Light and Sound

Question 1

what type of wave is visible light?

Answer 1

TRANSVERSE

Question 2

what can visible light be?

Answer 2

reflected and refracted

Question 3

what does reflection of visible light allow us?

Answer 3

to see most objects. light bounces off them into our eyes.

Question 4

what happens when light is reflected from an uneven surface?

Answer 4

when light is reflected from an uneven surface, such as a piece of paper, the light reflects off at all different angle and you get a DIFFUSE REFLECTION

Question 5

what happens when light is reflected from an even surface?

Answer 5

when light is reflected from an even surface (smooth and shiny like a mirror) then its all reflected at the same angle and you get a clear reflection.

Question 6

what is the law of reflection?

Answer 6

• angle of incidence = angle of reflection

- this applies to EVERY REFLECTED RAY.

Question 7

what is the normal?

Answer 7

• an imaginary line thats perpendicular (at right angles) to the surface at the point of incidence (the point where the wave hits the boundary)
• the normal is usually shown as a dotted line.

Question 8

where is the angle of incidence?

Answer 8

the angle of incidence is the angle between:

• the incoming wave and
• the normal

Question 9

where is the angle of relfection?

Answer 9

the angle off reflection is the angle between:

• the reflected wave and
• the normal

Question 10

when are virtual images formed?

Answer 10

virtual images are formed when the light rays bouncing off an object onto a mirror are diverging, so the light from the object appears to be coming from a completely different place.

Question 11

how is a light wave refracted?

Answer 11

• waves travel at different speeds in mediums of different densities.
• EM waves travel more slowly in denser medium usually. Sound waves travel faster in denser substances.
• so when a wave crosses a boundary between two substances, e.g. from glass to air, it changes speed.

Question 12

what happens if a wave hits a boundary “face on”?

Answer 12

it slows down but carries on in the same direction

Question 13

what happens if a wave hits a boundary at an angle?

Answer 13

• it will hit the denser layer and slow down. this leads to the wave refracting and bending TOWARDS the normal.
• the wave changes direction - its been refracted.

Question 14

how do you draw a ray diagram for a refracted ray?

Answer 14

1. start by drawing the boundary between the two materials and the normal
2. draw an incident ray that meets the normal at the boundary.
3. the angle between the ray and the normal is the angle of incidence (if you’re given this ray draw it carefully with a protractor)
4. now draw the refracted ray on the other side of the boundary. if the second material is denser than the first, the refracted ray bends towards the normal. the angle between the refracted ray and the normal (angle of refraction) is smaller than the angle of incidence
5. if the second material is less dense, the angle of refraction is larger than the angle of incidence.

Question 15

what happens to rays passing through a glass block?

Answer 15

they are refracted twice.

Question 16

PRACTICAL: refraction, using a light source and a rectangular block of a particular material resting on top of a piece of paper.

Answer 16

1. shine a light ray at an angle into the block. some of the light is reflected, but a lot of it passes through the glass and gets refracted as it does so.
2. trace the incident and emergent rays onto the paper and remove the block. you can draw in the refracted ray through the block by joining the ends of the other two rays with a straight line
3. you should see that as the light passes from the air into the block (a denser medium), it bends towards the normal. this is because it slows down.
4. when the light reaches the boundary on the other side of the block it passes into a less dense medium. so it speeds up and bends away from the normal. (some of the light is also reflected at the boundary).
5. the light ray that emerges on the other side of the block is now travelling in the same direction it was to begin with, its been refracted towards the normal and then back again by the same amount.

Question 17

what do triangular prisms do?

Answer 17

DISPERSE WHITE LIGHT.

• different wavelengths of light refract by different amounts, so white light (which is a mixture of all visible frequencies) disperses into different colours as it enters a prism and the different wavelengths are refracted by different amounts.
• a similar effect happens as the light leaves the prism, which means you get a nice rainbow effect.

Question 18

what does every transparent material have?

Answer 18

a refractive index.

Question 19

what does the refractive material of a transparent material tell you?

Answer 19

how fast light travels in that material.

Question 20

what is the refractive index of a material defined as?

Answer 20

refractive index (n) = speed of light in a vacuum (c) / speed of light in that material (v)

n = c / v

Question 21

what does Snell’s Law say?

Answer 21

when an incident ray passes into a material:

n = sin i / sin r

so if you know any two of n, i or r, you can work out the missing one.

Question 22

PRACTICAL: find the refractive index of glass using a glass block.

Answer 22

1. draw around a rectangular glass block on a piece of paper and direct a ray of light through it at an angle. trace the incident and emergent rays, remove the block, then draw in the refracted ray.
2. you need to then draw in the normal at 90° to the edge of the block, at the point where the ray enter the block
3. use a protractor to measure the angle of incidence (i) and the angle of refraction (r). remember, these are the angles made with the normal.
4. calculate the refractive index (n) using Snell’s Law n = sin i / sin r

Question 23

PRACTICAL: use semicircular blocks to show Total Internal Reflection

Answer 23

1. light going from a material with a higher refractive index to a material with a lower refractive index speeds up and so bends away from the normal - e.g. when travelling from glass into air.
2. if you keep increasing the angle of incidence (i), the angle of refraction (r) gets closer and closer to 90°
3. eventually i reaches a critical angle C for which r = 90°. the light is refracted right along the boundary.
4. above this critical angle, you get total internal reflection - no light leaves the medium.
5. an experiment to demonstrate this uses a semicircular block instead of a rectangular one. the incident light ray is aimed at the curved edge of the block so that it always enters at right angles to the edge. thus means it doesn’t bend when it enters the block, only when it leaves from the straight edge.
6. to inestigate the critical angle, C, mark the positions of the rays and the block on paper and use a protractor to measure i and r for different angels of incidence.

Question 24

(in semicircular blocks) if the angle of incidence is LESS THAN CRITICAL ANGLE…

Answer 24

most of the light passes out but a little bit is internally reflected

Question 25

(in semicircular blocks) if the angle of incidence is EQUAL TO CRITICAL ANGLE…

Answer 25

the emerging ray comes out along the surface. theres a lot of internal reflection

Question 26

(in semicircular blocks) if the angle of incidence is GREATER THAN CRITICAL ANGLE…

Answer 26

no light comes out. its all internally reflected, i.e. total internal reflection.

Question 27

how can you use Snell’s Law to find critical angles?

Answer 27

you can find the critical angle, C, of a material using this equation:

sin c = 1 / n

the higher the refractive index, the lower the critical angle.

Question 28

how do optical fibres use total internal reflection?

Answer 28

• optical fibres made of glass consist of a central core surrounding by cladding with a lower refractive index.
• he core of the fibre is so narrow that light signals passing through it always hit the core-clading boundary at angles higher than C, so the light is always totally internally reflected. it only stops working if the fibre is bent too sharply.

Question 29

how do prisms use total internal reflection?

Answer 29

• total internal reflection also allows us to use prisms to see objects that aren’t in our direct line of site. this is how a periscope works.
• the ray of light travels into one prism where it is totally internally reflected by 90°
• it then travels to another prism lower down and is totally internally reflected by another 90°
• the ray is now travelling parallel to its initial path but at a different height.

Question 30

what are sound waves?

Answer 30

• sound waves are longitudinal waves caused by vibrating objects.
• the vibrations are passed through the surrounding medium as a series of compressions.

Question 31

what frequencies can a human ear hear?

Answer 31

• the sound wave may eventually reach someone’s eardrum, at which point the person might hear it.
• the human ear is capable of hearing sound with frequencies between 20Hz and 20000 Hz.

Question 32

what happens to sound waves in a denser medium?

Answer 32

• because sound waves are caused by vibrating particles, in general the denser the medium, the faster sounds travels through it.
• this also means it can’t travel through a vacuum, where there aren’t any particles.

Question 33

where does sound generally travel faster?

Answer 33

sound generally travels faster in solids than in liquids, and faster in liquids than in gases.

Question 34

where will sound waves be reflected?

Answer 34

• sound waves will generally be reflected by hard flat surfaces.
• things like carpets and curtains act as absorbing surfaces, which will absorb sounds rather than reflect them.

Question 35

where will sound waves be refracted?

Answer 35

• sound waves will also refract (change direction) as they enter different media.
• as they enter denser material, they speed up.
• however, since sound waves are always spreading out so much the change in direction is hard to spot under normal circumstances.

Question 36

what is an oscilloscope and what does it display?

Answer 36

• a sound wave receiver, such as a microphone, can pick up sound waves travelling through the air
• to display these soundwaves, and measure their properties, you can plug the microphone into an oscillioscope.
• an oscilloscope is a decide which can display the microphone signal as a trace on a screen.
• the appearance of the wave on the screen tells you whether the sound is loud or quiet, and high or low pitched. you can even take detailed measurements to calculate the frequency of the sound by adjusting the settings of the display.

Question 37

what happens the louder the sound?

Answer 37

loudness increases with amplitude.

• the greater the amplitude of a wave, the more energy it carries.
• in sound this means it’ll be louder.
• louder sound waves will also have a trace with a larger amplitude on an oscilloscope

Question 38

what happens the higher the frequency?

Answer 38

the higher the frequency, the higher the pitch.

• frequency is the number of complete vibrations each second and is measured in Hz.
• you can compare the frequency of waves on an oscilloscope, the more complete cycles are displayed on the screen, the higher the frequency (if the waves are being compared on the same scale)
• if the source of sound vibrates with a high frequency the sound is high pitched, e.g. a speaking mouse.
• if the source of sound vibrates with a low frequency the sound is low pitched e.g. a mooing cow.

Question 39

PRACTICAL: find the period of a wave on an oscilloscope to get its frequency.

Answer 39

• the horizontal axis on the oscilloscope display is time.
• the time between each division on the scale can be adjusted to get a clear, readable trace.
• adjust the time division setting until the display shows at least 1 complete cycle.
• read off the period, the time taken for one complete cycle.

Question 40

PRACTICAL: use an oscilloscope to measure the speed of sound

Answer 40

• by attaching a signal generator to a speaker you can generate sounds with a specific frequency. you can use two microphones and an oscilloscope to find the wavelength of the sound waves generated.
  1. the detected waves at each microphone can be seen as a separate wave on the oscilloscope.
  2. start with both microphones next to the speaker, then slowly move one away until the two waves are aligned on the display, but exactly one wavelength apart.
  3. measure the distance between the microphones to find the wavelength.
  4. you can then use the formula v = f x λ to find the speed (v) of the sound waves passing through the air - the frequency (f) is whatever you set the signal generator to in the first place
• speed of sound in air is 340m/s, so check results roughly agree with this.

Question 41

what is the speed of sound in air?

Answer 41

340 m/s