Refraction Day 2- Speed of Light

- Index of Refraction- Calculating the Index of Refraction- Calculating the Speed of Light- Total Internal Reflection

Speed of Light

The speed of light is different in various media, but it is always less than that of the speed of light in a vacuum.

Recall: Wagon Example

A wagon changes direction when travelling at an angle from pavement onto sand because one front wheel slows down while the other wheels continue moving at a higher speed

Index of Refraction

The index of refraction for a medium is defined as the ratio of the speed of light in a vacuum to the speed of light in that medium.

n = c (speed of light in a vacuum)

v (speed of light in a medium)

n has no unit (as the unit of m/s for both c and v cancel each other out)

Calculate the Index of Refraction The speed of light in water is 2.25 x 108m/s.

Calculate the index of refraction for water.

n = c (speed of light in a vacuum) v (speed of light in a medium)

n = 3.00 x 108m/s 2.25 x 108m/s

n = 1.33

Therefore the index of Refraction for water is 1.33.**Always include a therefore statement for full

marks**

Calculate the Speed of Light Calculate the speed of light in diamond.

n = c rearrange so v = c v n (index of refraction

look in in a table = 2.42)

v = 3.00 x 108m/s 2.42

= 1.24 x 1o8m/s

Therefore the speed of light in diamond is 1.24 x 1o8m/s

Total Internal Reflection

Light bends AWAY from the normal when it speeds up at the boundary of two media i.e moves from acrylic (plastic) to air

As the angle of incidence increases so does the angle of refraction. When the angle of refraction becomes

90° it no longer leaves that first medium.

Total Internal Reflection

Total Internal Reflection

Critical Angle: The angle of incidence that results in an

angle of refraction of 90°

Total Internal Reflection

If the angle of incidence is increased past this “critical angle” then the ray will become internally reflected.

Total Internal Reflection

When this occurs the refracted ray disappears and only a reflected ray is visible. “total internal reflection”

Total Internal Reflection

Occurs When These Conditions Are Met: 1. Light is travelling more slowly in the

first medium than the second. WHY? 2. The angle of incidence is large enough

that no refraction occurs in the second medium. Instead, the ray is reflected back into the first medium.

Total Internal Reflection Demonstration

Total Internal Reflection: Diamonds

Diamonds have a high index of refraction, this combined with the way the diamond is cut with many faces creates that desired “sparkly” look.

Once a light ray enters the diamond it tends to bounce around several times before exiting through the top face (largest) of the diamond.

Total Internal Reflection: Diamonds

A high refractive index means that the critical angle is pretty small, in the case of diamonds 24.4°. WHY? Thus any ray coming in at an angle

greater than 24.4° will be internally reflected.

Total Internal Reflection: Diamonds

The cut of the diamond is very important to how the light is able to move and reflect. (The more times it reflects internally the sparklier it is)

Total Internal Reflection: Fibre Optics

This technology uses light to transfer information through a glass cable.

Again the critical angle must be small so that light entering the cable will have an angle of incidence greater than this and internally reflect i.e. High-Purity Glass, Lucite

(plastic)

Total Internal Reflection: Fibre Optics

Uses of fibre optics; Communications (phones,

computers, TVs) Instrument panels of cars Medicine (i.e. Endoscope)

▪ 2 bundles of fibre optics, 1 lights the body while the other carries the reflected light back so the image can be seen on a screen.

Fiber Optics Video

Total Internal Reflection: The Triangular Prism

A glass triangle has a critical angle of 41.1° any incident ray greater than this is internally reflected.

The ray that emerges from the triangle can be either 90° or 180° from the incident ray depending on the angle of incidence used.

Total Internal Reflection:Retro-Reflectors

A retro-reflector is an optical device in which the emergent ray is parallel to the incident ray. i.e. Road signs and bike reflectors are

retro-reflectors so that they are very visible at night (they use the light from vehicle headlights)