# TIR

continued from Snells Laws

So at this stage you should have noticed a few things about refraction

When a ray goes from a less dense medium to a dense medium the ray gets refracted _______ the normal

When a ray goes from a dense medium to a less dense medium the ray gets refracted ________ from the normal

Using these 2 pearls of wisdom, copy the diagram below and sketch out where you think each ray will approximately go ?

Did you have any difficulty ?

Do see any problems or special phenomena ??

what will happen if the rays leave the water at ever increasing angles of incidence ?

Is there a maximum angle through which the ray can be refracted ?

2. Total internal reflection

In answer to the last question, yes there is .... the angle of refraction maximum value is 90o, after this the ray does not leave the medium by definition.

Total internal reflection happens when light tries to leave a transparent material into a less dense medium but its angle of incidence is greater than the critical angle for that media interface. The light get reflected back into its original medium

Look at the picture opposite, I cant remember what type of creature it is (ask a biologist), but you can see its reflection in the surface of the water, almost better than you can see the actual creature.

Find a semicircular transparent object and shine a light box through it

trace out the rays that leave the block

Why is a semicircular block important for this experiment ?

look for semi-circle glass block

ask!!

The ray gets reflected back into the medium when going from a denser medium into a less dense one !

Critical angle.

so we can rewrite the snells law formula

for the maximum angle i= 90o

Sin 90o = 1

so we could equally write

Relationship between critical angle and refractive index.

If we need to find the critical angle or refractive index knowing one will lead to us knowing the other

Note this is ideally for when going into a vacuum

check out this for refraction and TIR

http://vnatsci.ltu.edu/s_schneider/physlets/main/refraction.shtml

As an exercise

Page ___ Exercise 4d .... 1 - 4

Optical Fibers

The 2009 Nobel prize in physics has been awarded to some of those whose work with light laid the foundations of the modern digital age.

The first half of the prize went to Charles Kao, who in the 1960s made it possible for the world to talk via the light inside optical fibres.

An optical fibre is a glass or plastic fiber that carries light along its length. They are as thin as a human hair and carry digital information over long distances. Light is kept in the core of the optical fiber by total internal reflection.

The second half was awarded to Willard Boyle and George Smith for the invention of the charge-coupled device (CCD) image sensor chip – a crucial component in today’s digital cameras. Read more here or here.

Advantages of Optical Fibers

1. No Electromagnetic interference
2. Data Security (because No Electromagnetic interference)
3. Nonconductive cables
4. Large Bandwidth (up to 400 MHz/km)
5. Long distance transmission (less need for repeaters) requires less energy
6. Smaller than copper wires

Transmission of light through optical fibres.

When light is travelling through a medium that is denser than the material outside it is possible to get the light to travel only within the medium. This is achieved by making the light glance off the surface at an angle from the normal that is greater than the critical angle (or less than the glancing angle).

Total Internal Reflection will occur as long as the incident angle of the light is greater than the Critical angle.

Optical fibre are mainly used to transmit information over long distances and with high bit rates.

Their benefits are numerous: the signal transmitted on the fiber is not disturbed by any electromagnetic wave created by power cables or electric machines. It also provides more security, as these cables can be fully dielectric. Besides, they provide a weight and space saving due to their small diameter, only 250 μm.

An optical fiber is made up of three main parts: the core, the cladding and the coating. In the center, the “core” is made of doped silica and is surrounded by the “cladding”, made of natural silica.

The light signal propagates along the core and the signal is reflected on the surface between the core and the cladding.

The purpose of the cladding might seem pointless as it cannot have a refractive index less than that of air (or a vaccum) and so by putting a cladding on the refractive index difference between the materials is less. This will make the critical angle bigger and make the light strike the glass at a more glancing angle than is needed. The point is if you do not have cladding then if 2 optic fibres were in contact at a point, light could leave the optic fibre, resulting in scrambled communications at the very least.

An acrylic “coating”, generally made of two layers, is protecting the sillica part against abrasion during installation.

Demonstration.

might be able to reconstruct in a piece of glass or perspex rod, heated and twisted ?

Uses of optical fibres:

• telecommunications

• medicine (endoscopes).

Appropriate calculations.

1. What is the maximum angle allowed between for TIR to take place
2. How could you maximise this angle
3. if you had a piece of _______ of gauge ______ what would be the tightest circle you could wrap of this optic fiber

Reflective road signs.

look at this road sign in Kilmessen, Co Meath. It stand out, it is clear to see the message of this sign.

This is with the flash on, the light is reflected back out to make the contrast stark,

this is achieved by a cross section of honey comb that reflects the light by total internal reflection,

Mirages.

occur on flat surfaces a distance in front of us. It is caused by the air being of different temperatures on a profile down from the outer limits of the athmosphere down to the surface of the earth. These different temperatures coupled with the differing densities of air leads to slightly different refractive indices for light coming down from the sky at an incidental angle

Prism reflectors.

Binoculars

At the front of each telescope is a lens, called the objective. Its role is to gather light from whatever it is you're looking at and bring it to a focus in the eyepiece, where the light is formed into a visible image and magnified to take up a large portion of the retina. The magnification depends on the focal length of the eyepiece, and for binoculars it is usually between 5x and 10x.

The image produced by this telescope will be upside down and backwards, but for astronomical viewing this is not a major inconvenient. In space there is no up and down or left and right. However, for watching birds or following the action at a baseball game a right- side-up picture is essential. This is why binoculars use corrective elements between the objective and the eyepiece, called prisms.

Prisms used in binoculars are blocks of glass that function as mirrors, but without a mirror's reflective backing. They come in two models and use different types of glass, and we willdiscuss about this later in the article. For now let's just mention their role, that is to bring the light beams from the objective closer together by means of internal reflection, and also turn the image right-side up and orient the view properly left to right.