Optics

Question 1

Give the 2 conditions required for total internal reflection

Answer 1

1) the incidence substance has a larger refractive index than the other substance
2) the angle of incidence exceeds the critical angle

Question 2

Define the critical angle and give the equation to find it

Answer 2

At the critical angle, the angle of refraction is 90° because the light Ray emerges along the boundary.

(Because sin90=1) sin ic = n₂ / n₁

Question 3

Give the equation for the refractive index of a substance

Answer 3

Refractive index = speed of light in a vacuum / speed of light in the transparent substance

n = sin i / sin r

Question 4

Explain why the core of an optical fibre must be very narrow

Answer 4

To prevent multipath dispersion

Question 5

Explain why cladding is required round each optical fibre core

Answer 5

So that due to the large reflective index, the light is internally reflected and to stop light from crossing from one core into another

Question 6

Define refraction

Answer 6

Refraction is the change of direction that occurs when light passes non-normally across a boundary between 2 transparent substances

Question 7

In which direction relative to the normal does light bend when it passes from:

i) a more refractive substance to a less refractive substance
ii) a less refractive substance to a more refractive substance

Answer 7

i) Towards the normal

ii) Away from the normal

Question 8

Other than refraction, what do you also get when a light ray meets a refractive substance?

Answer 8

Partial reflection

Question 9

State Snell’s law

Answer 9

n = sinθi / sinθr

Question 10

If a ray of light enters a glass block at angle i₁ and is refracted to angle r₁, as it leaves the glass block, it has an angle of incidence i₂ and leaves at an angle of r₂.
What is angle i₁ equal to?

Answer 10

The angle r₂

Question 11

If a ray of light enters a glass block at angle i₁ and is refracted to angle r₁, as it leaves the glass block, it has an angle of incidence i₂ and leaves at an angle of r₂.
What is angle i₂ equal to?

Answer 11

The angle r₁

Question 12

Give the equation relating the angles of incidence and refraction to the speed of light and thus the refractive index of the substance

Answer 12

sinθi / sinθr = c / cₓ
nₓ = c / cₓ
where cₓ is the speed of light in the substance and nₓ is the refractive index of that substance

Question 13

Explain why white light is dispersed into its constituent colours when it passes through a prism

Answer 13

Because white light is composed of light with a continuous range of wavelengths, from red (650nm) to violet (350nm)
The glass prism diffracts lights by different amounts depending on the its wavelength, the shorter the wavelength in air, the greater the refraction
This is because the speed of light in glass depends on wavelength

Question 14

Describe why diamonds sparkle when white light is directed at them

Answer 14

Because they have a very high refractive index of 2.417, so it separates the colours more than any other substance. It also has a critical angle of 24.4° so a light ray in a diamond may be totally internally reflected many times before it emerges, which means the colours in white light spread out more and more

Question 15

State 2 uses for total internal reflection

Answer 15

1) A communications optical fibre

2) A medical endoscope

Question 16

Who was first to suggest the wave nature of light and why was it rejected?

Answer 16

Christian Huygens in the 17th Century but he proposed it at the time of Sir Isaac Newton’s corpuscualr theory of light. Since Newton’s reputation was much greater, his theory remained unchallenged for over a century

Question 17

Define coherent

Answer 17

Waves of the same frequency and phase relationship

Question 18

Describe Young’s fringes

Answer 18

Alternate bright and dark fringes formed as the diffraction pattern for double slits in front of a monochromatic light source

Question 19

Explain the bright fringes produced from a double slit in front of a coherent light source

Answer 19

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

Question 20

Explain the dark fringes produced from a double slit in front of a coherent light source

Answer 20

The light from one slit cancels the light from the other slit. The light waves arrive 180° out of phase

Question 21

Give the equation for the fringe separation for a double slit experiment

Answer 21

w = λD / s
where w is the fringe separation, λ is the wavelength of light, D is the distance between the slits to the screen and s is the slit spacing

Question 22

Give the equation required for the constructive interference at point P on a screen from light emerging from the slits S₁ and S₂

Answer 22

S₁P - S₂P = mλ

where m is an integer

Question 23

Give the equation required for the deconstructive interference at point P on a screen from light emerging from slits S₁ and S₂

Answer 23

S₁P - S₂P = (m + ½)λ

where m is an integer

Question 24

Give the conditions for w = λD / s to hold true

Answer 24

1) The light from each slit must be coherent

2) s

Question 25

Give the wavelengths for red and blue light

Answer 25

Blue ≈ 400 nm

Red ≈ 700 nm

Question 26

Give 3 light sources and state their use as a monochromatic light source

Answer 26

1) Vapour lamps and discharge tubes - produces light with a dominant colour. Although other wavelengths are emitted, the dominant colour is much more intense than any other colour, so can in effect be used as a monochromatic light source
2) Light from a filament lamp or the Sun - is composed of the colours of the spectrum and therefore covers a continuous range from 350-650nm. A filter must be used to make it more monochromatic
3) Light from a laser - is highly monochromatic and coherent

Question 27

Why is diffraction of light important in the design of optical instruments, including telescopes?

Answer 27

Diffraction can distort an image
A telescope can show features we couldn’t detect directly, partly due to less diffraction occurs when waves pass through a wide gap than a smaller gap (e.g. the eye pupil), so less diffraction occurs when using a telescope

Question 28

What can be used to observe the diffraction of water waves through a gap?

Answer 28

A ripple tank

Question 29

When using a ripple tank with a single slit, what 2 things cause the diffracted waves to spread out more?

Answer 29

• If the gap is made narrower

- The wavelength is made larger

Question 30

Describe the diffraction pattern formed on a white screen when a parallel beam of light is directed at a single slit

Answer 30

• The pattern shows a central fringe with further fringes either side.
• The intensity of the fringes is greatest at the centre of the central fringe
• The central fringe is twice as wide as each of the outer fringes but the outer fringes are the same width

Question 31

Give the equation for the width of the central fringe for a single slit diffraction pattern

Answer 31

W = 2Dλ / a

where D is the distance to the screen and a is the width of the single slit

Question 32

Describe a diffraction grating

Answer 32

A diffraction grating consists of a plate with many closely spaced parallel slits ruled on it.
When a parallel beam of monochromatic light is directed normally at a diffraction grating, light is transmitted by the grating in certain directions only

Question 33

Why is the light transmitted by a diffraction grating in certain directions only?

Answer 33

• The light passing through each slit is diffracted
• The diffracted light waves from adjacent slits reinforce each other in certain directions only (including the incident light direction) and cancel out in all other directions

Question 34

State 2 ways in which the angle between each beam and the 0 order beam is increased

Answer 34

• Light of a longer wavelength is used (e.g. using a blue filter instead of a red filter
• A grating with closer slits is used

Question 35

Give the equation for the angle of the nth order for a diffraction grating with grating separation d

Answer 35

dsinθ = nλ

Question 36

Give the equation of the maximum number of orders for a diffraction grating

Answer 36

The maximum number of orders is given by the value of d / λ rounded down to the nearest integer

Question 37

State the 3 types of spectra

Answer 37

• Continuous
• Line emission
• Line absorption

Question 38

Describe a line emission spectra

Answer 38

Coloured lines on a black background, with the coloured line representing the wavelength of light emitted.
The lines are characteristic of the element that produced them
The lines get closer together at higher frequencies

Question 39

Describe a line absorption spectra

Answer 39

Black lines on a coloured background, with the black lines representing the wavelength of light absorbed.
The lines are characteristic of the element that absorbed them
The lines get closer together at higher frequencies

Question 40

For 2 different substances of refractive indices n₁ and n₂, give the equation for the law of refraction

Answer 40

n₁sinθ₁ = n₂sinθ₂