2 Light and Sound Flashcards
Locate the compressions and rarefactions
What is true about the position of the image behind a plane mirror
- the image is as far behind the mirror as the object is in front
- a line joining equivalent points on the object and image passes through the mirror at right angles.
Name a luminous object- an object which emits its own light.
a bulb
What is the relationship between refractive index, incident angle and refracted angle (Snell’s Law)?
Note: the angle i = angle in air
When measuring an angle of incidence, what is the most common error?
Angle of incidence is not 56°
Angle of incidence is between the incident ray and the normal
90° - 56° = 34°
incident angle = 34°
In bike reflectors, white light enters the front of the red reflector (all other colours are absorbed) and only red light emerges parallel to the incident ray.
Explain how TIR allows light to be bent by 180° allowing light to be reflected straight back to the car producing it.
- white light enters block at 0° - no refraction
- red light reaches back boundary at 45°
- 45°> the critical angle of 42°
- TIR occurs and red light reflects at 45° bending a total of 90°
- red light reaches next boundary at 45°
- 45°> the critical angle of 42°
- TIR occurs and light bend a total of 90°
- red light strikes front face of reflector at 0° (no refraction) and emerges parallel to incident ray
What type of wave is visible light?
It is a transverse wave- oscillations are 90° to direction of travel
What is the order of the visible spectrum?
red
orange
yellow
green
blue
indigo
violet
What can be altered to change the number of complete waves on the screen of a CRO?
time base
What shows that the chirp of each bird is the same pitch?
same number of waves in the same time
same frequency
Ultrasound is used in echo finding. Give one example of how human use ultrasonic echoes.
Trawlers send ultrasonic pulses into the ocean and time how long it takes for a reflection to return from the ocean floor.
distance = speed of sound in water (1500m/s) x time of echo
depth of ocean = depth / 2
What is the frequency of sound?
Number of vibrations per second
measured in Hertz Hz
Why does sound travel faster in solids compared to gases?
- Solids have stronger forces between particles compared to gases
- particles are closer together in a solid compared to a gas
- It is easier for vibrations to be passed between particles in a solid compared to a gas
You can find the refractive index of a block by following these instructions. What equipment is needed and what must be measured and calculated?
- Place the rectangular block in the centre of the paper and carefully trace its position
- Shine ray of light from ray box at an angle to the air/glass boundary
- Mark incident ray and emerging ray with two crosses using the pencil
- Remove the block and using a ruler trace the incident ray and emergent ray up to the air/glass boundary
- Join the points where the light entered and left the block, and draw a normal using a protractor.
Equipment: ray box, rectangular block of glass, paper, pencil, ruler, protractor
- Measure the incident and refracted angle using the protractor.
- Calculate the refractive index n = sin i/ sin r
- Repeat steps 1-6 for different angles, calculate the refractive index and find a mean
ALTERNATIVE:
- Repeat steps 1-6 for five different angles, plot sin i against sin r and draw a line of best fit through the origin.
- Find the gradient of the line, this equals the refractive index
An endoscope is one use of visible light. Light is sent down an optical fibre and an image of internal organs can be viewed by a doctor.
What prinicple of light does this application use?
TIR or total internal reflection of light
What happens to the speed, direction and wavelength of light when it enters a more dense medium at an angle to the normal?
Light slows down, the waves bunch up and the wavelength decreases, the waves change direction- bend towards the normal.
In a vacuum, the speed of light is 300 000 000 m/s. In air it is effectively the same. What is the refractive index of air and how does this compare to any other medium?
The refractive index of air is 1.00.
The speed of light in any other medium will be lower than 300 000 000 m/s and so the refractive index will be greater than 1.00
- i.e. the refractive index of plastic is 1.50*
- the refractive index of water is 1.33*
What property of a sound wave indicates the loudness of the sound?
amplitude
Binoculars use TIR to bend light through prisms and direct it to the back of the eye. Explain what happens in each prism?
angle of incidence = 45°
critical angle of prism is 42°
Therefore TIR occurs
If incident angle = 45°, then reflected angle= 45°
Therefore light is bent a total of 90°
What is the relationship between the critical angle and refractive index?
What is the unit for pitch of sound?
Hertz Hz
pitch = frequency
What is the difference between an real image and a virtual image?
Real image- rays from the object meet to form the image (they pass through it), it can be formed on a screen
Virtual image - no rays pass through the image, it cannot be formed on a screen
How is sound different from visible light?
sound is a longitudinal wave, light is a transverse wave
sound cannot travel through a vacuum, light can
sound travels much slower in air (340 m/s), light is much faster at 300 000 000 m/s
What type of wave is a sound wave?
Longitudinal
Medium oscillates (to and fro) parallel to direction of wave travel
If reflection is light bouncing, what is refraction?
Refraction is light bending (changing direction)
Determine the amplitude of the wave.
volts per division = 5 V/ div
amplitude = 2 division
2 divisions x 5V/division= 10V
Define the angle of refraction
the angle between the refracted ray and the normal
Light which leaves an object below the surface of water refracts away from the normal at the water/air boundary. Because of this the object looks less deep than it really it
Explain how refraction creates a real and apparent depth?
- Light leaving a point on the box is refracted at the water/air boundary so as to appear to be coming from a point vertically above the real position of the fish.
- The refraction creates the illusion that the fish is nearer the surface than it really is- apparent depth
- The refraction produces a virtual image of the fish above the real position- real depth
A prism is a triangular block of glass. The sides of the prism are not parallel.
When white light is shone through the prism, different wavelengths of light are deviated by different amounts.
What is observed and what is the effect called?
The white light splits into a range of colours called a spectrum.
The effect is called dispersion
Note: This shows that white light is a mixture of colours of the rainbow
Label the wavelength of the sound wave below
distance from middle of one compression to the middle of the adjacent compression.
compression to compression
What happens when light strikes a black object
All light is absorbed
Why does the light totally internally reflect down the glass tube?
The angle of incidence is greater than the critical angle, therefore TIR occurs
What happens to the speed, direction and wavelength of light when is enter a less dense medium?
Light speeds up, the waves spread out and the wavelength increases, the waves bend away from the normal
Explain how TIR is used in a periscope.
- Light from object enters the top 45-45-90 block at 0° (perpendicular to the boundary)- no refraction
- Light hits hypoteneuse at 45°
- 45° > critical angle of 42°
- TIR occurs and light is reflected at 45°
- Light bends a total of 90° and emerges from next boundary at 0° - no refraction
- Light bends 90° at a second triangular block and enters the eye.
- Virtual Image is seen behind second block.
Which sound has the higher frequency?
Mouse has a higher frequency sound
More waves per unit time
How does the following analogy of soldiers marching in rank from concrete to mud help explain refraction.
- soldiers slow down when going into mud, light slows down when going into a more dense medium
- rows of soldiers bunch up when in the mud, wavelength of light decreases when light enters more dense medium
- direction of soldier bends towards the normal when they enter the mud, ray of light bends towards the normal when entering a more dense medium
How do you find the image in a plane mirror using a protractor, pencil and paper.
- draw a ray from a point on the object to the mirror
- draw a normal line 90° to the mirror surface where this ray strikes the mirror
- measure the incident angle using a protractor
- draw the reflected ray at the same angle on the other side of the normal line
- repeat step 1-4 for a new ray in a different direction.
- using dotted line, follow both reflected rays back behind the mirror.
- Where the cross is the position of the image
When white light is passed through a prism, which colour of light is deviated the least?
red
How can you calculate the refractive index from the following graph?
The gradient of a sin i against sin r graph equals the refractive index
gradient = 0.9/0.5 = 9/5 = 1 and 4/5 = 1.80
n = 1.80
Reflections in Maths are the same as reflections in a plane mirror in Physics.
Where is the mirror located if the object is triangle ABC?
The mirror is located at the x axis
How is frequency and time period related?
frequency = 1 / time period
time period = 1 / frequency
Determine the frequency of the wave.
time base = 0.5 s/ division
peak to peak = 4 divisions
Time period = 4 x 0.5s / division = 2s
f = 1 / time period = 1/2 = 0.5 Hz
When white light is passed through a prism, which colour is deviated the most?
violet
How can the echo method be used to measure the speed of sound through air?
- Stand 170 m from large wall or cliff face.
- begin clapping at constant rate and listen for echo
- speed up or slow down rate of clapping until your clap is at same time as you hear the echo. (sound travels there and back between claps = 340m!!)
- time 10 full claps. (340m x 10 = 3400 m)
speed = 3400m / time
(time should be 10 second as 3400 m / 10 s = 340 m/s)
What do the following graphs tell us about the relationship between the incident angle and refracted angle?
- i is not proportional to r (1st graph is not a straight line)
- sin i is proportional to sin r (2nd graph is a straight line through the origin)
Out of the liquids listed below, which will bend light the most?
Sodium chloride will bend light the most because it has the largest refractive index.
How can you measure the speed of sound through air in a laboratory?
- Use microsecond timer connected to two microphones which are places 2.00 metres apart (using metre rules)
- make sound (hammer on metal) behind first microphone
- first microphone starts timer
- second microphone stops timer
- repeat 5 times and find mean
speed of sound = 2.00 m / average time
Diamond has a very high refractive index of 2.40. This means that the critical angle is very small at 25°.
Why does this make diamond lustre?
TIR occurs when i > critical angle
If the critical angle of diamond is only 25°, then more light is reflected within the diamond.
Diamonds are cut into a shape which allows light entering the top of the diamond to reflect within it and emerge back through the top - making it lustre!
In terms of TIR, what is the critical case?
When light is shining from a more dense medium to a less dense medium at exactly the critical angle.
In this case the light will run along the boundary with a refracted angle of 90°
What can be altered to make a sound wave amplitude larger or smaller on the screen of a CRO?
volts per division
Describe the image in a plane mirror
- same size
- upright
- virtual
- laterally inverted
Determine the amplitude of the wave.
volts per division = 2 V/ div
amplitude = 2 division
2 divisions x 2V/division= 4V
Determine the time period of the wave.
time base = 10 microseconds / division
10 x 10-6 s
peak to peak = 3.3 divisions
(crest to crest)
T= 3.3 div x 10 microseconds/ div= 33 microseconds
33 x 10-6 s
What is the critical angle?
The critical angle is the largest angle of incidence for which refraction can still occur.
What are the issues with measuring speed of sound in air using a starting pistol and stopwatch?
How can the method be improved?
- large reaction time using stopwatch
- time reading can be very small and so this creates large percentage error in time readings
Improvement:
Larger distances can be used with increases the time reading and reduces the percentage error (distance > 200m is best)
What must be connect to a CRO to display a sound wave?
a microphone
Determine the time period of the wave
time base = 10 ms/ division
Peak to peak = 3.2 divisions
time period = 3.2 x 10 ms = 32 ms
What happens to the speed, direction and wavelength of light when it enters a more dense medium at angle of 0° (along normal)?
Light slows down, the waves bunch up and the wavelength decreases but the direction does not change
