When light strikes a transparent surface, it undergoes partial reflection, refraction, and absorption. Refraction, the bending of light due to a change in speed as it crosses media boundaries, is governed by two fundamental laws, the second of which is Snell's Law.
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What happens when light is incident on the
surface of a transparent object?
Consider a glass slab and suppose the narrow
beam of light is incident on one side.
The beam of light ‘partially reflects’,
‘partially refracts’ and it's even
‘partially absorbed’ by the object.
In the past, we have already discussed laws of reflection.
Do you remember those laws?
The first one says that the light ray which
is incident is reflected in such a way
that the angle of incidence and the angle of reflection are equal.
And the second one says that the incident ray,
the reflected ray and the normal
all lie in the same plane.
Just like for reflection, there are some laws
for refraction.
And what is refraction?
Refraction is the change in direction of light
as it passes across the boundary separating
two media caused by its change in speed!
If the refractive medium is optically denser
compared to incident medium, then the refracted
ray will bend towards the normal.
And if it is optically rarer, it will bend away from the normal.
Note that the light is incident obliquely on the surface!
When the incident light is perpendicular to
the surface, it does not change its direction!
There are two laws of REFRACTION.
Among them, the first one is same as the
second law of REFLECTION!
Consider light travelling from some medium
1 to another medium 2, which is optically
denser than medium 1.
So its speed in medium 2 decreases and
it bends towards the normal.
The first law of refraction says that,
the incident ray, refracted ray and the normal
at the point of incidence all lie in the same plane.
It means that if I were to draw this on a
piece of paper, they all will be on the
plane of paper and not out of the paper
What about the second law?
The second law is also called
Snell’s law of refraction.
Snell’s law gives us the
relation between the incident and refraction angles
and the refractive indices of the mediums.
What is the refractive index of a medium?
It is the extent to which the refractive medium
increases or decreases the speed of light.
Mathematically, it is the ratio of the speed
of light in the incident medium to the speed
of light in the refractive medium.
It's denoted by ‘n’ and the two numbers
in the base are the two mediums.
We read this as, refractive index of medium two
with respect to the incident medium 1.
Also ‘‘v one’’ and ‘‘v two’’
are speeds of light in medium one and two respectively.
We can also define it as the extent to which
light changes its direction in the refractive medium.
Ok, let’s discuss the Snell’s law of refraction now.
In simple words, it tells us that the ratio
of the sine of the angle of incidence to the
sine of the angle of refraction is equal to
the refractive index of the second medium
with respect to the first medium!
Now look at the term on the right!
It is constant for light of a given colour
and for the given pair of media!
This is the basic idea of Snell’s law!
And there are different ways in which we can understand it.
In the previous video, we’d seen what 'N two one' means!
It is equal to the ratio of speed of light in the incident medium
to the speed of light in refractive medium.
So we can also say that 'sine i' over 'sine r'
is ‘v one' over 'v two’.
There’s yet another way to understand this.
We may want to understand the ratio in terms
of the absolute refractive indices of the medium!
Absolute refractive index if you remember,
is the refractive index of a medium with respect
to vacuum!
For instance, if the refractive medium is water,
then it is written like this:
So can we find this ratio in terms of
‘n one’ and ‘n two’?
Yes we can!
We just use some very basic math!
We can write this ratio like this.
Where ‘c’ is the speed of light in vacuum!
Yes, if you simplify this ratio, we will get
‘v one' over 'v two’.
And can you guess what the numerator and
the denominator are equal to?
That's right, this is nothing but the absolute refractive
index of medium 2 and this is the absolute
refractive index of medium 1.
That was another way of understanding the Snell’s law!
We can also say that ‘sine i' over 'sine r’
is ‘n two' over 'n one’.
Where ‘n two’ and ‘n one’ are the
absolute refractive indices of media 2 and 1 respectively!
Hope you understood this well!
Let’s move on to a new page!
So these were the two laws of refraction of light.
