Optics Flashcards
Give the 2 conditions required for total internal reflection
1) the incidence substance has a larger refractive index than the other substance
2) the angle of incidence exceeds the critical angle
Define the critical angle and give the equation to find it
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
Give the equation for the refractive index of a substance
Refractive index = speed of light in a vacuum / speed of light in the transparent substance
n = sin i / sin r
Explain why the core of an optical fibre must be very narrow
To prevent multipath dispersion
Explain why cladding is required round each optical fibre core
So that due to the large reflective index, the light is internally reflected and to stop light from crossing from one core into another
Define refraction
Refraction is the change of direction that occurs when light passes non-normally across a boundary between 2 transparent substances
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
i) Towards the normal
ii) Away from the normal
Other than refraction, what do you also get when a light ray meets a refractive substance?
Partial reflection
State Snell’s law
n = sinθi / sinθr
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?
The angle r₂
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?
The angle r₁
Give the equation relating the angles of incidence and refraction to the speed of light and thus the refractive index of the substance
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
Explain why white light is dispersed into its constituent colours when it passes through a prism
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
Describe why diamonds sparkle when white light is directed at them
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
State 2 uses for total internal reflection
1) A communications optical fibre
2) A medical endoscope
Who was first to suggest the wave nature of light and why was it rejected?
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
Define coherent
Waves of the same frequency and phase relationship
Describe Young’s fringes
Alternate bright and dark fringes formed as the diffraction pattern for double slits in front of a monochromatic light source
Explain the bright fringes produced from a double slit in front of a coherent light source
The light from one slit reinforces the light from the other slit. The light waves from each slit arrive in phase with each other
Explain the dark fringes produced from a double slit in front of a coherent light source
The light from one slit cancels the light from the other slit. The light waves arrive 180° out of phase
Give the equation for the fringe separation for a double slit experiment
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
Give the equation required for the constructive interference at point P on a screen from light emerging from the slits S₁ and S₂
S₁P - S₂P = mλ
where m is an integer
Give the equation required for the deconstructive interference at point P on a screen from light emerging from slits S₁ and S₂
S₁P - S₂P = (m + ½)λ
where m is an integer
Give the conditions for w = λD / s to hold true
1) The light from each slit must be coherent
2) s
Give the wavelengths for red and blue light
Blue ≈ 400 nm
Red ≈ 700 nm
Give 3 light sources and state their use as a monochromatic light source
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
Why is diffraction of light important in the design of optical instruments, including telescopes?
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
What can be used to observe the diffraction of water waves through a gap?
A ripple tank
When using a ripple tank with a single slit, what 2 things cause the diffracted waves to spread out more?
- If the gap is made narrower
- The wavelength is made larger
Describe the diffraction pattern formed on a white screen when a parallel beam of light is directed at a single slit
- 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
Give the equation for the width of the central fringe for a single slit diffraction pattern
W = 2Dλ / a
where D is the distance to the screen and a is the width of the single slit
Describe a diffraction grating
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
Why is the light transmitted by a diffraction grating in certain directions only?
- 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
State 2 ways in which the angle between each beam and the 0 order beam is increased
- 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
Give the equation for the angle of the nth order for a diffraction grating with grating separation d
dsinθ = nλ
Give the equation of the maximum number of orders for a diffraction grating
The maximum number of orders is given by the value of d / λ rounded down to the nearest integer
State the 3 types of spectra
- Continuous
- Line emission
- Line absorption
Describe a line emission spectra
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
Describe a line absorption spectra
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
For 2 different substances of refractive indices n₁ and n₂, give the equation for the law of refraction
n₁sinθ₁ = n₂sinθ₂
