Turning Points: Waves

Question 1

What were the differences in Newton’s theory of light and Huygens?

Answer 1

Newton believed that light was made out of particles (known as corpuscles) whereas Huygens believed that light is a longitudinal wave travelling through ether.

Question 2

What is the corpuscular theory?

Answer 2

Light was made out of corpuscles that travelled in a straight line from the source.

Question 3

What did the corpuscular theory prove?

Answer 3

the formation of sharp shadow when an opaque object intercepted sunlight.

Question 4

What are the key features of Huygens’ theory?

Answer 4

Light acts as a wave, every point of waveform acts as a source of secondary wavelets

Question 5

How does a dark and bright fringes on screen support Huygens’ theory?

Answer 5

each point at slit produces secondary wavelets. The wavelets overlap the screen. Path difference etc..

Question 6

Describe the process of Fizeau’s experiment to determine speed of light?

Answer 6

He shone a light at a partially reflecting mirror which directed between the teeth of rotating toothed wheel. The distant mirror reflects back towards the wheel and observer.

Question 7

What happened as soon as Fizeau started to rotate the wheel?

Answer 7

the teeth broke up the light beams and into pulses of light is observed.

Question 8

What happened when Fizeau started to increase the frequency of wheel?

Answer 8

A speed was discovered as light leaving through one gap returned at an instant as the next tooth blocked it path.

Question 9

What is Fizeau’s equation of speed? and meaning of each

Answer 9

c = 4DNf
D - distance from wheel to distant mirror
N- number of teeth
f - frequency

Question 10

What did Maxwell predicted about electromagnetic wave?

Answer 10

Electromagnetic wave exist when a changing magnetic field creates a changing electric field and so on…

Question 11

What is m0 (muu nought) and what it relates to?

Answer 11

Permeability of free space. (H - Henry)
It relates to the magnetic flux density of a magnetic field to the electric current that creates it.

Question 12

What is E0 (epsilon nought) and what it relates to?

Answer 12

permittivity of free space (F - Farad)
It relates to the electric field strength to the charge that creates it

Question 13

Why did Hertz’s transmitted included an induction coil and a capacitor?

Answer 13

These produced an alternating high voltage to create a spark, which kept reversing direction and created radio waves.

Question 14

What did Hertz do to detect the electromagnetic waves?

Answer 14

He used a spark gap detector made up of wire loop, the ends made up of brash sphered and seperated by small gap.

Question 15

What did Hertz finally discovered?

Answer 15

EM waves spreading from the sparks created by transmitter which induces emf hence a current, in wire loop, creates sparks between brass spheres.

Question 16

What else did Hertz do and concluded?

Answer 16

He created stationary radio waves from superposition of the incident and reflected waves. He measured the wavelength by node to node distance and used c = f * wavelength

Question 17

List all EM waves from lowest wavelength to highest

Answer 17

Gamma, x-rays, ultraviolet, visible light, microwaves, radiowaves

Question 18

What is black body radiation?

Answer 18

It is an electromagnetic radiation. Spectrum with peak depending on temperature

Question 19

What is Wein’s displacement law?

Answer 19

The peak wavelength was inversely proportional to the kelvin temperature.

Question 20

What led to the ultraviolet catastrophe? (predictions)

Answer 20

The wave theory states as the wavelength of radiation decreases, the intensity increases leading to infinite amount of ultraviolet radiation

Question 21

What was Planck’s solution to the catastrophe? and equation

Answer 21

suggested that EM waves was quantised, EM waves travelled in packets called quanta. Hence E=hf

Question 22

Give 1 reason why photoelectric effect can’t explain the wave theory (about threshold frequency)

Answer 22

Wave theory suggest that any frequency of light should cause photoelectric emission because the energy absorbed by each electron gradually increases whereas opposes the idea of threshold frequency

Question 23

Give 1 reason why photoelectric effect can’t explain the wave theory (about immediate)

Answer 23

photoelectric effect is immediate whereas wave theory suggest time needed for electrons to reach work function

Question 24

Give 1 reason why photoelectric effect can’t explain the wave theory (about intensity)

Answer 24

increasing the intensity of light does not increase the speed of the photoelectric emission (where the theory states)

Question 25

Give 1 reason why photoelectric effect can’t explain the wave theory (about kinetic energy)

Answer 25

photoelectrons are released at range of kinetic energies

Question 26

What did Einstein claimed the EM waves were

Answer 26

photons

Question 27

What are the three points that Einstein proved the wave theory wrong?

Answer 27

1. All electrons same amount of energy
2. when photon interacts with electron, all the energy is transferred to it
3. Intensity equal to number of photoelectrons released per second.

Question 28

What happens when the potential supplied across metal surface is positive.

Answer 28

kinetic energy of photoelectrons will decrease as work must be done against the electrostatic force of attraction towards surface.

Question 29

What is the stopping potential?

Answer 29

the potential difference need to supply to the metal to stop photoelectron with maximum kinetic energy

Question 30

What is the equation relating stopping potential?

Answer 30

hf = work function + stopping potential*charge (e)

Question 31

What is de-Broglie hypothesis and what equation relates?

Answer 31

all particles have a wave-like nature and particle nature.
wavelength = h/mv

Question 32

What proved de-Broglie’s hypothesis

Answer 32

electron diffraction

Question 33

When light passes through a diffraction grating, what happens with larger wavelength?

Answer 33

It gives larger angle of diffraction for each maximum hence more widely spaced concentric rings

Question 34

What happens when accelerating the voltage increases?

Answer 34

speed of electrons increases and their de-Broglie wavelength decreases so fringe spacing decreases (concentric rings)

Question 35

What is the resolving power of microscopes?

Answer 35

ability distinguish structures which are close to each other

Question 36

How does TEM microscopes work?

Answer 36

electrons are accelerated by electron gun, passing though magnetic lenses, passing through a thin sample so electron don’t slow down. The lenses deflect electron towards the axis.

Question 37

What are the three types of lenses of TEM and their uses?

Answer 37

• Condenser lens - deflects electrons so forms a wide parallel beam
• Objective lens - form an image of sample
• Projector lens - magnify the image and project on fluorescent screen

Question 38

What are the reasons the resolving power are limited in TEM?

Answer 38

sample thickness,
electrons travelling at range of speed ( some electrons lose kinetic energy )

Question 39

What is the diameter of atom

Answer 39

0.1nm

Question 40

What does STM microscopes use?

Answer 40

uses quantum tunnelling of electrons in order to form an image on the surface of an object

Question 41

How does STM microscopes operate?

Answer 41

a very fine tipped probe which moves across a surface of an object and stays constant potential. ( electrons travel one direction) As probe moves across, size of gap vary

Question 42

What happens when size of gap is large in STM microscopes?

Answer 42

tunnelling current will decrease as tunnelling of electrons less likely to occur vice versa for small gap

Question 43

What is tunnelling current?

Answer 43

The movement of electrons measured

Question 44

2 things that needs to be constant in STM

Answer 44

height mode and current mode