Optics Flashcards
When light passes from a less dense to more dense medium, the light bends which way?
Towards the normal.
When light passes from a more dense to less dense medium, the light bends which way?
Away from the normal.
The refractive index of a medium is given by what formulae?
N = Sin( i ) / Sin( r ) = c1/c2 = λ1/λ2
The critical angle of a substance is given by what formula?
Sin( ic ) = n2/n1
Why must the core in an optical fibre be narrow?
To prevent multipath dispersion.
What is refraction?
Refraction is the change of direction that occurs when light passes non-normally across a boundary between two transparent substances.
When is light totally internally reflected?
When the angle of incidence is more than the critical angle.
What does the equation w = λD/s describe?
The relationship between slit spacing ‘s’, fringe width ‘w’, wavelength of light ‘λ’ and the slit-to-screen distance ‘D’.
Describe how to set up a Double-Slit Experiment.
A light source is used to emit light. A coloured screen is placed in front of the light source to ensure all the light is of one wavelength/frequency. Then the light must pass through and be diffracted by a single slit. This makes the light coherent. The light then passes through two parallel slits that are closely spaced. These emit waves that are coherent in that they have a constant phase relationship and the same frequency. These waves of light interfere with one another, causing a pattern of dark and intense fringes to be seen on the projected screen. At places where there at light fringes there has constructive interference or enforcement meaning the waves have arrived in phase with one another. At dark fringes there has been destructive interference or cancellation as the waves arrive 180 degrees out of phase.
What is the path difference?
The path difference is the difference between the distance travelled by two waves in the double slit experiment to get to the same place.
What does the path difference tell us?
If the path difference is an integer amount ‘m’ of the wavelength ‘λ’ then the waves will reinforce at that point and create a maxima if the path difference is (m+1/2) lots of λ then there will be cancellation as the waves will be 180 degrees out of phase, creating a minima.
How should one measure the fringe width ‘w’?
Measure across several fringes from the centre of a dark fringe to the centre of another dark fringe, as the centres of dark fringes are easier to discern. Divide your measurement by the number of fringes you measured across.
What assumption is made during the derivation of w = λD/s?
That the screen is far enough away from the slits such that the angles from the slits to the screen can both be assumed to be 90 degrees.
How do the double slit fringe patterns of red and blue light differ?
Because of the fact that w = λD/s, we can see that as wavelength increases or decreases and D and s stay the same, the fringe width will change accordingly.
How does laser light differ from ordinary light?
- Laser light is highly monochromatic.
- A laser gives off inherently coherent light.
Describe the diffraction of light through a single slit.
- The central fringe is twice as wide as the other fringes.
- The peak intensity of each fringe decreases with distance from the central order fringe.
- Each of the outer fringes is the same width.
- The outer fringes are much less intense than the central fringe.
Compare the idealised double slit interference pattern and the actual double slit interference pattern.
Ideally we would see many fringes of equal intensity and equal spacing from one another. This is purely due to interference. However, because of how light diffracts when passing through a slit we see the original pattern bounded by the single slit diffraction pattern.
What is a diffraction grating?
A diffraction grating consists of a plate with many closely spaced parallel slits ruled into it.
How does a diffraction grating diffract light?
The light passing through each waves is diffracted and these waves then reinforce each other in certain directions only, including the direction of the incident light (called the zero order beam).
What relationship does dsinθ = nλ describe?
The relationship between ‘d’ slit difference, ‘sinθ’ the angle between the wavefront and the plane of the slits and ‘nλ’ the distance between the wavefront and the slit which emitted the wave second.
What can we discern from a continuous spectrum?
Because a hotter light source emits more of high frequency light, by looking at the wavelength of the most intense part of the spectrum we could work out the temperature of the light source.
