Optics

Question 1

What is the definition of refraction?

Answer 1

The change of direction which occurs when light passes at an angle across a boundary between 2 transparent substances.

Question 2

What happens to the light ray when it travels from air to glass? What happens when light rays travels from glass to air?

Answer 2

It bends towards the normal. It bends away from the normal.

Question 3

What is the formula for refractive index n? What is the definition of refractive index?

Answer 3

sin i / sin r. It measures how much a material slows down light.

Question 4

Why does refraction occur?

Answer 4

The speed of light waves is different in each substance.

Question 5

what is formula for refraction using speed?

Answer 5

n = c/cs, where c is the speed of light, and cs is the speed of light in the substance.

Question 6

What changes, and what doesn’t change during refraction?

Answer 6

Wavelength and the speed of the wave changes, but the frequency of the wave remains the same.

Question 7

what is snell’s law?

Answer 7

n1sinθ1 = n2sinθ2. where θ1 is the angle of incidence, and θ2 is the angle of refraction.

Question 8

What is the refractive index of air?

Answer 8

1

Question 9

What wavelength in air gives the greatest amount of refraction?

Answer 9

The shortest wavelength.

Question 10

When does the light ray refract along the boundary?

Answer 10

when the angle of incidence is increased to a value called the critical angle.

Question 11

When can total internal reflection occur?

Answer 11

1) When the incident substance has greater refractive index than the other substance.
2) When the angle of incidence exceeds the critical angle.

Question 12

What is the angle of refraction at the critical angle

Answer 12

90 degrees, so sin 90 becomes 1.

Question 13

Where are optical fibres used?

Answer 13

In medical endoscopes to see inside of the body and in communications to carry light signals.

Question 14

What happens to the light ray inside the optical fibres?

Answer 14

The light ray is totally internally reflected along the fibre. At each point where light reaches the boundary, the angle of incidence exceeds the critical angle of the fibre.

Question 15

Why do the fibres need to be transparent? Why has each fibre got a layer of cladding around its core?

Answer 15

Fibres need to be transparent to minimise the absorption of light. The layer of cladding has a lower refractive index to reduce light loss from the core. Light loss would reduce the amplitude of the pulses. Signal can be degraded by absorption, leading to lost information.

Question 16

What would happen when two fibres are in direct contact, and there is no cladding?

Answer 16

Light would cross from one fibre to the other if there was no cladding. Such crossover would mean that the signals wouldn’t be secure, as they would reach the wrong destination.

Question 17

Why has the core got to be very narrow?

Answer 17

To prevent modal dispersion, which occurs in a wide core and leads to signal degradation. A pulse of light sent along a wide core can increase in length and broaden due to some rays taking longer to get to receiver. If it becomes too long, it would merge with the next pulse. Core is also narrow to ensure total internal reflection occurs, as angle of incidence is always greater than critical angle.

Question 18

When does material dispersion occur?

Answer 18

When white light is used instead of monochromatic light. Different frequencies travel at different speeds in the core, which means they get to receiver at different times, and also can lead to pulse merging. So, light used must be monochromatic to prevent pulse merging.

Question 19

What does the double slit experiment investigate?

Answer 19

Interference of light.

Question 20

What two things must there be for a source to be coherent

Answer 20

Source has to produce waves with constant phase difference and the same frequency.

Question 21

What is special about the two slits which makes the whole experiment work?

Answer 21

The two slits act as coherent sources of waves.

Question 22

Why must the fringes be displayed on a screen?

Answer 22

A beam of laser light will damage the retina.

Question 23

Why do bright fringes form?

Answer 23

Light from one slit reinforces the light from the other slit. Light waves from each slit arrive in phase with each other.

Question 24

Why do dark fringes form?

Answer 24

Light from one slit cancels the light from the other slit. Light waves from the two slits arrive 180 degrees out of phase.

Question 25

What is the formula for fringe separation?

Answer 25

w = λD/S, where λ is the wavelength of light, D is the distance from the slits to the screen, and s is the slit spacing. All in metres.

Question 26

For reinforcement at a point on a screen, what does the path difference have to be?

Answer 26

mλ, where m = 0,1,2. Light emitted simultaneously from s1 and s2 arrive in phase at P, so reinforcement occurs.

Question 27

For cancellation at a point on a screen, what does the path difference have to be?

Answer 27

(m + 1/2)λ. Light emitted simultaneously from s1 and s2 arrives out of phase by 180 degrees, so cancellation occurs at p. Path difference between waves leads to a phase difference.

Question 28

What is the fringe separation?

Answer 28

The distance from the centre of a bright fringe to the centre of the next bright fringe.

Question 29

Why is the fringe separation greater for red light than blue light?

Answer 29

Red light has a longer wavelength than blue light.

Question 30

What is one way in which lasers differ from non - laser light?

Answer 30

Laser light is highly monochromatic, which means we can specify its wavelength very accurately, correct to a nanometer.

Question 31

What is the second way in which lasers differ from non - laser light?

Answer 31

A laser is a convenient source of coherent light. So this means that we don’t have to make the light pass through a narrow slit first as we do with light from a non laser beam. Each emitted photon from a laser is in phase with each other, so a laser is coherent source of light. In comparison, for a non laser light, atoms emit photons at random so the photons in such a beam have random phase differences.

Question 32

What two things make diffraction through a gap greater?

Answer 32

1) The gap being made narrower.

2) The wavelength being made larger.

Question 33

What are the main properties of diffraction by a single slit?

Answer 33

1) Central fringe is twice as wide as each of the outer fringes.
2) Peak intensity of each fringe decreases with distance from the centre.
3) Each of the outer fringes are of the same width.
4) Outer fringes are much less intense than the central fringe.

Question 34

What is formula for single slit diffraction? What does increasing slit size do to central maximum fringe?

Answer 34

W = wavelength of light (λ) / Width of single slit (a) * 2D. Increasing slit size makes central fringe narrower and more intense. Decreasing slit size makes central fringe wider and less intense. Wavelength proportional to central maximum fringe size.

Question 35

What two things about the slits must be correct for an interference pattern to be observed?

Answer 35

Each slit has to be narrow enough to make light passing through it diffract sufficiently.

Two slits must be close enough so that the diffracted waves overlap on the screen.

Question 36

What is a diffraction grating?

Answer 36

A plate with many closely spaced parallel slits ruled on it. So light is transmitted by the grating in certain directions only.

Question 37

How does a diffraction grating work?

Answer 37

Light passing through each slit is diffracted. The diffracted light waves from adjacent slits reinforce each other in certain directions, and cancel out in all other directions.

Question 38

What is the central beam in the same direction as in a diffraction grating?

Answer 38

The incident beam

Question 39

How does the angle of diffraction between each transmitted beam and central beam increase?

Answer 39

When the light of the wavelength increases, and a grating with closer slits is used.

Question 40

What is the formula for diffraction grating?

Answer 40

dsinθ = nλ, where d = grating spacing, n = order number, and θ is the angle between the wavefront and the plane of slits.

Question 41

How do you calculate the number of slits per meter on the grating?

Answer 41

N = 1/d, where d is the grating spacing.

Question 42

How do you find the maximum number of orders produced?

Answer 42

Substitute in θ = 90 degrees in the grating equation, and calculate n using n = d/λ

Question 43

What is diffraction?

Answer 43

The spreading out of waves when they pass through a gap or over an edge.

Question 44

Explain young’s double slit experiment?

Answer 44

Light shone at a single slit and diffracts to illuminate a double slit. Two slits act as coherent wave sources. Light diffracts at the two slits and the two waves superpose, forming an interference pattern, due to a combination of constructive and destructive interference.

Question 45

How does having more slits in a diffraction grating help?

Answer 45

You get a sharper interference pattern, this means you can obtain more accurate results.

Question 46

When is diffraction through a single slit most observable and least observable?

Answer 46

Most when the gap is the same size as the wavelength. No diffraction when gap is much wider than wavelength.

Question 47

How does interference pattern vary when monochromatic light isn’t used?

Answer 47

Light used is made up of different wavelengths which diffract by different amounts. The fringes would be a lot less clear.

Question 48

What are some advantages of optical fibres and what is it used for?

Answer 48

Used in medical endiscopes to carry light and also to transmit phone and cable tv signals. Signal can carry more information because light has a high frequency Light also doesn’t heat up the fibre - almost no energy is lost as heat.

Question 49

What is signal degradation and what are two ways in which signal degradation can occur in optical fibres.

Answer 49

Through absorption and dispersion - either modal dispersion or material dispersion. Absorption is where fibre absorbs some of the signal’s energy. Both types of dispersion cause pulse broadening, which leads to pulses overlapping and leading to information loss.