Chapter 3: Refraction of Light Flashcards
What is the effect on light when it passes from a less dense medium to a more dense medium?
the light beam is bent towards the normal as the edge of the eam arriving first is slowed on entering the denser medium
What is the equation used to calculate the absolute refractive index of a medium?
refractive index = velocity of light in air / velocity of light in the medium
What are the refractive indices of the following?
- air
- water
- cornea
- crystalline lens
- crown glass
- flint glass
- diamond
- air = 1
- water = 1.33
- cornea = 1.37
- crystalline lens = 1.386 - 1.406
- crown glass = 1.5
- flint glass = 1.6
- diamond = 2.5
What instrument can be used to determine the refractive index of any material?
refractometer
What is Snell’s law?
the incident ray, refracted ray and normal all line in the same plan, and the angle of incidence, i, and angle of refraction, r, are related to the refractive index, n, of the media by:
refractive index = sin i / sin r
where the light is passing from a vacuum into the second medium.
if the interfrace is between 2 denser media of offering optical densities, the refractive index for the interface is
1n2 = n2/n1
What are 2 properties of light passing obliquely through a plate of glass?
- emerging ray is parallel to the incident ray
- the direction of light is unchanged but is laterally displaced
What proportion of light emerges on the other side of a lens or window with a refractive index of 1.5 and why?
92.16% - 4% is reflected at the first surface and further 4% of remaining light is reflected at the second surface
What instrument makes use of the small proportion of reflected light from glass?
teaching mirror of indirect ophthalmoscope - most light is refrcted across glass to the observer but some reflection enables teacher to have same view as the observer
Wat is the equation that can be used to calculate refracting power or vergence of a convex spherical curved surface (and what is important about the signs)?
surface power = (n2-n1)/r
r=radius of curvature of the surface in metres, surface power is in dioptres
surface power is positive for converging surfaces, negative for diverging surfaces
Why do objects in an optically dense mediu me.g. underwater appear displaced when viewed from a less dense medium?
refraction of the emerging rays which appear to come frmo a point i, the virtual image of object O
therefore objects in water seem less deep than they really are
What happens when a light ray strikes at 90 degrees to the interface between a denser and less dense medium?
is undeviated
What is meant by the critical angle?
the angle of incidence at which a ray of light will run parallel with the surface at the interface from a more dense to less dense optical medium (see angle c)
What is total internal reflection?
when rays of light passing from a more dense to less dense optical medium strike more obliquely than the critical angle (i.e. angle c is greater than the critical angle) fail ot emerge from the denser medium and are reflected back into it as from a mirror
What is the critical angle for:
a) the tear film/air interface?
b) crown glass/air interface?
- a) 48.5 degrees
- b) 41 degrees
What are 2 optical instruments that make use of TIR?
- Prisms - excellent reflectors
- Fibre optic cables (surgical intraocular light source, transmission of laser light from laser tube to delivery system of laser slit lamp)
What is a key point and function of total internal reflection within the eye?
- cornea: air interface
- prevents visualisation of parts of the eye e.g. ngle of the anterior chamber and peripheral retina by an examiner
How can an examiner view the angle of the anterior chamber despite total internal reflection preventing this otherwise?
gonioscopy: applying contact lens made of material with a higher refractive index than the eye and filling the space between eye and lens with saline, thus destroying the cornea/air refracting surface
What can overcome total internal reflection to allow visualisation of the peripheral retina by an examiner?
three-mirror contact lens - has higher refractive inex than eye, space between eye and lens filled with saline, destroying the cornea/air refracting surface
How does the refractive index of a medium differ for light of different wavelengths?
light of shorter wavelength (e.g. blue) is deviated more than light of longer wavelength (e.g. red)
What is the refractive index of a material usually taken to mean?
that for the yellow sodium flame
What measurement indicates the dispersive power of a medium?
the angle formed between red and blue light around the yellow after refraction
Is the dispersive power of a medium related to refractive index?
no
How is a raindrop produced based on dispersion and refraction?
- When sunlight enters a raindrop it is dispersed into its constituent spectral colours
- in certain circumstances the angle of incidence causes TIR to occur within the raindrop; when dispersed light finally emerges each wavelength (colour) ameks a different angle with the horizon
- the observer sees receives only a narrow pencil of rays from each drop
- the whole rainbow is the result of rays received from a bank of drops at increasing angle to the observer’s eye
What what position must an observer be in to see a rainbow?
the observer must look away from the sun
What causes the order of colours in the rainbow?
violet, the colour making the smallest angle to the horizon, is received from lower drops while red, making the greatest angle with the horizon, is received from the highest drops
this red is on the outside of the primary rainow
What causes a secondary rainbow to appear?
formed by rays that have twice undergone total internal reflection within the raindrops and the colours are in reverse order (violet on the outside)
