Total Internal reflection Ifa ray of light is passing from a medium of higher refractive index to a medium of lower refractive index under certain conditions all of the light can be reflected internally. This effect is traditionally investigated using a semicircular glass block.

If a light ray is incident on the semi-circular glass block along its radius there is no deviation of the ray because the light strikes normal to the glass THE SEMICIRCULAR GLASS BLOCK

θ1θ1 If the angle of incidence on the inner face of the block is less than a critical value. Refraction occurs as expected. As the the angle of incidence (θ 1 ) is increased the refracted ray moves further from the normal until the angle of refraction (θ 2 ) = 90 0 θ1θ1 θ is the critical angle. For the glass air boundary the value of the critical angle is around θ2θ2

When the angle θ 1 is greater than the critical angle all of the light is reflected. The light now obeys the laws of reflection rather than those of refraction θ1θ1

Deriving the value of the critical angle at the glass air boundary Using The critical angle θ c occurs when the angle of refraction is θ1θ n 2 = 1 and sin θ 2 =1 For glass with n= 1.5

Flash Investigate refraction and TIR. Click anywhere on the image. If your computer does not load the simulation here, it can be found at

denser mediumless dense medium θ n1n1 n2n2 More generally Where n 1 is the refractive index of the more dense material

Optical Fibre Optical fibres are replacing electrical cables for the transmission of information. Some Advantages: They can carry much more data Light signals are switched rapidly using pulsed lasers to encode information rather than fluctuating electric currents. Unlike currents the signals do not interfere with each other Less data loss occurs They are immune from e.m. interference

Optical Fibre A simple (uncladded) optical fibre consists of a fibre of glass of uniform refractive index. Light which strikes the fibre walls at an angle greater than the critical angle is totally internally reflected in this way light can be guided through the fibre by multiple reflections. The glass is extremely pure and has a refractive index of about Calculate the critical angle.

Optical Fibre With a simple fibre like this the critical angle is fairly small Angle greater than about 42 0 This means that quite a large range of possible angles of reflection can be involved in the transmission of a pulse of light.

Optical Fibre A way to improve on the performance of the fibre is to use a cladded fibre, the cladding being of a less dense material with a slightly lower refractive index than the core fibre. This is referred to as a STEP-INDEX fibre

Calculate the critical angle if the core fibre has a refractive index of 1.45 and the surrounding fibre has a refractive index of 1.38

Some Advantages 1.The inner fibre is protected. 2.When the fibres are bundled adjacent central fibres cannot touch to leak signal across the bundle 3.Only those rays close to the central axis of the fibre are transmitted (fewer spurious signals) 4.A reduction in the range of possible angles for TIR mean that the light rays in a pulse travel about the same distance and arrive at the same time. This is known as a reduction in multipath dispersal 5.The number of reflections is reduced. This leads to a)Less data loss b)The time for transmission is shorter.