Question

The beauty of nature is the beauty of physics.

↓↓ Question ↓↓

Why does the light moves away from the normal after emerging from denser medium to rarer medium?

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Answer 1

$\bold{\underline{\underline{\huge{\sf{AnsWer:}}}}}$

$\bold{\large{\sf{\red{Refraction}}}}$$\bold{\large{\sf{\blue{Of}}}}$$\bold{\large{\sf{\red{Light}}}}$

Starting answer with your tareef! xD

'The beauty of nature is the beauty of physics' amazing choice! It will more fun answering your question now! xD

The change observed in the direction of propagation (generation) of light when the light rays passes obliquely from one transparent medium to another medium is termed as Refraction of light.

Now why does the, light ray changes its path?

Is there any force acting on the rays of light or it's because they get tired and hence take a pause aside by and then continue with their journey? xD

Well, the thing is...When the light ray passes obliquely from one transparent medium to another, there's a change in velocity of the rays of light. The velocity of light is different in different media (plural - medium). So when it crosses a medium in which it's speed is higher (rarer medium) as compared to the medium in which it will enter (denser medium) the ray of light bends towards the normal, as the speed of ray decreases.

When this same ray of light, makes an exist from the denser medium and enters the rarer medium, the light ray bends away from the normal. That's because the velocity of light increases.

Why is there a change in Speed of light when it travels obliquely from one transparent medium to another?

It's all because of the obstruction in the path of the light rays at the interface. Let's take an example of a ball being thrown in water. I took this example because I understand this really well xD! You may look up for other examples too.

Consider the ball to be the ray of light and water to be the denser medium. Throw the ball from the rarer medium (air) into the water (denser medium) considering an imaginary line being formed at the interface of air and water. As soon as the ball (ray of light) comes in contact with the interface it is being obstructed and the velocity of the ball (light ray) reduces and as a result it moves towards the imaginary line.

Similarly, when this ball (ray of light) makes an exist from the denser medium (water) there is a change noticed in the velocity, which is greater than the velocity when the ball hit the interface and hence the ball moves away from the normal.

*Velocity of light is always greater in rarer medium.

So from all this I guess you must have now understood why the light moves away from the normal after emerging from denser medium to rarer medium.

Refractive index :

Let's consider $\bold{i}$ to be the angle of incidence and $\bold{r}$ to be the angle of refraction, then

$\bold{\dfrac{sin_i}{sin_r}}$ = constant

This is knows as Snell's Law.

The constant is called the refractive index of the second medium with respect to the first medium. Denoted as, $\bold{2_n_1}$.

Thus, $\bold{_2_n_1}$ = $\bold{\dfrac{sin_i}{sin_r}}$

Magnitudes :

If $\bold{v_1}$ is (magnitude of) the velocity of light in the first medium and If $\bold{v_2}$ (magnitude of) the velocity of light in the second medium, then

$\bold{_2_n_1}$ = $\bold{\dfrac{v_1}{v_2}}$

The same way, the refractive index of the first medium with respect to the second medium is denoted as,

$\bold{_1_n_2}$ = $\bold{\dfrac{v_2}{v_1}}$

But, if the first medium here then

$\bold{_2_n_1}$ is considered with respect to vacuum. It is called as the absolute refractive index of the medium 2 and is denoted by n.

Answer 2

$\huge\underline\mathfrak\red{Question-}$

Why does the light moves away from the normal after emerging from denser medium to rarer medium?

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The phenomenon used here is refraction of light.

It is the phenomenon of change in the path of light ray when it travels from on transparent medium to the another one.

$\sf\underline\green{Experimental\:verification-}$

*Refer to the attachment for diagram*

For propagation of light both the rarer and denser media are required. In the rarer medium speed of light becomes more and in denser medium speed of light becomes less.

Let we take a glass slab ( denser medium ), which is a rectangular, solid piece of glass. Let any incident rst strikes to its one surface AB at point O the incident point. Let NN' be normal at the point of incidence. The angle in between the incident ray and normal is the angle of incidence ( ∠i ).

Now ray enters from air ( rarer medium ) to glass ( denser medium ). As a result of this, the speed of light ray decreases and it bends towards the normal. In other sense, the ray doesn't travel along its actual path, which could be possible if no slab were present. This ray which has been deviated from its actual path is called the reflected ray. It makes an angle with the normal, called as the angle of refraction ( ∠r ).

Inside the slab, it travels towards another face CD and strikes at point O' . From here, it passes to air which is again a rarer medium. All this time, its speed increases and it deviates away from the normal and emerges out of slab. This time, we called it emergent ray. It also forms an angle with normal. It is called angle of emergence ot emergent angle ( ∠e ).

This emergent ray travels along same direction as there was its original direction. The only difference comes that it doesn't move on its original path. This time its direction of travelling is parallel to the actual, original path.

The emergent ray then deviated from its original path by a small perpendicular distance. The perpendicular distance by which the emergent ray deviates from its actual path is called the lateral shift or lateral displacement.

Lateral shift can also be defined as the perpendicular distance in between the emergent ray and incident ray. It is represented by letter d.

Numerically, it is observed that angle of incidence is equal to angle of emergence.

$\huge\boxed{\angle i = \angle r}$

The emergent ray becomes parallel to the incident ray.

$\sf\underline\blue{Laws\:of\:refraction-}$

★ 1st law of refraction : Incident ray, reflected ray and normal at the point of incidence all lie in same plane.

★ 2nd law of refraction : Ratio of sin of angle of incidence to the sin of angle of refraction is a constant quantity.

$\sf\dfrac{Sin(Angle\:of\:Incidence)}{Sin(Angle\:of\:refraction)}$ = Constant

$\leadsto$ $\sf\dfrac{Sin(\angle i)}{Sin(\angle r)}$ = Constant ( n ).

Where, ∠i = angle of incidence

∠r = angle of refraction

n = constant ( snell's constant )

#This constant is called also the snell's constant or refractive index. This law is popularly known as the Snell's law.

$\sf\underline\pink{Refractive\:Index-}$

It is an important physical quantity in terms of constant which provides the information about the extent of change of path of light when it travels from one transparent medium to another.

We know that ray of light always changes its path and when travels from one transparent medium to other. Change of path is due to the change in speed of light propagation.

In other words, if light ray travels from rarer to denser the speed decreases and vice versa. It means the change of path is basically the phenomenon of change in speed. So if the speeds of two media are known then by comparing them, change of path of light ray can be determined in any medium compared with other. This particular job is done by refractive index ( which behaves like an indicator giving information about change of path of light ).

Actually, the refractive index compares two media by their speeds of light. So it can be defined as :

"Refractive index is the ratio of speed of light in one medium to the speed of light in another medium."

Refractive index = $\sf\dfrac{Speed\:of\:light\:in\:medium\:1}{Speed\:of\:light\:in\:medium\:2}$

• $\sf{n_2_1}$ = Refractive index of medium 2 with respect to 1.

• $V_1$ = Speed of light in medium 1.

• $V_2$ = Speed of light in medium 2.