Waves

Question 1

what is a transverse wave?
including examples

Answer 1

particles oscillate perpendicular to direction of propagation
-EM waves
-water waves

Question 2

what is a longitudinal wave?
including examples

Answer 2

each particle oscillates parallel to direction of propagation
-sound waves

Question 3

what is a mechanical wave?

Answer 3

oscillation of particles in a physical medium (cannot be in a vacuum) and can be transverse or longitudinal

Question 4

what is an EM wave?

Answer 4

produced by acceleration of charged particles and can transmit energy through a vacuum.
- they are always transverse

Question 5

definition of time period and frequency

Answer 5

time period - time taken for one oscillation
frequency - number of waves passing any one point in one second

Question 6

what happens when unpolarised light is shone through a polarising filter?
what happens when a second filter is placed perpendicular to the first?

Answer 6

-all the waves move in the same plane absorbing any that aren’t in the same plane (very few oscillations are let through)

• when perpendicular, no light is let through because the plane of polarised light is also at right angles to the second filter.

Question 7

wave phases
1) what happens when waves are in phase? how do they interfere?
2) what happens when waves are 180 degrees out of phase (in anti-phase)? how do they interfere?

Answer 7

1) waves are oscillating together and causes constructive interference

2)waves are oscillating so that when wave 1 peaks, wave 2 troughs and causes destructive interference (nothing is seen)

Question 8

What is the principle of superposition

Answer 8

When two or more waves overlap, the resultant displacement at a point is equal to the sum of the individual displacements at that point

Question 9

What does coherent mean

Answer 9

Waves that have a fixed phase

Question 10

What are the conditions for constructive interference

Answer 10

The path difference between coherent sources is a a whole number of wavelengths (waves are in phase)

Question 11

What are the conditions for destructive interference

Answer 11

The path difference is half a wavelength (in antiphase) so waves are cancelled out

Question 12

What is a monochromatic light source

Answer 12

The waves have a single wavelength

Question 13

What does the results of Youngs double slit experiment look like on the screen

Answer 13

A bright maxima with spots either side getting dimmer and dimmer. There is a path difference of one wavelength between each spot

Question 14

Which colours of light diffracts the most and the least

Answer 14

Red diffracts the most (had a wider maximum)
Blue diffracts the least

Question 15

Diffraction
what happens when a light wave passes through a gap that is
1) smaller than its wavelength?
2) larger than its wavelength
3) equal to its wavelength?

Answer 15

1) no light passes through so there is no diffraction
2)diffracts a little bit but a lot of light passes through
3) diffracts a lot but not as much light passes through

Question 16

Conditions for a stationary wave

Answer 16

Two progressive waves with the same frequency travelling in opposite directions

Question 17

How many nodes/antinodes in the
First harmonic
Second harmonic

Answer 17

First harmonic - two nodes, one antinode
Second - three nodes two antinodes

Question 18

What happens to the energy in a stationary wave

Answer 18

It’s stored

Question 19

What is the phase difference in stationary waves

Answer 19

All particles between two adjacent nodes are in phase

Question 20

What is refraction

Answer 20

Change of direction a light ray undergoes when it enters a medium with a different optical density

Question 21

When does total internal reflection occur

Answer 21

When the angle of incidence exceeds the critical angle

Question 22

What happens if the angle of incidence is
Smaller than the critical angle
Equal to the critical angle
Greater than the critical angle

Answer 22

Smaller - light ray is refracted out of the material
Equal- refracted along the boundary of the material
Greater- total internal reflection

Question 23

what is absorption in optical fibres and what is the effect of it?

Answer 23

some of the signal’s energy is absorbed by the fibre which results in the amplitude of the signal being reduced, making it harder to detect

Question 24

what is modal dispersion and why does it happen?

Answer 24

when the received signal is broader than the signal sent due to some of the light taking a longer path. this results in information loss

Question 25

what is material dispersion and how can it be reduced?

Answer 25

different wavelength of light travel at different speeds which is shown more as it refracts off the cladding, so the signals will arrive at different times. use monochromatic light to reduce this