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Flashcards in Optics Deck (49)
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1
Q

What is the definition of refraction?

A

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

2
Q

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

A

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

3
Q

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

A

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

4
Q

Why does refraction occur?

A

The speed of light waves is different in each substance.

5
Q

what is formula for refraction using speed?

A

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

6
Q

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

A

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

7
Q

what is snell’s law?

A

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

8
Q

What is the refractive index of air?

A

1

9
Q

What wavelength in air gives the greatest amount of refraction?

A

The shortest wavelength.

10
Q

When does the light ray refract along the boundary?

A

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

11
Q

When can total internal reflection occur?

A

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

12
Q

What is the angle of refraction at the critical angle

A

90 degrees, so sin 90 becomes 1.

13
Q

Where are optical fibres used?

A

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

14
Q

What happens to the light ray inside the optical fibres?

A

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.

15
Q

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

A

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.

16
Q

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

A

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.

17
Q

Why has the core got to be very narrow?

A

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.

18
Q

When does material dispersion occur?

A

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.

19
Q

What does the double slit experiment investigate?

A

Interference of light.

20
Q

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

A

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

21
Q

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

A

The two slits act as coherent sources of waves.

22
Q

Why must the fringes be displayed on a screen?

A

A beam of laser light will damage the retina.

23
Q

Why do bright fringes form?

A

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

24
Q

Why do dark fringes form?

A

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

25
Q

What is the formula for fringe separation?

A

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.

26
Q

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

A

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

27
Q

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

A

(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.

28
Q

What is the fringe separation?

A

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

29
Q

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

A

Red light has a longer wavelength than blue light.

30
Q

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

A

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

31
Q

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

A

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.

32
Q

What two things make diffraction through a gap greater?

A

1) The gap being made narrower.

2) The wavelength being made larger.

33
Q

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

A

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.

34
Q

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

A

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.

35
Q

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

A

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.

36
Q

What is a diffraction grating?

A

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

37
Q

How does a diffraction grating work?

A

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.

38
Q

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

A

The incident beam

39
Q

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

A

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

40
Q

What is the formula for diffraction grating?

A

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

41
Q

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

A

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

42
Q

How do you find the maximum number of orders produced?

A

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

43
Q

What is diffraction?

A

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

44
Q

Explain young’s double slit experiment?

A

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.

45
Q

How does having more slits in a diffraction grating help?

A

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

46
Q

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

A

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

47
Q

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

A

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

48
Q

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

A

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.

49
Q

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

A

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.