5 Optics

Question 1

refractive index

Answer 1

sin i
n= ——
sin r

air into glass
-light bends towards the normal (goes into a more dense substance)

glass into the air
light bends away from the normal

no refraction when the incident ray is along the normal

Question 2

refractive index 2

Answer 2

c
ns= —-
cs

ns = ratio of speeds of light
c= speed in a vacuum
cs= speed of substnace

Question 3

critical angle

Answer 3

i < critical angle

(an arrow inside and outside the block)

ic= critical angle

(an arrow along the boundary)

i > critical angle

(reflected)

Question 4

what substance has a high refractive index?

Answer 4

diamond

Question 5

optical fibres

Answer 5

more dense —> less dense

the incident substance has a larger refractive index than the other substance

partial internal reflection always occurs at a boundary when the angle of incidence is less than or equal to the critical angle

Question 6

optical fibres

inference

Answer 6

the core has to narrow to prevent dispersion

violet light travels slower than red light

Question 7

medical endoscope

Answer 7

enclosed space
two bundles of fibres (one for light and another for viewing the image)

core has to be very optically clear t reduce absorption and thin

cladding = lower refractive index

Question 8

coherence

Answer 8

• same frequency and constant phase difference
• fringe separation depends on the colour of the light
• vapour or discharge tubes = dominant colour
• filament lamp = all colours of the spectrum

Question 9

single slit

Answer 9

diffraction of a single slit

central fringe is twice as wide
intensity reduces with distance

Question 10

diffraction grating

Answer 10

plate with many parallel slits

N = 1
—-
d
number of spacing

maximum number of orders is given by the value of distance divided by wavelength rounded down to the nearest whole number

Question 11

light sources

Answer 11

laser

• coherent (in phase)
• monochromatic

non laser light = polarised first

white light fringes

• blue fringes are closer together than red
• central fringe white (every colour)

Question 12

Refractive index of air

Answer 12

1

Question 13

Snell’s law

Answer 13

N2sin2 = n1sin1

Question 14

Total internal reflection

Answer 14

Sin θ = n2 / n1

Question 15

dispersion

Answer 15

dispersion. Visible light is actually made up of different colors. Each color bends by a different amount when refracted by glass. That’s why visible light is split, or dispersed, into different colors when it passes through a lens or prism.

Question 16

optical fibres

Answer 16

Cladding is one or more layers of materials of lower refractive index, in intimate contact with a core material of higher refractive index. It is used in optical fibers. The cladding causes light to be confined to the core of the fiber by total internal reflection at the boundary between the two.

Question 17

material dispersion

Answer 17

Material dispersion is a phenomenon in which different optical wavelengths propagate at different velocities, depending on the refractive index of the material used in the fibre core.

Question 18

modal dispersion

Answer 18

the phenomenon that the group velocity of light propagating in a waveguide structure depends on the waveguide mode

Question 19

pulse broadening definition

Answer 19

Pulse broadening is defined as the spreading of the light pulses as they travel down the fiber.

Question 20

pulse broadening and absorption

Answer 20

light rays travelling straight down the axis of the fibre core travel the shortest distance and so reach the far end of the fibre in the shortest possible time. light rays entering the core at a small angle to the axis will be reflected internally a few times before emerging. These rays have a longer path because the core has a non zero diameter. because they’ve travlled a longer path they take a greater time to travel the length of the fibre.

rays that enter the fibre at the same time will emerge from the other end at different times - short duration pulses of light used for digital signalling will emerge as wider duration pulses - and it’s a limit on the rate you can send digital information through a long fibre because in that situation you would like one pulses, one pulse following another in as short a time as possible.