Chapter 3: Refraction of Light

Question 1

What is the effect on light when it passes from a less dense medium to a more dense medium?

Answer 1

the light beam is bent towards the normal as the edge of the eam arriving first is slowed on entering the denser medium

Question 2

What is the equation used to calculate the absolute refractive index of a medium?

Answer 2

refractive index = velocity of light in air / velocity of light in the medium

Question 3

What are the refractive indices of the following?

• air
• water
• cornea
• crystalline lens
• crown glass
• flint glass
• diamond

Answer 3

• 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

Question 4

What instrument can be used to determine the refractive index of any material?

Answer 4

refractometer

Question 5

What is Snell’s law?

Answer 5

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

Question 6

What are 2 properties of light passing obliquely through a plate of glass?

Answer 6

• emerging ray is parallel to the incident ray
• the direction of light is unchanged but is laterally displaced

Question 7

What proportion of light emerges on the other side of a lens or window with a refractive index of 1.5 and why?

Answer 7

92.16% - 4% is reflected at the first surface and further 4% of remaining light is reflected at the second surface

Question 8

What instrument makes use of the small proportion of reflected light from glass?

Answer 8

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

Question 9

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)?

Answer 9

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

Question 10

Why do objects in an optically dense mediu me.g. underwater appear displaced when viewed from a less dense medium?

Answer 10

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

Question 11

What happens when a light ray strikes at 90 degrees to the interface between a denser and less dense medium?

Answer 11

is undeviated

Question 12

What is meant by the critical angle?

Answer 12

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)

Question 13

What is total internal reflection?

Answer 13

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

Question 14

What is the critical angle for:

a) the tear film/air interface?
b) crown glass/air interface?

Answer 14

• a) 48.5 degrees
• b) 41 degrees

Question 15

What are 2 optical instruments that make use of TIR?

Answer 15

1. Prisms - excellent reflectors
2. Fibre optic cables (surgical intraocular light source, transmission of laser light from laser tube to delivery system of laser slit lamp)

Question 16

What is a key point and function of total internal reflection within the eye?

Answer 16

• cornea: air interface
• prevents visualisation of parts of the eye e.g. ngle of the anterior chamber and peripheral retina by an examiner

Question 17

How can an examiner view the angle of the anterior chamber despite total internal reflection preventing this otherwise?

Answer 17

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

Question 18

What can overcome total internal reflection to allow visualisation of the peripheral retina by an examiner?

Answer 18

three-mirror contact lens - has higher refractive inex than eye, space between eye and lens filled with saline, destroying the cornea/air refracting surface

Question 19

How does the refractive index of a medium differ for light of different wavelengths?

Answer 19

light of shorter wavelength (e.g. blue) is deviated more than light of longer wavelength (e.g. red)

Question 20

What is the refractive index of a material usually taken to mean?

Answer 20

that for the yellow sodium flame

Question 21

What measurement indicates the dispersive power of a medium?

Answer 21

the angle formed between red and blue light around the yellow after refraction

Question 22

Is the dispersive power of a medium related to refractive index?

Answer 22

no

Question 23

How is a raindrop produced based on dispersion and refraction?

Answer 23

• 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

Question 24

What what position must an observer be in to see a rainbow?

Answer 24

the observer must look away from the sun

Question 25

What causes the order of colours in the rainbow?

Answer 25

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

Question 26

What causes a secondary rainbow to appear?

Answer 26

formed by rays that have twice undergone total internal reflection within the raindrops and the colours are in reverse order (violet on the outside)