section 3: Waves

Question 1

important units:

Answer 1

-degree (º)
-hertz (Hz)
-metre (m)
-metre/second (m/s)
-second (s)

Question 2

waves:

Answer 2

-gaves transfer energy and information without transferring matter; the particles oscillate about a fixed point
-transverse waves
-longitudinal waves

Question 3

transverse waves:

Answer 3

-have peaks and troughs
-vibrations are at right angles (perpendicular) to the direction of travel
-e.g. light

Question 4

longitudinal waves:

Answer 4

-consist of compressions (particles pushed together) and rarefractions (particles moved apart)
-vibrations are in the same direction (parallel) as the direction of travel
-e.g. sound

Question 5

important definitions: amplitude, wavefront, frequency, wavelength and time period

Answer 5

-amplitude: the distance from the equilibrium position to the maximum displacement
-wavefront: a line joining point on a wave at the same point in their wave cycle at a given time
-frequency: the number of waves that pass a single point per second
-wavelength: the distance between a point on one wave and the same point on the next wave
-time period: the time taken for one complete wave to pass a fixed point

Question 6

formula linking speed of a wave, frequency and wavelength:

Answer 6

-the speed of a wave is equal to the product of the frequency and wavelength:
speed = frequency x wavelength
v = f λ

Question 7

formula linking frequency and time period:

Answer 7

-the frequency of a wave is equal to the recirprocal of the time period, measure in Hertz (Hz):
frequency = 1/time period
f = 1/T

Question 8

the Doppler Effect:

Answer 8

-if a wave source is moving relative to an observer, there will be a change in the observed frequency and wavelength due to the Doppler Effect
-this is because the wavefronts either get bunched together or space apart
-an example of this is when the siren of an ambulance is high-pitched as it approaches you, and low-pitched as it goes away

Question 9

reflection:

Answer 9

-all waves can be reflected when they travel from a medium of low optical density (such as air) to one of much higher optical density (such as glass)
-the law of reflection states that:
angle of incidence = angle of reflection
-frequency, wavelength and speed are all unchanged

Question 10

refraction:

Answer 10

-all waves can be refracted, which is when the speed of a wave changes when it enters a denser new medium
-if the wave enters a denser medium, its speed decreases and it bends towards the normal
-if the wave enters a less dense medium, its speed increases and it bends away from the normal
-in all cases, the frequency stays the same but the wavelength changes and as a result, the velocity (speed) must change

Question 11

electromagnetic spectrum:

Answer 11

order: radio waves, microwaves, infrared radiation, visible light, ultraviolet, x-ray and gamma ray
-colours: roygbiv
-they increase in frequency
-they decrease in wavelength
-all em waves travel with the same high speed in vacuum and approximately the same speed in air
3 x 10^8 m/s

Question 12

electromagnetic waves: uses

Answer 12

-radio waves: (red) broadcasting and communications
-microwaves: (orange) cooking and satellite transmissions
-infrared: (yellow) heaters and night vision equipment
-visible light: (green) optical fibres and photography
-ultraviolet: (blue) fluorescent lamps
-x-rays: (indigo) observing the internal structures of objects and materials, including for medical applications
-gamma rays: (violet) sterilising food and medical equipment

Question 13

electromagnetic waves: dangers + protective measures

Answer 13

-there are detrimental effects of excessive exposure of the human body to em waves:
-microwaves: internal heating of body tissue (shielding to prevent them from reaching the user)
-infrared: skin burns (using insulating materials to reduce the amount of IR radiation reaching your skin)
-ultraviolet: damage to surface cells and blindness (sun cream and sun glasses prevent over-exposure)
-gamma rays: cancer, cell mutation (using protective shielding made of very dense materials such as lead)

Question 14

light:

Answer 14

-ligh waves are transverse waves and can be reflected and refracted
-reflection of light can be shown when light reflects at a plane mirror and forms an image -> this can be represented by a ray diagram
-refraction of light can be shown when light is passed through a glass slab at an angle to its normal
-when light enters a more optically dense medium, the angle of incidence (the angle between the incident ray and the normal) is greater than the angle of refraction (the angle between the refracted ray and the normal) -> can also be represented by a ray diafram
-the opposite is true when light enters a less optically dense medium

Question 15

practical: investigate the refraction of light, using rectangular blocks, semi-circular blocks and triangular prisms

Answer 15

1. connect the ray box to the power supply and insert the single slit slide so that it produces a clear and thin beam of light
2. place one of the blocks onto the sheet of paper and draw around it
3. remove the block and then mark the position on the outline that you are going to shine the light ray at with a cross
4. using a protractor, draw a normal to that point (perpendicular line)
5. mark on a selection of different angles of incidence by measuring angles from the normal line
6. replace the block on top of the outline, and then shine the ray of light along each incident line. For each angle, mark the position on the other side of the block where the light exits
7. turn off the ray box and remove the block
8. using a ruler, connect up the entry position and the exit position for each angle of incidence
9. using a protactor, measure the angles of refraction for the different angles of incidence
10. repeat for the other two shaped blocks and compare the results

Question 16

Snell’s Law:

Answer 16

-relates the angle of incidence and the angle of refraction to the refractive index of a medium by n1sin(i) = n2sin(r) where n is the optical density and i is the angle of incidence and r is the angle of refraction

Question 17

formula linking refractive index, angle of incidence and angle of refraction:

Answer 17

n = sin(i)/sin(r)

Question 18

practical: investigate the refractive index of glass, using a glass block

Answer 18

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

Question 19

total internal reflection:

Answer 19

-at a certain angle of incidence called the critical angle, the light will travel along the boundary between the two media
-total internal reflection occurs when the angle of incidence is greater than the critical angle and the light reflects back into the medium
-for total internal reflection to occur, the light must also be travelling from a more optically dense medium into a less optically dense medium (most common examples is glass to air)
-the critical angle c can be related to the refractive index by:
n = 1/sin sin(c) or sin c = 1/n

Question 20

optical fibres:

Answer 20

-an optical fibre is a long thin rod of glass surrounded by cladding which uses total internal reflection to transfer information by light, even when bent
-they are used extensively in medicine (endoscopes, inside-body flexible cameras) and communications (high speed data transfer)
-sound waves are longitudinal waves and can be reflected and refracted