1. REFRACTION AND INTERNAL REFLECTION

6. REFRACTION
- Bending of light rays when they pass obliquely through the boundary of two media of different optical density.

7. OPTICAL DENSITY Measure of the speed of light through the medium
The greater the optical density, the slower is the speed of light in the medium.

8. REFRACTION OF SOUND WAVES

9. The effect of refraction during sunrise and sunset

14. In the diagram above, if there was no atmosphere then when the Sun is at position A, it would already be below the horizon of the observer.
However, refraction causes the Sun's rays to bend down towards the observer so that it appears to be at point B. This delays sunset by some minutes. Refraction also causes the lower limb of the Sun to be raised more than the upper limb giving it a squashed appearance.

15. Refraction caused by the presence of water droplets in the air.

19. REFRACTION AND INTERNAL REFLECTION IN CRYSTALS

20. FAST – Bends AWAY from Normal

21. SLOW – Bends TOWARDS Normal

23. INDEX OF REFRACTION (n)
n = velocity of light in air = sin i
velocity of light in medium sin r
where i angle of incidence and r is angle of reflection

24. LAW OF REFRACTION (SNELL’S LAW)
ni sin θi = nr sin θr
The incident light (θi ) is refracted or bent at an angle θr as it passes from the air to the water. A small portion of the incident light is also reflected at the surface.
Air and water are two different media: a wave will travel faster in air then in water. Scientists have defined a variable which is characteristic of each medium named the index of refraction and usually denoted n.

25. Refraction: from a less dense to a more dense material
This is the case of light passing from air to water.
Case 1: θi = 0°
then θr = 0°: a beam falling perpendicularly to the interface is not refracted as shown by the blue arrow.
Case 2: implication of n0 < n1
sin θr < sin θi , hence θr < θi : the beam when passing through the interface gets closer to the normal as shown by the yellow arrow.
Case 3: = 90°
sin θr is maximum, hence θr has a critical value for which the refraction is maximum as shown by the red arrow.