Noadswood Science, 2013

Optics To be able to calculate the refractive index using the critical

angle of a substance

Wednesday, April 19, 2023

Optical Fibres Optical fibres can carry information coded in light or infrared

signals – they can carry more information than an ordinary cable of the same diameter

Information such as computer data and telephone calls can be converted into electrical signals – this information can also be converted into either visible light signals or infrared signals, and transmitted by optical fibres

Optical fibres can carry more information than an ordinary cable of the same thickness – the signals in optical fibres do not weaken as much over long distances as the signals in ordinary cables

Optical Fibres An optical fibre is a thin rod of high-quality glass (very little

light is absorbed by the glass)

Light getting in at one end undergoes repeated total internal reflection, even when the fibre is bent, and emerges at the other end

Optical Fibres Find the critical angle for the glass block for total internal

reflection to occur where the light getting in at one end undergoes repeated total internal reflection, even when the fibre is bent, emerging at the other end...

Set up a ray box so a light ray goes through the glass block – rotate the glass block until the

light ray comes out from the side

Optical Fibres Light getting in at one end undergoes repeated total internal

reflection, even when the fibre is bent, and emerges at the other end

The critical angle for glass is about 42°

Diamond’s have a critical angle of 24° – this is why they sparkle so much, due to many internal reflections

Total Internal Reflection Total internal reflection occurs when light is coming out of

something dense, such as glass, water or perspex

If the angle is shallow enough the ray will not come out at all, but it reflects back into the material – this is total internal reflection

Total Internal Reflection Angle of incidence is less than critical angle

Most of the light passes through the air, but a little bit of it is internally reflected

Total Internal Reflection Angle of incidence = critical angle

The emerging ray comes out along the surface, with quite a lot of internal reflection

Total Internal Reflection Angle of incidence is greater than the critical angle

No light comes out as it is all internally reflected (total internal reflection)

Optical Fibres - Uses Optical communications have several advantages over

electrical signals in wires: - The signal does not need boosting as often A cable of the same diameter can carry much more

information The signals cannot be tapped into, or suffer interference

from electrical sources

Normally no light would be lost at each reflection, however some light is lost due to imperfections in the surface, so boosters are needed every few km

Critical Angle The value of the critical angle (c) depends on the refractive

index of the material

A dense material with a high refractive index has a low critical angle

This is why diamonds sparkle so much (much more light is internally reflected as it has a small critical angle)

Refractive index = 1 ÷ sin c

Endoscopes Endoscopes are used to look inside people – they are a

narrow bunch of optical fibres with a lens system at each end (with another set of optical fibres to carry light down inside)

The image is displayed on a monitor, meaning operations can be undertaken without the need to cut large holes in the patient

Optics Complete the optics worksheet

Questions1. It is totally internally reflected

2.

Questions3. The first bundle is used to shine light into the stomach. The

second is used to see the inside of the stomach; a tiny lens over the bundle forms an image on the ends of the fibres, and the image can then be seen directly.

4. Laser light can be used to cut or burn away and destroy diseased tissue. It can also seal off (cauterise) leaking blood vessels