1. P1d(ii) Light and Lasers
P1d(ii) Light and Lasers
You will learn about:
Total Internal Reflection (TIR)
How optical fibres are used in medicine and communications

2. Critical Angle www.PhysicsGCSE.co.uk
Recall that the angle of incidence, θi is measured from the Normal.
If the angle of incidence is SMALLER than the critical angle the light will refract through the material.
If the angle of incidence is EQUAL to the critical angle the light will refract at 90° to the Normal along the boundary.
If the angle of incidence is LARGER than the critical angle the light will totally internally reflect within the material.
For TIR to occur within a fibre optic cable you can see that the
angle of incidence has to be greater than the critical angle.
Normal

3. Fibre Optics www.PhysicsGCSE.co.uk
REMEMBER:
If the light did not undergo total internal reflection (TIR) then the light signal would not reach the receiver.
Light can be trapped within many materials such as water, perspex or more commonly glass. The light can be trapped because these materials are more dense than the surrounding air, and, the light is reflected at the boundary between the material and the air. An optic fibre is a SOLID thin piece of glass. They are used because they are very flexible and relatively cheap to make.
You can see that many signals can be sent through fibre optic cables. This is very useful for communication purposes.
All of the light inside the fibre optic is reflected through it.
TOTAL…
INTERNAL…
REFLECTION

4. Endoscopy DIRECTION OF LIGHT www.PhysicsGCSE.co.uk
Endoscopes utilise the technology of fibre optics. Doctors are able to use an endoscope to see inside the body without surgery. Keyhole surgery can now also utilise the same technology.
Endoscopy
Light passes along the outer fibres and lights up inside the body at the end of the manoeuvrable tip.
Light is reflected by the body and passes down the inner fibres to the eyepiece which the Doctor is looking through.
DIRECTION OF LIGHT
REMEMBER:
Within the endoscope the light travels by TIR.
The light exits the endoscope.
The light reflects off the body.
Some of that light enters into the inner fibre of the endoscope.
This light, by TIR, travels to the eyepiece.

5.
Questions
What is the law that links the angle of incidence and the angle of reflection?
Use this image to explain how a fish is able to see another larger fish behind it.
Light travels through a glass block. It reaches the glass-air boundary with an angle of incidence of 30°. If the critical angle for this glass is 42° state what happens to the light.

6.
Questions
What is the law that links the angle of incidence and the angle of reflection? Angle of Incidence = Angle of Reflection or θi = θr
Use this image to explain how a fish is able to see another larger fish behind it. Light from the larger fish totally internally reflects at the water-air boundary and reaches the eye of the fish in front.
Light travels through a glass block. It reaches the glass-air boundary with an angle of incidence of 30°. If the critical angle for this glass is 42° state what happens to the light. The angle of incidence is smaller than the critical angle so the light will refract. It will leave the glass block and enter the air.