse wave

Answer 3

• vibrations are perpendicular to the direction of energy transfer
• they have peaks and troughs
• water waves
• electromagnetic waves (light)

Question 4

what is a longitudinal wave

Answer 4

• vibrations are parallel to the direction of energy transfer
• compressions and rarefactions
• sound waves

Question 5

definition of frequency

Answer 5

the number of vibrations per second - measured in Hz (Hertz)

Question 6

definition of time period

Answer 6

the time for one vibration - measured in seconds

Question 7

definition of amplitude

Answer 7

maximum displacement of a vibration from its rest position

Question 8

definition of wavelength

Answer 8

distance from a point on a wave to the same point on the next wave - measured in metres

Question 9

relationship between time period and frequency

Answer 9

time period is 1/frequency

Question 10

what is wave speed

Answer 10

speed at which energy is transferred throughout a medium

Question 11

how do you measured speed of sound in air

Answer 11

• Person one should stand a large distance away from person two and make an obvious visible sound.
• The distance between them (at least 100m) should be measured with a tape measure.
• Person two should start their stop clock when they see the sound being made and stop it when they hear the sound.
• The speed of sound can then be calculated from speed = distance / time.

Question 12

how can you measure speed of ripples on water surface

Answer 12

• Using a ripple tank you can measure the distance across ten waves using a ruler and then divide by 10 to find one wavelength.
• You can read the frequency from the signal generator that is driving the ripple tank making the waves.
• The speed can then be calculated using speed = frequency x wavelength.

Question 13

how do you determine speed of waves on a string

Answer 13

• Stretch a string over a pulley using masses.
• Use a frequency generator to generate a wave in the string.
• Measure the wavelength when a clear wave is shown and note the frequency on the generator.
• Use the wave speed equation to determine the speed.

To vary speed, frequency of generator can be varied or increase tension on string by increasing the masses on the end.

Question 14

difference between diffuse and specular reflection

Answer 14

• specular is when light is incident on smooth surfaces - reflected in a predictable manner and always reflect parallel to each other
• diffuse is when light is incident on rough surfaces - parallel rays do not remain parallel and are scattered in different directions

Question 15

law of reflection

Answer 15

angle of incidence is equal to the angles of reflection

Question 16

what is a virtual image

Answer 16

virtual image is one where the light appears to come from but does not actually come from - like in a mirror - whereas a real image is where the light rays are really coming from

Question 17

what is refraction

Answer 17

• change in speed in a wave when it reaches the boundary between two different materials
• it happens as materials have different optical densities
• they cause the wave to change direction

Question 18

what happens to frequency during refraction

Answer 18

nothing - the source of the wave does not change so nor does the frequency

Question 19

what happens to wavelength during refraction

Answer 19

• if wave speeds up, wavelength will increase
• if wave slows down, wavelength will decrease

Question 20

what effects does refraction of light have

Answer 20

refraction at water/air boundaries cause objects to appear closer to the water surface then they actually are - this is why straw looks broken in a glass of water

Question 21

what happens to parallel rays of light when they enter a convex lens

Answer 21

parallel rays converge at the focal point, creating the focal length, the distance from the centre of the lens to the focus

Question 22

convex diagrams

Answer 22

To accurately draw lens diagrams:
1. Ray from top of object horizontal to lens then down through F on other side.
2. Ray from top of object directly through the centre of the lens.
At this point the rays should cross and this is where the “top” of the image is.
A third ray can be drawn down through F nearest the object (if possible) then horizontal through the lens
to confirm the position of the image.

Question 23

what happens to parallel waves of light when they enter a concave lens

Answer 23

parallel rays are made to diverge away from the lens - focal point is on the same side of the lens as the object

Question 24

concave diagrams

Answer 24

To accurately draw lens diagrams:
3. Ray from top of object horizontal to lens then back through F on the same side (this bit dotted).
4. Ray from top of object directly through the centre of the lens.
At this point the rays should cross and this is where the “top” of the image is.

Question 25

convex and concave symbols

Answer 25

() and )(

Question 26

what happens to white light when it is shone through a prism

Answer 26

• separate colours of spectrum are seen
• called dispersion
• different colours of light therefore have different wavelengths so change speed by different amounts, and are refracted by various amounts

Question 27

why do opaque objects appear the colour they do

Answer 27

coloured objects only reflect the wavelengths of light (colour) that they appear and absorb all others

white object reflect everything and black object absorbs everything

Question 28

what is a transparent object

Answer 28

transparent objects transmit most light incident on them

Question 29

what is a translucent object

Answer 29

translucent objects transmit light that is incident on them but only partially

Question 30

what is a filter

Answer 30

only allows light of wavelengths of the colour that it is pass through it and all other wavelengths are absorbed - e.g. red filters absorb all other colours that are not red

Question 31

what happens to colour of object if it viewed through a colour filter

Answer 31

If the filter allows the wavelength of light corresponding to the object to pass it will look the colour it is. If the filter does not allow the wavelength of light corresponding to the object to pass it will look black.