Refraction of light:
Have you observed when light is incident on water surface its direction changes or not? Dose it changes its direction of propagation when goes from one transparent medium to another transparent medium? Now we discuss in detail by taking example in our day to day life.
You must observed that, when sunlight enters the water its direction changes in water. Also, when you put the glass balls in a glass filled with water, you will observed that the size of the glass balls seems to be increased. Also, when you put the coin inside the glass filled with water, the coin appears to be large in size. Why this kind of things happening? The reason behind this is only the refraction of light.
When light travels from one transparent medium to another transparent medium its direction changes in second medium which is called as refraction of light. The direction of changing the ray depends on the refractive index of the medium & speed of light in that medium. Thus, all this examples explains that light does not travel in straight line in all the media. Its direction of travelling depends on the refractive index of the media.
Activity:
- Take a pencil & deep into the glass filled with water.
- Observe the size of pencil in air medium and water medium.
- What you seen at the air-water interface?
- Perform the same activity for glass filled with kerosene and oil, note down the difference.
Activity:
- Take a printed paper & place a glass slab on it.
- Observe the size of the words on paper through glass slab.
- Move the glass slab slowly what will happen?
Refraction through a rectangular glass slab:
Take a glass slab & place it on the white paper sheet of drawing board and draw its outlines on paper.
Now remove the slab & note the drawn rectangle as PQRS.
Now , fix the two drawing pins above the side PQ as A & B such that the line joining them will be vertically incident on the side PQ as shown in fig.
Now, place the glass slab on the pattern PQRS drawn and from the RS side observe the image of drawing pins A & B and in that direction fix the drawing pins C & D as shown in fig.10.6.
Later, remove the slab & join the line AB & extend it to side PQ where it meets at point O.
Similarly, join the line CD & extend it to side RS where it meets at point O’ as shown in fig.10.6.
Now join the OO’ & draw normal MM’ at O & NN’ at O’ respectively. Also extend the line ABO in straight forward up to S.
After above all construction, we observe that the ray of light AB entering at air-glass interface at O changes its direction as OO’ in glass medium. This OO’ ray again changes its direction at glass-air interface as O’CD in air medium.
It is observed that the incident ray and emergent ray are parallel to each other. This is due to the extent of bending of ray is same at air-glass interface & glass-air interface.
It is observed that, when incident ray travels from air to glass medium, the refracted ray bends towards the normal.
And when light ray travels from glass to air medium, it bends away from the normal.
This is due to the speed of light is different in different media.
From above experiment, the laws of refraction concluded are as follows:
i) The incident ray, refracted ray and normal to the interface separating two media all lie in the same plane.
ii) The ratio of sine of angle of incidence to the sine of angle of refraction is constant and this constant is called as refractive index of the second medium with respect to first medium.
This law is also called as Snell’s law of refraction& it is valid for 0<i<90.
Then, sini/sinr = constant
i-Angle of incidence
r- Angle of refraction
sini/sinr= Refractive index of second medium with respect to first
iii) Incident ray, transmitted ray and normal lies in same plane.
The Refractive Index:
We have already seen that, the ray of light travelling from one transparent medium to second transparent medium, changes its path in second medium. The amount of change in path takes place during refraction for a given pair of media is expressed in terms of refractive index which is constant for a given medium.
The refractive index of the medium depends on the speed of light in that medium. The speed of light in vacuum is largest and which is 3* 108 m/s. In air it decreases slowly and more in water and glass.
Refractive index of the medium
Let us consider that, the ray of light PO is incident on the air-glass interface & it refracted as OQ. Let V1 and V2 be the speed of light in air & glass medium respectively.
Then the refractive index of second medium with respect to first is the ratio of speed of light in medium 1 to the speed of light in medium 2.
It is given by, μ21 = speed of light in medium 1/ speed of light in medium 2
= V1/V2
Similarly, the refractive index of medium 1 with respect to medium 2 is given by,
Μ12 = speed of light in medium 2/ speed of light in medium 1 = V2/V1
Absolute Refractive Index:
If the medium 1 is air or vacuum then refractive index of medium 2 with respect to air or vacuum is called as absolute refractive index of the medium and it is denoted by
μm = speed of light in air/ speed of light in medium = c/v
Where c- speed of light in air or vacuum
v- Velocity of light in medium
Material Medium |
Refractive Index |
Material Medium |
Refractive Index |
Air | 1.0003 | Crown glass | 1.52 |
Ice | 1.31 | Dense flint glass | 1.65 |
Water | 1.33 | Ruby | 1.71 |
Alcohol | 1.36 | Diamond | 2.42 |
Absolute refractive index of different media
Do you know?
Optically rarer and optically denser medium:
- A medium in which the speed of light is more is called as optically rarer medium. Air is rarer medium as compared to water and glass.
- A medium in which speed of light is less is called as optically denser medium. Glass is optically denser medium as compared to air and water.
- When a ray of light travels from rarer medium to denser medium, it bends towards the normal at the point of emergence.
- When a ray of light travels from denser medium to rarer medium, it bends away from the normal at the point of emergence.
- The refractive index is the ratio of two similar quantities (sine of angles), it has no units. It is a pure no.
Questions:
- Is there a phenomenon of refraction in forming rainbow? How?
- Why the speed of light is different for kerosene, oil, water. Give valid reason.
- Refractive index of the medium depends on which properties & how?
- Why the diamond is the hardest one?
- Light enters from air to ice having refractive index 1.31. What is the speed of light in ice, if speed of light in vacuum is 3*108m/s?
- What are the optical properties of the medium? Give some properties.