Optics (topic 5) Flashcards
Normal (definition)
Imaginary line perpendicular to the boundary between a material or surface
Refraction (definition)
Change of direction that occurs when light passes at an angle across a boundary between two transparent substances
How does the density of a material (comparatively to the density of the material it was in) affect how light refracts when crossing the boundary? (2)
- Light ray bends towards the normal in a more dense substance
- Light ray bends away from the normal in a less dense substance
Method for investigating refraction by glass (5)
- glass block on paper
- draw around glass block with pencil + draw a normal
- Shine a ray of light at an angle into the point where normal reaches glass block
- Mark P (incident ray) and Q (ray leaving block)
- measure angle of refraction from P/refracted ray to normal
What is always true for a refracted ray of light in a glass block?
- angle of refraction < angle of incidence
- ratio of sin i/sin r is the same for each ray (Snell’s law)
What is Snell’s law?
The ratio of sin i/sin r is the same for every angle of i
Refractive index (definition + equation)
The ratio between angle of incidence and angle of refraction in a substance (sin i/sin r)
Apart from refraction, what also occurs when shining a ray of light into a glass block?
Partial reflection
Comparison of refractive index from air>glass and glass>air
If n for air>glass = n
glass>air = 1/n
(as i(2) = r(1) and r(2) = i(1))
Why does refraction occur, and how does this relate to refractive index?
Speed of light waves is different in each substance
(smaller speed = greater refractive index)
What changes about light when crossing a boundary, and what doesn’t change?
Speed of light changes
Wavelength changes
Frequency DOES NOT change
Equation for refraction at a boundary between two transparent substances
n(1)sin theta(1) = n(2) sin theta(2)
where theta (1) is i and theta (2) is r
Refractive index of air/a vacuum
Approximately 1
(Exactly 1 for a vacuum)
Why can a prism be used to split a beam of white light into a spectrum?
White light is made up of light with a continuous range of wavelengths
Glass prism refracts light by different amounts depending on wavelength
Rule for wavelength and diffraction, and why this happens (2)
Shorter wavelength in vacuum = greater diffraction
Because speed of light in a material depends on wavelength e.g. violet light travels slower than red light in glass
How does the refraction/reflection of light from glass to air change as the angle of incidence is changed?
Refracts away from the normal up until the critical angle (where it is reflected along the boundary (90* - the critical angle). An angle of incidence larger than that of the critical angle, total internal reflection takes place
What is total internal reflection?
At the boundary between two substances (e.g. glass and air), a ray behaves like it has reached a plane mirror and reflects back into the glass instead of refracting into air.
Equation or critical angle and why it works (2)
sin theta(c) = n2 / n1
As sin theta(2) = sin(90) = 1
What are the conditions for total internal reflection?
- Incident substance has a larger refractive index than other substance (n1 > n2)
- angle of incidence is greater than critical angle
Why do diamonds sparkle when white light is directed at them? (2)
Very high refractive index = separates colours more
Low critical angle = light is totally internally reflected many times, which means colours spread out even more
What are optical fibres used for?
1) medical endoscopes to see inside body
2) communications to carry signals
Why do slits give an interference pattern?
They act as coherent light sources (emit light with constant phase difference and same frequency)
What happens when a single slit is too wide? (2)
Each part produces a separate fringe pattern, which is displaced from the others.
The darker fringes become smaller than the lighter fringes, so contrast is lost between dark and bright fringes.
Young’s slits pattern
Alternating bright and dark fringes, evenly spaced and parallel to the double slits
How is a bright fringe formed?
Light from one slit reinforces light from the other slit
How is a dark fringe formed?
Light from one slit cancels light from the other slit
What is fringe separation?
Distance from the centre of one bright fringe to the centre of the next bright fringe
Fringe separation equation
w = (lambda)D / s
What is path difference (Young’s slits)?
The difference in distance from each slit to one point on the screen
Why do bright fringes appear?
Reinforcement as the path difference for the light from the slits is a whole number of wavelengths
Why do dark fringes appear?
Cancellation as the path difference for the light from slits is a whole number of wavelengths + 1/2 a wavelength
Why measure w from dark fringes?
centre of dark fringes is easier to locate
Why does a single slit create a coherent source?
Diffraction makes the waves behave as if they are coming from a single point
Why can a normal light not create an interference pattern with slits?
Not coherent - emits waves at random
Which colour of light has the largest wavelength?
Red
Which colour of light has the largest frequency?
Violet
Would two sources that are not in phase produce an interference pattern?
Not necessarily - they can still produce a pattern if they have a constant phase difference and the same frequency
Examples of monochromatic light sources (3)
- vapour lamps/discharge tubes - e.g. sodium vapour lamp (mostly - wavelengths are dominated by orange/yellow)
- laser light
- white light through a colour filter
Examples of non-monochromatic light sources (2)
- the sun
- filament lamp
Why are lasers the best choice to demonstrate double slit interference? (2)
- highly monochromatic (specify wavelength to a nanometre)
- highly coherent (so single slit is not needed)
Describe fringes formed by white light (3)
- Central fringe is white as every component colour overlaps
-outer fringes form spectra of overlapping fringes with red on outside and blue on inside, as red has widest fringes
- outer fringes are wider
What is diffraction?
Spreading of waves through a gap or by an edge
How does diffraction improve telescopes?
Telescope lenses are wider than the human eye so less diffraction occurs, allowing us to see more detail
How can the amount of diffraction be changed? (2)
- narrower gap
- larger wavelength
Describe the single slit diffraction pattern
- Central fringe is brightest and widest (2x width of others)
- outer fringes decrease in intensity with distance from centre
- all outer fringes same width
How can the width of a fringe in a slit pattern be increased?
- Narrower slit
- Larger wavelength
Width of (outer) fringe for single slit pattern
w = (lambda)D / a
where “a” is width of single slit
Why might an interference pattern not be observed with Young’s slits? Why does this prevent fringes forming? (2 + reason)
- Slits too wide
- Slits too far apart
Light from slits does not overlap so no pattern forms
What is a diffraction grating?
Plate with many closely spaced parallel slits ruled onto it
Why does a parallel beam of monochromatic light create beams when passed through a diffraction grating? (2)
Light passing through each slit is diffracted.
Diffracted light waves from adjacent slits reinforce each other only in some directions, and cancel everywhere else.
How to increase angle of diffraction between each beam transmitted through a diffraction grating? (2)
- longer wavelength
- closer slits
equation for nth order beam from a diffraction grating
d sin (theta) = n(lambda)
where d = 1/number of slits
equation for maximum order beam from diffraction grating
n = d/(lambda)
- round DOWN to nearest whole number
How does a communications optical fibre work? (3)
- light enters through transmitter and reaches receiver at the other end
- TIR happens around bends as cladding has lower refractive index
What properties should communications optical fibre have? why? (3)
- Highly transparent - minimises absorption of light which would reduce amplitude
- Cladding of lower refractive index - prevents light loss through refraction which would reduce amplitude; also prevents signals mixing between fibres in contact
- Narrow - prevents multi-path dispersion where light becomes longer in wide core and merges with next pulse
Why does pulse dispersion happen? (2)
- Core is too wide which causes some light to cover greater distance in the same time, causing pulses to merge
- White light spreads out based on component colours’ speed in glass, creating a longer pulse
How does a medical endoscope work? (3)
- made up of two bundles of fibres which are inserted into a cavity and used to see into the body (light sent through one bundle, lens on other)
- light from image travels up other fibre where it can be observed
- must be coherent bundle (i.e. same relative positions - cannot cross)
