3.2 refraction , diffraction and interference

Question 1

Define coherence

Answer 1

Coherent waves have fixed phase difference and the same frequency and wavelengths

Question 2

Why is a laser useful in showing interference and diffraction?

Answer 2

It produces monochromatic ( same wavelength/ colour ) light so diffraction and interference patterns are more defined

Question 3

What was Young’s double-slit experiment?

Answer 3

A single light source is directed towards two slits , which each act as a coherent light source , the light then interferes constructively and destructively to cause an interference pattern

Question 4

Describe the interface pattern using white light

Answer 4

A white central maximum at the zero order with alternating spectral fringes on either side of decreasing intensity with violet closest to the zero order and red the furthest

Question 5

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

Answer 5

Where light diffracts as it passes through the slit where the waves are in phase constructive interference takes place making bright fringes and where the waves are completely out of phase destructive interference occurs making dark fringes

Question 6

What happens when the slit is widened?

Answer 6

Less diffraction so the zero order maximum is narrower and more intense

Question 7

What is the approximate refractive index of air?

Answer 7

1

Question 8

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

Answer 8

Towards

Question 9

When does total internal reflection occur?

Answer 9

When light is on the boundary of a less optically dense medium and the angle of incidence is greater than the critical angle

Question 10

What is the purpose of cladding in a step index optical fibre?

Answer 10

1) protects core from scratches which would allow light to escape degrading the signal
2) Allows total internal reflection as it has a lower refractive index than the core

Question 11

How does signal degradation by absorption in the optical fibre affect the received signal?

Answer 11

Part of the signals energy is absorbed by the fibre so the amplitude is lower

Question 12

What is pulse broadening?

Answer 12

When the received signal is wider than the original, this can cause overlap in signals meaning information lost

Question 13

How does modal dispersion cause pulse broadening ?

Answer 13

Light rays enter the fibre at different angles causing them to take different paths, meaning some take longer than others causing pulse broadening

Question 14

What is material dispersion?

Answer 14

When light with different wavelengths is used some wavelengths slow down more than others causing them to reach the end at different times causing pulse broadening

Question 15

How can modal dispersion be reduced?

Answer 15

By using a single mode fibre reducing the number of different possible paths making pulse broadening smaller

Question 16

How can material dispersion be reduced?

Answer 16

By using a light with only 1 wavelength / monochromatic light

Question 17

How can both absorption and dispersion be reduced?

Answer 17

Use an optical fibre repeater to regenerate the signal now and then

Question 18

State the advantages of using optical fibres over traditional copper fibres

Answer 18

1) signal can carry more information as light has a high frequency
2) No energy lost as heat
3) No electrical interference
4) Cheaper
5) Very fast

Question 19

What path does a light ray take when the angle of incidence is equal to the critical angle?

Answer 19

It goes along the boundary so the angle of refraction is 90 degrees

Question 20

What formula can be used for the critical angle of two materials whose refractive indices are known?

Answer 20

sinC = n2/n1 n2>n1

C = critical angle
n1= refractive index of material 1
n2 = refractive index of material 2

Question 21

What is the critical angle of water to air if the refractive index of water is 1.33 ?

Answer 21

sinC = 1/1.33 sin^-1(1/1.33) C = 48.8

Question 22

What is snell’s law of refraction?

Answer 22

n1sini=n2sinr i= angle of incidence r = angle of refraction

Question 23

What formula is used to determine the refractive index of a material?

Answer 23

n= c/v
c= speed of light in a vacuum 
v = speed of light in material

Question 24

State 2 applications of diffraction gratings?

Answer 24

1) Splitting up light from stars to make line absorption spectra used to identify elements in the star
2) X-ray crystallography , a crystal sheet acts as the diffraction grating the X-rays pass through- used to find the spacing between atoms

Question 25

Derive the formula dsinϑ=λ

Answer 25

1) for the first order maximum the difference between 2 adjacent rays of light is 1λ , the angle between the normal to the grating and the light ray is ϑ
2) A right angled triangle is formed with sides d and λ, the upper angle is ϑ
3) for the first maximum sinϑ= λ/d . rearrange to dsin= λ

Question 26

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

Answer 26

The wavelength of light has increased so it will diffract more, the orders will become further apart

Question 27

What is diffraction?

Answer 27

The spreading out of waves as they pass through or around a gap

Question 28

How did Young’s double slit experiment provide evidence of the wave nature of light?

Answer 28

Diffraction and interference are wave properties so the interference pattern of light shows light has wave properties

Question 29

What are 4 safety precautions that must be followed when using a laser ?

Answer 29

1) Wear laser safety goggles
2) Dont shine the laser at reflective surfaces
3) Display a warning sign
4) Never shine the laser at person

Question 30

What formulas is linked with Young’s double slit experiment?

Answer 30

w = λD/s 
w-fringe spacing
λ-wavelength of light used 
D-distance from screen to slits
s-slit separation

Question 31

What is path difference?

Answer 31

The difference in distance travelled by 2 waves

Question 32

How could you investigate stationary waves?

Answer 32

Place a speaker at the end of a long glass tube , place powder along the bottom of the glass tube , the powder will be shaken at the antinodes and settle at the nodes. The distance between 2 nodes is half a wavelength

Question 33

What is phase?

Answer 33

The position of a certain point on a wave cycle ( units are in degrees radians or fractions of a cycle)