1. Quiz 2: Sunday 11th October
( Refraction & Diffraction)
Test 1: Sunday 18th October
(Chapter 13 & 14)-All

2. REFRACTION
Is the bending(change of direction) of light when it passes from one material into another.
The speed of light decreases when light enters a dense medium.
Refracted ray bends towards the normal.

3. The pencil appears broken when in water.
The rays reaching our eyes bend at the surface.
The beam bends when it enters glass
The fish appears near the surface of water that it real is. Rays from the fish refract at the surface.

4. Coin appears to be near the surface of water
Coin in Water
Coin appears to be near the surface of water
Apparent depth
Real depth
Glass of water

5. Glass to Air Air to glass
(Less dense to dense medium)
Glass to Air
(Dense to less dense)
Speed of light decreases and refracted ray
Speed of light increase and refracted ray
Bends Towards the Normal
Bends Away from the Normal
A normal is a line drawn 90o to the surface at the point where the light enters the medium.

6. Refraction by glass block
Exit ray
Ray Box
Glass Block
Incident light

7. Complete the paragraph below
Refraction is caused by the change of ____________of light when it moves from one material to another.
When light moves from less dense to dense material the speed of light____________and the refracted ray bends_______________the normal.
Also, when light moves from dense to less dense material the speed of light____________. The refracted ray bends______________ the normal.
The ratio of speed of light in a material to the speed of light in a vacuum is called__________________ of the material.

8. Boundary Medium 1 Medium 2
Refracted ray
Medium 1
Medium 2
Incident ray
Which medium is denser. Explain your answer.

9. TEACHING POINT Plot a graph of sin i against sin r.
Define refractive index of a material.

10. The gradient of the graph gives the refractive index(n)
A graph of sini against sinr is a straight line through the origin.
sini
sinr
The gradient of the graph gives the refractive index(n)

11. Draw graph of sin i (y- axis) against sin r (x-axis)
Complete the table for sin i and sin r.
i/ o
r / o 30 20
35o
23o
40o
26o
45o
29o
50o
32o
55o
34o
60o
36o
sin i
sin r
0.50
0.34
0.57
0.39
0.64
0.44
0.70
0.49
0.76
0.53
0.81
0.56
0.86
0.59
Draw graph of sin i (y- axis) against sin r (x-axis)

12. Draw graph of sin i (y-axis) against sin r (x- axis)
(0.68, 1.0)
Slope = n
(0.14, 0.2)
Refractive index (n) =
= 1.48

13. When the incident angle is 900, no refraction occurs
When the incident angle is 900, no refraction occurs. The rays of light goes straight unchanged

14. Class work: 1. Write down a formula for refractive index of a material given: (i) relative speeds (ii) Relative angles (iii) Page 187# Explain what is meant by (i) Monochromatic light (ii) Critical angle (iii) Total internal reflection (iv) State two conditions for total internal reflection to occur. (V) Page 190# State one use of total internal reflection 4. Draw a diagram of an optical fibre to show how a signal is transmitted

15. REFRACTIVE INDEX(n)
Refractive index is the ratio of speed of light in a vacuum to the speed of light in a material.
The speed of light in glass 2x108 m/s. Calculate the refractive index of glass if the speed of light in air is 3x108 m/s
𝑛= 3𝑥 𝑥 10 8
n=1.5 (No units)

16. Exercise 1. Speed of light in 3 x 108 m/s
Exercise 1.
Speed of light in 3 x 108 m/s
The speed of light a material is 2.5 x 108 m/s. Find the refractive index of materials.
The refractive index of crown glass is 1.4. Find the speed of light in crown glass.
Find the speed of light in a water of refractive index 4/3.

17. Angle of incidence
Snell’s Law
Normal i
Incident ray
The ratio of the sine of angle of incidence to the sine of angle of refraction is a constant called refractive index.
Air
Glass r
Refracted ray
Angle of refraction

18. Glass to air Air to glass

19. 300 find refractive index 𝑛= 0.500 0.342 = 1.46 20o Air Glass
Normal
300
Incident ray
Air
Glass
Refracted ray
𝑛= sin 30 sin 20 =
𝑛= = 1.46
20o

20. Find r. 600 n= 1.5 r Angles are always given to 1 decimal place Air
Normal
600
Incident ray
Air
Glass
n= 1.5
1.5= 𝑠𝑖𝑛60 𝑠𝑖𝑛𝑟
Refracted ray r
sin 𝑟 = 𝑠𝑖𝑛 = 0.577
r = sin-1(0.577) = 35.20
Angles are always given to 1 decimal place

21. CRITICAL ANGLE(C): The critical of glass is 420.
The angle of incidence which gives angle of refraction is 900 when light is travelling from dense to less dense medium
Or: Minimum angle at which total internal reflection occurs
Watch video
Click Link
The critical of glass is 420.

22. MONOCHROMATIC LIGHT
Light of a single wavelength
Example : Laser light

23. TOTAL INTERNAL REFLECTION(T.I.R)
Complete reflection of light when angle of incidence is greater than critical angle when light is traveling from dense to less dense.
This occurs when:
Light is travelling from dense to less dense medium.
The angle of incidence is greater than critical angle.

24. Total internal reflection
Watch Video

25. Calculating Critical Angle
It is possible to calculate the refractive index using the critical angle
r= 90°
Air
Glass c
r = 900
i = C n

26. Calculate the critical angle of a glass of refractive index of 1.5
Calculate the critical angle of a water of refractive index of 1.3.
2Calculate the refractive index of a ,material whose critical angle is 470.

27. Light from air to glass at 90o. No Refraction A & D
Critical angle of glass in prism 42o. Draw the path taken by light shown below.
Light from air to glass at 90o. No Refraction
A & D B
450 A
The angle of incidence of 450 greater than critical angle of 420 Total internal reflection occurs.
B & C
450 C D

28. Critical angle of glass in prism 42o
Light from air to glass at 90o
No Refraction
Light attempting to go from glass to air but angle of incidence greater than critical angle.
450
Total Internal Reflection
Light from glass to air at 90o
No Refraction

29. 1. Reflective road signs. Applications of Total Internal Reflection
2. Optical Fibres used in
-communication
-Endoscopes
-Periscopes
Advantages of optical fibres over copper conductors
Less interference.
cheaper raw material
occupy less space / more information (carried) in the same space
flexible for inaccessible places/
do not corrode

30. Optic Fibre Glass cladding of low refractive index
Glass core of high refractive index

31. Total internal; Reflection in Optic fibre
Glass of low refractive index N N
Normal

32. Visible Spectrum
White light is made up of the seven colours of the rainbow.
Red, orange, yellow, green, blue, indigo, and violet.

33. Dispersion The splitting of white light into seven colours by a prism
Least refracted
Most refracted

34. RAINBOWS
We see rainbows because the light is first refracted by raindrops and then reflected off the back of the raindrop.

36. RESOURCES Click on the link below
Sample questions on Refraction of waves.
Sample questions on Diffraction
Sample question on refraction of light