REFRACTION:
What causes a pencil to appear broken when you view if in water.
What causes a drinking straw to appear broken when viewed through a glass with
water?
When the drinking straws are dipped in different liquids , they appear differently
on each glass. Why?
Here is an answer to the simple questions.
The above phenomenon is called refraction.
What is refraction?
Its is the change in direction of light as it enters a material of different optical
density due to change in its speed.
Examples:
From air to glass
From water to air
From glass to air
From water to glass
The refraction of light can be demonstrated using a rectangular prism block
a).When the light hits the rectangular prism glass at an angle,it changes direction
or bends at the point of incidence
b).When light hits the glass at right angle, it passes through the glass without
change in direction, though the speed changes.
TERMS USED IN REFRACTION:
When light passed through a rectangular prism block it behaves in way shown in
the diagram below.
Be able to draw the diagram as accurately as show.
The incident ray and the emergent ray must always be parallel
1. INCIDENT RAY: The ray that is moving towards the boundary.
2. ANGLE OF INCIDENCE: Angle between the normal and the incident ray
3. Normal: An imaginary line that is perpendicular to the boundary at the
point of incidence
4. REFRACTED RAY: It is the bent ray that moving away from the boundary
5. ANGLE OF REFRACTION: It is the angle between the normal and the
refracted ray.
LAWS OF REFRACTION
1. The incident ray , the normal and the refracted ray, lie on the same plane.
2. The angle of incidence and the angle of refraction lie on the opposite sides
of the the normal at the point of incidence
3. Light travelling from a less dense material to more dense material, bends
towards the normal at the point of incidence
From air to glass, light bends towards the normal ( check point O)
4. Light travelling from a more optically dense material to a less optically
dense material, bends away from the normal. (check point B)
Eg. From glass to air.
NB
Always put arrows on your rays
The normal must always be perpendicular to the surface at the point of
incidence and it must be a broken line
DRAW BIG AND WELL LABELLED DIAGRAMS
REFRACTIVE INDEX
It is the ratio of the sine of the angle of incidence to the sine of the angle of
refraction.
𝒔𝒊𝒏 𝒊
REFRACTIVE INDEX =
𝐬𝐢𝐧 𝒓
Example
Light enters a rectangular prism block. The angle of of incidence of the light ray is
53° and the angle of refraction is 28°. Calculate the refractive index of the glass.
𝒔𝒊𝒏𝒊
Refractive index =
𝒔𝒊𝒏𝒓
𝐬𝐢𝐧 𝟓𝟑°
=
𝒔𝒊𝒏𝟐𝟖
REFRACTIVE INDEX can also be defined as:
The ratio of the speed of light in vacuum to the speed of light through a material.
𝒔𝒑𝒆𝒆𝒅 𝒐𝒇 𝒍𝒊𝒈𝒉𝒕 𝒊𝒏 𝒗𝒂𝒄𝒖𝒖𝒎
Refractive index =
𝒔𝒑𝒆𝒆𝒅 𝒐𝒇 𝒍𝒊𝒈𝒉𝒕 𝒊𝒏 𝒎𝒂𝒕𝒆𝒓𝒊𝒂𝒍
Refractive index can also be defined as:
The ratio of the real depth to the apparent depth.
𝒓𝒆𝒂𝒍 𝒅𝒆𝒑𝒕𝒉
Refractive Index =
𝒂𝒑𝒑𝒂𝒓𝒆𝒏𝒕 𝒅𝒆𝒑𝒕𝒉
For any given material , the refractive index is always constant.
Refractive index give the ability of the material to bend light.
Different materials have different refractive index.
The value of the refractive index determines how much the material will bend
light.
The larger the value of the refractive index , the more it will bend the light.
The refractive index of glass is greater than that of water. This means light bends
light more than water.
The material with a larger refractive index is said to be more optically denser.
So glass is more optically dense than water and air
REFRACTIVE INDEX OF SOME MATERIALS
MATERIAL REFRACTIVE
INDEX
Water 1.33
Crown glass 1.55
Air 1.00
Perspex 1.49
Diamond 2.42
Parrafin Oil 1.44
Write the examples on calculation of refractive index from your books.
Answer the questions on calculation of refractive from your books.
