1.1. To recap ray diagramsTo recap ray diagrams

2.2. To understand the concept of refractionTo understand the concept of refraction

3.3. To be able to conduction refractive index To be able to conduction refractive index calculationscalculations

Book Reference : Pages 188-189Book Reference : Pages 188-189

Within the optics topic it is convenient to Within the optics topic it is convenient to show the passage and behaviour of light as show the passage and behaviour of light as “ray diagrams”. These should:“ray diagrams”. These should:

1.1.Be drawn with a pencil and rulerBe drawn with a pencil and ruler

2.2.Contain exactly 1 arrow head per ray Contain exactly 1 arrow head per ray “branch”“branch”

3.3.Show the normal (typically as a dotted Show the normal (typically as a dotted line) at right angles to whatever it is normal line) at right angles to whatever it is normal to!to!

Refraction is the change in direction of a wave Refraction is the change in direction of a wave due to a change in its speed. Most commonly due to a change in its speed. Most commonly seen when a wave passes from one to anotherseen when a wave passes from one to another

The classic practical for refraction involves The classic practical for refraction involves sending a single ray of light from air through sending a single ray of light from air through glass blocks at different angles... glass blocks at different angles...

Refraction2 animation

Incident ray

Refracted ray

Emergent ray

Angle of incidence (i)

Angle of refraction (r)

From the classic practical we reach the From the classic practical we reach the following conclusions:following conclusions:

The light ray bends The light ray bends towardstowards the normal when the normal when travelling from travelling from lessless optically dense to optically dense to moremore optically optically dense materials (e.g. dense materials (e.g. air to glassair to glass))

The light ray bends The light ray bends awayaway from the normal when from the normal when travelling from travelling from moremore optically dense to optically dense to lessless optically optically dense materials (e.g. dense materials (e.g. Glass to airGlass to air))

When parallel to the normal, refraction appears to not When parallel to the normal, refraction appears to not apply since the ray continues along the normal. apply since the ray continues along the normal. Refraction (as in change of speed) still occurs just has Refraction (as in change of speed) still occurs just has no effect on the directionno effect on the direction

Incident and emergent rays are parallelIncident and emergent rays are parallel

On exit from glass block, new incident ray (iOn exit from glass block, new incident ray (i22) is equal ) is equal to first angle of refraction rto first angle of refraction r11 Similarly for r Similarly for r22 and i and i11

i1

i2

r1

r2

If we calculate:If we calculate:

Sin iSin iSin rSin r

For a range of angles we find that For a range of angles we find that the result is a constant. We call this the result is a constant. We call this the the refractive index refractive index of the materialof the material

n = n = Sin iSin iSin rSin r

refractive index :refractive index :

measure of the bending of a ray of light when passing measure of the bending of a ray of light when passing from one medium into another. (Relative to a vacuum)from one medium into another. (Relative to a vacuum)

Snell’s LawSnell’s Law

Willibord SnellWillibord Snell

Air 1.0003Eye, Aqueous humor 1.33Eye, Cornea 1.38Eye, Lens 1.41Eye, Vitreous humor 1.34Glass, Arsenic Trisulfide 2.04Glass, Crown (common) 1.52Water (0° C) 1.33346Water (100° C) 1.31766Water (20° C) 1.33283Sugar Solution 30% 1.38Sugar Solution 80% 1.49Honey, 21% water content 1.484Diamond 2.417Alcohol, Ethyl (grain) 1.36