Optics

Question 1

What is a Coherent Light Source

Answer 1

Coherent light source produces waves with constant phase difference and identical frequency

Question 2

Why is using a laser useful for showing interference and diffraction

Answer 2

It produces monochromatic light at a fixed wavelength which shows more defined interference patterns

Question 3

Describe Youngs Double Slit Experiment

Answer 3

A Single Light Source is directed towards two slits, which each act as a coherent light source.
A bright central maximum is produced
The light constructively interferes every path difference of a full wavelength producing a bright light
The light destructively interferes every path difference of 1/2 of the wavelength producing a dark fringe
This produces dark and bright alternating fringes of decreasing intensity either side of the bright central maximum

Question 4

Describe the interference pattern created when a single slit experiment uses white light

Answer 4

A bright white central maximum’
Flanked by alternating spectral fringes of decreasing intensity
With Violet being closest to order 0
Indigo
Blue
Green
Yellow
Orange
Red being the furthest
The pattern the. repeats

Question 5

Why does an interference pattern form when light is passed through a single slit

Answer 5

The light Diffracts as it is passed through the slit of same magnitude to the wavelength
The waves that are in phase constructively interfere making bright fringes

The waves that are out of phase destructively interfere making dark fringes

A secondary bright fringe is formed either side the bright central maximum when two waves constructively interfere. That is 1/2 the width and a lot less intense each further successive bright fringe is a lot less intense and narrower

Question 6

What would increasing the slit width do to the central diffraction maximum (single slit)

Answer 6

as W = (Landa * D)/s
W and S are inversely proportional hence if you increase s W will decreasing making the separation between the fringes smaller and in tern the central diffraction maximum narrower and more intense

Question 7

What is the refractive index of air?

Answer 7

1

Question 8

When light enters a more dense medium does it bend towards or away from the normal?

Answer 8

Faster
Away
Slower
Towards

more dense -> more friction -> moves slower

Towards the normal

Question 9

When does total internal reflection occur

Answer 9

When the angle of incidence is higher then the critical angle
Happens when entering a substance with a lower refractive index

Question 10

What’s the purpose of cladding in optical fibres

Answer 10

Protects the core
Allows total internal reflection as it has a lower refractive index then the core

Question 11

What is modal Dispersion

Answer 11

When some of the signal reaches the end of the fibre before other parts due to each wave taking a different number of reflections.

• use monochromatic light

Question 12

What is pulse dispersion

Answer 12

White light is used vs monochromatic
Light is detracted into different colours with different wavelengths while under going total internal reflection, this means that different colours have different retention times , due to internally reflecting a different number of times, to the end of the fibre compared to other colours causing delays and information loss.

Question 13

Explain Bundle coherence

Answer 13

In application such as endoscopes
it is important the image produced is identical to that of the start
So it is essential that the hundreds of fibres are in the same coherent pattern at either side of the bundle.

Question 14

How can absorption of light and dispersion be reduced in optical fibres

Answer 14

Use an optical fibre repeater to regenerate a correct signal

Question 15

Why are optical fibres better then traditional copper wires

Answer 15

. Signal can carry a higher rate of information due to the use of light over electrons
. No energy lost as heat
. No electrical interference

Question 16

What happens when the angle of incidence is equal to the critical angle

Answer 16

The light goes along the boundary
Angle of refraction = 90 degrees

Question 17

Critical angle =

Answer 17

arcsine ( n2/n1)
where n1>n2
n= refractive index

Question 18

What is the water to air critical angle

Water has a refractive index of 1.33

Answer 18

Question 19

What is snells law?

Answer 19

Refractive index =
sin (Angle of incidence ) / sin (angle of refraction)
Speed of light in a vacuum / speed of light in substance

n1(sin(i) = n2(sin(r)

Question 20

Answer 20

Question 21

A higher refractive index =

Answer 21

More optically dance material

Question 22

State two applications for diffraction gratings

Answer 22

Splitting up light from stars to make line absorption spectra -used to identify element present in the star

X Ray crystallography, a crystal sheet is used as a diffraction grating
used to find the spacing between atoms

Question 23

Explain the formula d(sin()) = n * Landa

Answer 23

1) First order maximum - path difference between 2 adjacent rays is 1 wavelength
2) A Right angle triangle is formed
3) For the first maximum Sin() = Landa/ d
4) The other maxima occur when the path difference between the two rays of light is n* Landa
5) Replace n*landa with it to get

d(sin()) = n * Landa
d = distance between slits

Question 24

When light passing through a diffraction grating is changed from blue to red do the orders get closer?

Answer 24

The wavelength has increased

d(sin()) = n * Landa

so dSin() has increased resulting in larger values of the angle so the orders have gone further apart

Question 25

What is a Diffraction

Answer 25

The Spreading of waves when they pass through or around a gap

Question 26

Hoe did Youngs double slit provide evidence for waves nature of light

Answer 26

Shows Diffraction and interference which both are theoretically impossible with particle theory of light.

Question 27

List 4 Safety precautions when using a laser

Answer 27

Wear safety goggles
Don’t shine laser at reflective surfaces
Display warning signs
Always shine laxer in the same direction

Question 28

What formula is associated with young’s double slit experiment

Answer 28

Fringe spacing =
(Landa * Distance from screen to split )/ Slit Separation

Question 29

Can Sound waves and em waves interfere?

Answer 29

Yes

Question 30

How does Light refracting as it entered a substance prove wave theory

Answer 30

“Bending effect” occurs when part of the wave reaches the medium boundary before the other

. If light is moving towars a material of higher refraction index it refracts towards the normal at the boundary

.If light is moving towards a material of lowe refraction index it refacts away from the normal at the boundary

If light were a particle the only factor that would change is the speed and direction would stay constant

Question 31

How would you investigate Diffraction?

Answer 31

Add water to a clear plastic tray so the ripple generator is touching the surface of the water
Use Metal L junctions to create a narrow slit
A suitable distance form the ripple generator
Using tracing paper tho copy the pattern shown on the screen when lid is closed and measure the angle to the normal
Repeat for multiple slit sizes

Question 32

For optical fibres, would a larger core diameter or a shorter core diameter allow for a greater transmission rate.

Answer 32

Shorter
Narrower core leads to less modal dispersion - as the range of paths lengths between reflections decrease (pulse broadening)
This means that the pulses are shorter in length, faster in reaching the end and therefore producing a greater rate of transmission

Question 33

For any material red light has a lower refractive index then green light, and blue has a higher refractive index

If a green light ray is incident on a boundary at its critical angle

Explain what would happen to the red light, and what would happen to the blue light if both were shone at the same angle of incidence

Answer 33

Refractive indices change by the same factor ie Sin(Critical angle) = n2/n1 n2 and n1 decrease by the same scale factor

Hence Path by blue and red light are the same as the blue light

because the critical angle is the same

Question 34

Explain how the following diffraction pattern supports the idea that electrons within the beam are behaving as waves.

Answer 34

Successive bright rings around the central maximum are caused by the superposition and constructive interference between 2 in phase waves.

This interference pattern is only produced by waves that undergo diffraction

Particle behaviour would only produced a small spot of light at the centre (or scatter randomly)

Question 35

A single slit diffraction using green monochromatic light

The monochromatic light is changed to red light

Describe the change in pattern observed on the screen.

Answer 35

Fringe Spacing increases with Larger Wavelengths of light

Producing a wider Central Maximum

and Subsequent maxima are further apart from each other

Question 36

A filter only lets waves with 600-700nm of wavelength through

Suggest how this affects a single slit diffraction.

Answer 36

Successive orders of maxima more broader fringes then if monochromatic light were used,

Different wavelengths create orders of maxima on different points on the screen in the same region

Central maximum width stays unchanged