Refraction and total internal reflection

Lesson 2

The Endoscope

There are two optical fibres

One for light, to illuminate the inside of the patient

One for a camera to send the images back to the doctor. Key hole surgery

Step into the photo

You are the patient in the photos

Describe your operation in medieval timesDescribe you operation in modern times with keyhole surgery

Surgeons at Worthing and Southlands pioneer ‘keyhole’ operating techniques for the treatment of

stomach cancers. Medieval surgeons open up a patient

To see what is wrong.

Fibre optics

n = 2.41

Diamonds sparkle

Calculate the critical angle for a diamond

Sinθc = n2/n1

θc = 24.5

Snell’s Law

• The refractive index (n) and v explained on next slide

What is the refractive index

• The refractive index (n)of a medium is the speed of light in vacuum (C) divided by the speed of light in the medium (V)

• It is the "optical density" of the medium. It is always greater than 1

n = C/V

Vacuum: n = 1 exactlyAir: n = 1.0003, often approximated by 1

Water: n = 1.34, average over visible rangeGlass: n = 1.5 is typically used in optics homework problems

Fused silica (pure SiO2 glass): n = 1.4

Vacuum: n = 1 exactlyAir: n = 1.0003, often approximated by 1

Water: n = 1.34, average over visible rangeGlass: n = 1.5 is typically used in optics homework problems

Fused silica (pure SiO2 glass): n = 1.4

Common refractive indexes

Quick Refraction

The speed of light in a certain glass is 1.8 10 8 m s-1 . What is the refractive index of the glass? 

2. The refractive index of diamond is 2.4. What is the speed of light in diamond?

3. The refractive index changes with the colour of the light leading to dispersion. If the refractive index for blue light in a certain glass is 1.639 and for red light is 1.621, calculate the angle between the rays if they were both incident at 50o

n = C/VC = 3 x 108 in air n (air) = 1

Correct Answers

ng = c/vg = 3.0 108 / 1.8 108 = 1.67 (no unit as refractive index is a number)

 

2. nd = c/vd so vd = c/nd = 3 108 m s-1 / 2.4 = 1.25 10 8 m s-1

  1 sin θ1 = n2 sin θ2

3. For blue light, sin θ1 = sin 50° / 1.639 = 0.4674 and θ = 27.86o

For red light, sin θ1 = sin 50° / 1.621 = 0.4726 and θ = 28.20o

The angle between the two rays is the difference so = 0.34o.

[NB with a prism red is deviated least (“red tries to go straight ahead” pronounce ‘ahead’ as “a red”), useful to learn. The diffraction grating spreads out the colours with blue deviated most.)

n = C/V

n1 =1 n2 =0.4674

Which way will the light refract?The rules

1)Draw the normal line going through the prism (90 degrees from the surface)

2) If light is going into something more dense it will bend towards the normal

3)Draw a normal line for the light coming out of the prism

4)Light will bend away from the normal if going into a less dense material

n = 1.5n =

1.4n =1.6

n = 1.5n =

1.4n =1.6

n = 1.5n

=1.4n =1.6

n = 1.7

n = 1.7

n =1.7

Wave fact revision!

• Frequency f is determined by the source of the waves (the “transmitter”)

• Wave speed v is determined by the medium in or on which the wave propagates.

• So wavelength follows from v = fλ

• At boundaries (reflection, refraction etc), f must stay the same.

On a rope

Superposition

• When two waves arrive at the same point and at the same time, the resultant displacement is given by the algebraic sum of the two individual displacements. (‘Algebraic sum’ means that you have to take account of positive and negative values.)

+

+

=

=

Reinforcement

Cancellation

Superposition of waves worksheet

Superposition in water

Create your own wave superposition in your tub.

Take a photograph and print off a picture

Show on the picture where the waves have a

supercrest, where they have a

supertrough Where a crest meets a trough and the resultant

displacement is zero

Paste pictures in here (at least 4 per page)