What causes waves to refract when they pass across a boundary?
What happens to the speed of light waves when they enter a material such as water?
In which direction do light waves bend when they travel out of glass and into air?
Why does a glass prism split sunlight into the colours of a spectrum?
What do we mean by rays?
What is Snell’s law?
How is refractive index related to the speed of light waves?
What are the conditions for total internal reflection?
How is the critical angle related to refractive index?
Why do diamonds sparkle?
You will be able to answer these questions
Refraction is the changing direction of light when it goes into a material of different density
Set up a pyramid prism to see how different frequencies of light bend by different amounts while going through the prism
The Normal line
The normal line is 90 degrees from the plane surface.
Right angle to the mirror
Normal on a curved surface
Which way does the light travel?
When going in to the block?
Towards the normal
When coming out of the block
Away from the normal?
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 exactly Air: n = 1.0003, often approximated by 1 Water: n = 1.34, average over visible range Glass: n = 1.5 is typically used in optics homework problems Fused silica (pure SiO2 glass): n = 1.4 Common refractive indexes
The speed of light in a certain glass is 1.8 10 8 m s -1 . What is the refractive index of the glass?
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 50 o
n = C/V C = 3 x 10 8 in air n (air) = 1
n g = c/v g = 3.0 10 8 / 1.8 10 8 = 1.67 (no unit as refractive index is a number)
n d = c /v d so v d = c/n d = 3 10 8 m s -1 / 2.4 = 1.25 10 8 m s -1
1 sin θ 1 = n 2 sin θ 2
3. For blue light, sin θ 1 = sin 50° / 1.639 = 0.4674 and θ = 27.86 o
For red light, sin θ 1 = sin 50° / 1.621 = 0.4726 and θ = 27.20 o
The angle between the two rays is the difference so = 0.86 o .
[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 n 1 =1 n 2 =0.4674
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
Total Internal Reflection Normal line This has to be equal or greater than 90 degrees Sin 90 = 1 Sin θ c = n 2 /n 1 Θ c is the critical angle n 1 = refractive index of the glass (1.5) n 2 = refractive index of the air (1.001) Sin-1 (1.001 / 1.5) Critical angle = 41.8 n 1 Sin θ 1 = n 2
n = 2.41 Diamonds sparkle Calculate the critical angle for a diamond Sin θ c = n 2 /n 1
Refraction Questions 1. Explain why substances with a high refractive index like diamond, sparkle. 2. A pulse of white light is sent straight down a fibre optic cable 1 km long. The refractive index for blue light is 1.639 and for red light 1.621. What time interval will there be between the two components when t hey reach the far end? 3. Calculate the critical angle of an optical fibre: a) without cladding if the glass has a refractive index of 1.56. b) when cladding is added of n = 1.49 c) what advantage is this? n = C/V Sin θ c = n 2 /n 1 C = 3 x 10 8 m/s n air = 1.01 Speed = Distance/ time
1) As any angle larger than critical is totally internally reflected, the smaller the critical angle the easier it is to get internal reflections which cause the sparkle. The speed of the blue light, V b = c/1.639 = 1.830 × 10 8 m s -1 The speed of the red, V r = c/1.621 = 1.851 × 10 8 m s -1 time = distance / speed time taken by the blue light = 1.0 × 10 3 / 1.830 × 10 8 = 5.46 μs time taken by the red = 1.0 × 10 3 / 1.851 × 10 8 = 5.40 μs the time lag is 0.06 μs. 3. a) (just glass) sin θ c = 1/n glass = 1/1.56 = 0.641 so C = 39.9 o glass and cladding sin θ c = 1.49/1.56 and C = 72.8 o .
Total Internal Reflection Normal line This has to be equal or greater than 90 degrees Sin 90 = 1 Sin θ c = n 2 /n 1
To do at home Read pages 188- 194 Answer question on page 208 Q1, Q2 (sketch the diagram)