7. waves

Question 1

Define transverse waves

Answer 1

A wave in which the particles of the medium vibrate perpendicularly to the direction of the wave velocity

Question 2

Define longitudinal waves

Answer 2

A wave in which the particles of the medium vibrate parallel to the direction of the wave velocity

Question 3

What are ripple tanks used to demonstrate-

Answer 3

Ripple tanks can be used to demonstrate reflection, refraction and diffraction

Question 4

Define-
Displacement
Amplitude
Wavelength

Answer 4

1. Displacement (x) of a wave is the distance from its equilibrium position. It is a vector quantity; it can be positive or negative
2. Amplitude (A) is the maximum displacement of a particle in the wave from its equilibrium position
3. Wavelength (λ) is the distance between points on successive oscillations of the wave that are in phase
   These are all measured in metres (m)

Question 5

Define-
Period
Frequency
Speed

Answer 5

1. Period (T) or time period, is the time taken for one complete oscillation or cycle of the wave. Measured in seconds (s)
2. Frequency (f) is the number of complete oscillations per unit time. Measured in Hertz (Hz) or s-1.
3. Speed (v) is the distance travelled by the wave per unit of time. Measured in metres per second (m s-1).

Question 6

Explain phase

Answer 6

The phase difference tells us how much a point or a wave is in front or behind another
This can be found from the relative positive of the crests or troughs of two different waves of the same frequency
When the crests or troughs are aligned, the waves are in phase
When the crest of one wave aligns with the trough of another, they are in antiphase
Phase difference is measured in fractions of a wavelength, degrees or radians
The phase difference can be calculated from two different points on the same wave or the same point on two different waves
The phase difference between two points:
In phase is 360o or 2π radians
In anti-phase is 180o or π radians

Question 7

Explain wave energy-

Answer 7

Waves transfer energy between points, without transferring matter
When a wave travels between two points, no matter actually travels with it:
The points on the wave simply vibrate back and forth about fixed positions
Waves that transfer energy are known as progressive waves
Waves that do not transfer energy are known as stationary waves

Question 8

How to calculate the period and the frequency- from cathode-ray oscilloscope display

Answer 8

The period of the wave can be determined from the time-base This is how many seconds each division represents measured commonly in s div-1 or s cm-1
Use as many wavelengths shown on the screen as possible to reduce uncertainties
Dividing the total time by the number of wavelengths will give the time period T (Time taken for one complete oscillation)
The frequency is then determined through 1/T

Question 9

How to calculate the speed of the wave-

Answer 9

v=fλ
v= speed
f= frequency
λ/ lamda= wavelength

Question 10

Name the two waves in terms of transferring energy-

Answer 10

Transverse waves transfer energy
Stationary waves do not transfer energy

Question 11

Define the term intensity in waves

Answer 11

The amount of energy passing through a unit area per unit time is the intensity of the wave

Question 12

How do we calculate the intensity of the wave

Answer 12

Intensity= power/ area
intensity= Watt m to the power -2
power= watt
area= m2

The intensity of a progressive wave is also proportional to its amplitude squared and frequency squared

Intensity ∞ Area2
Intensity ∞ frequency 2

Question 13

Differentiate between transverse and longitudinal waves with examples

Answer 13

Transverse waves- the particles oscillate perpendicular to the direction of the wave velocity.
Transverse waves show areas of troughs and crests.
examples- 1. Electromagnetic waves e.g. radio, visible light, UV
2. Vibrations on a guitar string
transverse waves can be polarised.

Longitudinal waves- the particles oscillate parallel to the direction of the wave velocity.
Longitudinal waves show areas of compression and rarefaction.
examples- sound waves and ultrasound waves
These waves cannot be polarised.

NOTE- When the graph is of displacement and time the wave shown can be either transverse or longitudinal
but when the graph is between displacement and direction of travel then the wave shows can only be transverse.

Question 14

Define doppler shift-

Answer 14

the change in frequency of a wave observed when the source of the wave is moving towards or away from the observer.

Question 15

The difference in frequencies when the source moves towards and away from the observer

Answer 15

The frequency is increased when the source is moving towards the observer

The frequency is decreased when the source is moving away from the observer

Question 16

How to calculate the doppler shift

Answer 16

f0 = fs (V / V +- Vs)
f0= observed frequency
fs=source frequency
V=wave velocity
Vs= source velocity

If the source is moving towards, the denominator is v - vs
If the source is moving away, the denominator is v + vs

Question 17

Write all the common properties of electromagnetic waves-

Answer 17

They are all transverse waves
They can all travel in a vacuum
They all travel at the same speed in a vacuum (free space) — the speed of light 3 × 108 ms-1

NOTE-
Since they are transverse, all waves in this spectrum can be reflected, refracted, diffracted, polarised and produce interference patterns
Since they are transverse, all waves in this spectrum can be reflected, refracted, diffracted, polarised and produce interference patterns

Question 18

use of radio waves-

Answer 18

communication -radio and tv

Question 19

use of microwaves-

Answer 19

heating food
communication- wifi

Question 20

use of infrared waves-

Answer 20

thermal images- medical
motion sensors
heating food
fibre optic communication

Question 21

use of visible light-

Answer 21

to see things
taking photographs and videos

Question 22

use of ultraviolet waves-

Answer 22

fluorescent bulbs
getting a suntan

Question 23

use of x-rays-

Answer 23

x-ray images- medical, security airport

Question 24

use of gamma rays-

Answer 24

sterilising medical equipment
treating cancer

Question 25

Electromagnetic spectrum-

Answer 25

RAdio waves - lower energy, lower frequency, longer wavelengths
MIcro waves
INFrared waves
VIsible light
ULtraviolet
X-rays
GAmma rays - higher energy, higher frequency and shorter wavelengths

Question 26

Electromagnetic spectrum and their wavelengths values-

Answer 26

look at save my exams and memorise it its important

Question 27

About polarisation and transverse wavs

Answer 27

Transverse waves can be polarised, this means:
Vibrations are restricted to one direction
These vibrations are still perpendicular to the direction of propagation/energy transfer

A plane polarised waves has oscillations in only one direction.

Question 28

Why can’t longitudinal waves be polarised?

Answer 28

Longitudinal waves (e.g. sound waves) cannot be polarised since they oscillate parallel to the direction of travel

Question 29

How can you polarise the waves-

Answer 29

Waves can be polarised through a polariser or polarising filter. This only allows oscillations in a certain plane to be transmitted

note- only unpolarised waves can be polarised- polarised waves cannot be polarised.

Question 30

What are other ways than polariser and polarising filter that light can be polarised -

Answer 30

Light can also be polarised through reflection, refraction and scattering

Question 31

What falls by half after the light gets polarised-

Answer 31

intensity of the light

Question 32

what are the two filters that light passes through when its getting polarised-

Answer 32

First filter is polariser
second is analyser

Question 33

What are the conditions falling under analyser- to follow malus law

Answer 33

If the analyser has the same orientation as the polariser, the light transmitted by the analyser has the same intensity as the light incident on it
If they have a different orientation, we must used Malus’s law

Question 34

What is malus law- its equation

Answer 34

Malus’s law states that if the analyser is rotated by an angle θ with respect to the polariser, the intensity of the light transmitted by the analyser is.

I= I0 cos² (θ)
I= intensity of the light transmitted
I0= maximum intensity
θ= angle between polarised light and transmission axis

NOTE- memorize at the table of transmission depending on polariser orientation