Chapter 5 - Optics Flashcards
What is refraction?
Change in direction that occurs when light passes at an angle across a boundary between two transparent substances. Due to change in speeds
What is the normal?
Line perpendicular to a boundary between two materials or a surface
What happens to a ray of light at the boundary of a more dense substance?
Light ray slows down and therefore bends towards the normal.
What happens to a ray of light at the boundary of a less dense substance?
Light ray speeds up and therefore bends away from normal.
What is approximately the refractive index of air?
1
What is the equation for refractive index involving speeds of light?
n = speed of light in a vacuum/ speed of light in the transparent substance
What is snells law for refraction at a boundary?
n2/n1 = sini/sinr
A light ray crosses the boundary between water of refractive index 1.33 and a glass of refractive index 1.50 at an angle of incidence of 40 degrees. Calculate the angle of refraction of this light ray.
n1 = 1.33 , n2 = 1.50 , sini = 40 degrees
sinr = 1.33 x sin40 / 1.5
= 0.57
r = 35 degrees
Do smaller wavelengths refract less or more?
More
Dispersive effect occurs because of the speed of light in glass depends on the wavelength. Violet light travels more slowly than red light, so has a greater refractive index.
What is the critical angle?
Angle where light ray refracts along the boundary. If angle of incidence exceeds the critical angle total internal reflection occurs.
What are the conditions for total internal refraction to occur?
- ) The incident substance has a larger refractive index than the other substance
- ) Angle of incidence exceeds the critical angle
What does an optical fibre do? How does this work?
Allows pulses of light that enter at one end, from a transmitter, to reach the receiver on the other end.
Light rays are internally refracted each time it reaches a boundary - as the angle of incidence exceeds the critical angle.
What features of optical fibres allow it to carry out it’s function?
- Core surrounded by layer of cladding of lower refractive index to allow total internal refraction to occur and to reduce light loss from the core (that would reduce amplitude). Also prevents light from passing to other fibres in direct contact.
- Highly transparent to minimise absorption of light, which would reduce amplitude of the pulses.
- Core must be narrow to prevent modal dispersion
- Monochromatic light used to prevent material dispersion
What is modal dispersion? What is done to prevent this
Occurs in wide core fibres because light travelling axis of the core travels a shorter distance per metre of fibre than the light that repeatedly undergoes total internal reflection
Core is made very narrow to prevent this
What is material dispersion?
Speed of light in glass of optical fibre depends on wavelength of the light. Differences in speed would cause white pulses in optical fibres to become larger, as e.g. violet falls behind red.
Monochromatic light used to prevent this.
What experiment is carried out to prove interference of light?
Youngs double slit experiment
Briefly describe young’s double slit experiment
Illuminate a narrow single slit using light source which illuminates two closely spaced parallel slits (double splits) using light source. The two slits act as coherent sources of wave (emit waves of constant phase difference and the same frequency). Alternate white and dark fringes (young’s fringes) can be seen on a white screen placed where the diffracted light from the double slit overlaps.
Why are the fringes formed in young’s double slit experiment?
Due to interference of light from the double slits
Why are bright fringes formed?
Light from one one slit constructively interferes with the light from the other slit (arrive in phase)
Why are dark fringes formed?
Light from one slit destructively interferes with the light from the other slit (arrive 180 degrees out of phase/ anti-phase)
What is the equation for fringe spacing?
fringe spacing = wavelength x distance from the screen to the slits/ split spacing
Give the equation for path difference in this situation
S(1)P - S(2)P = m x wavelength
What would the equation be if the light emitted from S(1) and S(2) arrives out of phase at P?
S(1)P - S(2)P = (m+1/2) x wavelength
Why are lasers dangerous?
Lens of eye can focus the laser to a very fine spot (aided by it’s monochromatic property) on the retina and an intense concentration of light would destroy the retina.
What does coherent mean?
Waves of constant phase difference and same frequency
What is diffraction?
Spreading out of waves when they pass through a gap or by an edge
Briefly describe diffraction of light by a single slit
Monochromatic/white light shone at a slit. The diffracted light forms a pattern that can be observed on a white screen. Shows a central fringe (x2 the size) with further fringes either side of the central fringe that are less intense (all the same size)
What is the equation for width of central fringe in single slit diffraction?
w = wavelength of light/ width of the single slit x (2 x distance from the slit to screen)
What is the effect of using greater wavelength light on the diffraction pattern?
Fringes get wider
What is the effect of making the slit narrower on the diffraction pattern?
Makes fringes wider
What makes waves diffract more?
- Gap is made narrower
- wavelength is made larger
What is a diffraction grating?
Plate with many closely spaced parallel slits on it
When a parallel beam of monochromatic light is directed normally at a diffraction grating, why is light only transmitted by the grating in certain directions?
Because:
- Light passing through each slit is diffracted
- Diffracted light waves from adjacent slits reinforce each other in certain directions only.
What is the central beam, emitted by the diffraction grating - in the same direction as the incident beam called?
zero order beam
- other transmitted beams are numbered outwards from zero order beam.
What increases the angle of diffraction between each transmitted beam and central beam?
- Light of longer wavelength
- grating with closer slits
What is the equation for finding wavelength of light being used in a diffraction grating?
dsinx = n x wavelength
x = angle to the normal made by the maximum n = order of maximum d = distance between the slits
Using the diffraction grating equation what re the effects of:
- greater wavelength
- greater distance between slits
- Greater wavelength makes sinx bigger, so the pattern spreads out more
- D being bigger makes sinx smaller, so pattern will spread out less
What are some applications of diffraction gratings?
- Line absorption spectra
- X-ray crystallography
