Let the X–Y plane be the boundary between two transparent media. Medium 1 in z ≥ 0 has a refractive index of and medium 2 with z < 0 has a refractive index of . A ray of light in medium 1 given by the vector is incident on the plane of separation. The angle of refraction in medium 2 is :

Engineering

Physics

Plane Surface Refraction

Question

Let the X–Y plane be the boundary between two transparent media. Medium 1 in z ≥ 0 has a refractive index of $\sqrt{2}$ and medium 2 with z < 0 has a refractive index of $\sqrt{3}$ . A ray of light in medium 1 given by the vector $\vec{A} = 6 \sqrt{3} \hat{i} + 8 \sqrt{3} \hat{j} - 10 \hat{k}$ is incident on the plane of separation. The angle of refraction in medium 2 is :

45°

60°

30°

75°

X–Y Plane

µ₁sinθ₁ = µ₂sinθ₂

${cosθ}_{1} = \frac{10}{\sqrt{{(6 \sqrt{3})}^{2} + {(8 \sqrt{3})}^{2} + 100}} = \frac{10}{\sqrt{400}} = \frac{10}{20}$

$\cos θ_{1} = \frac{1}{2}$

$\sqrt{2} \sin 60 = \sqrt{3} {sinθ}_{2}$

$\sqrt{2} \times \frac{\sqrt{3}}{2} = \sqrt{3} {sinθ}_{2}$

${sinθ}_{2} = \frac{1}{\sqrt{2}}$

θ₂ = 45º

This problem involves finding the angle of refraction when a light ray passes from one medium to another. The key concept is Snell's Law, which relates the angles of incidence and refraction to the refractive indices of the two media.

Step 1: Understand the Geometry and Given Data

The boundary is the x-z plane (z=0). Medium 1 (z ≥ 0) has refractive index n₁ = $\sqrt{2}$ . Medium 2 (z < 0) has refractive index n₂ = $\sqrt{3}$ .

The incident light ray in Medium 1 is given by the vector: $\vec{A} = 6 \sqrt{3} \hat{i} + 8 \sqrt{3} \hat{j} - 10 \hat{k}$ .

Since the boundary is the x-z plane, the normal at the point of incidence is along the negative y-axis (pointing into Medium 2) or positive y-axis. We will consider the normal pointing from Medium 1 to Medium 2, which is along the negative y-axis ( $\hat{n} = - \hat{j}$ ).

Step 2: Find the Angle of Incidence (θᵢ)

The angle of incidence is the angle between the incident ray vector and the normal to the surface. First, let's find the direction of the incident ray.

The magnitude of the incident vector $\vec{A}$ is: $| \vec{A} | = \sqrt{{(6 \sqrt{3})}^{2} + {(8 \sqrt{3})}^{2} + {(- 10)}^{2}} = \sqrt{108 + 192 + 100} = \sqrt{400} = 20$

The unit vector along the incident ray is: $\hat{a} = \frac{\vec{A}}{| \vec{A} |} = \frac{6 \sqrt{3} \hat{i} + 8 \sqrt{3} \hat{j} - 10 \hat{k}}{20} = \frac{3 \sqrt{3}}{10} \hat{i} + \frac{2 \sqrt{3}}{5} \hat{j} - \frac{1}{2} \hat{k}$

The normal vector is $\hat{n} = - \hat{j}$ .

The angle of incidence θᵢ is given by the dot product between the unit incident vector and the unit normal vector: $\cos (θ_{i}) = | \hat{a} \cdot \hat{n} |$ (We take the absolute value to get the acute angle).

$\hat{a} \cdot \hat{n} = (\frac{3 \sqrt{3}}{10} \hat{i} + \frac{2 \sqrt{3}}{5} \hat{j} - \frac{1}{2} \hat{k}) \cdot (- \hat{j}) = - \frac{2 \sqrt{3}}{5}$

Therefore, $\cos (θ_{i}) = | - \frac{2 \sqrt{3}}{5} | = \frac{2 \sqrt{3}}{5}$

Now, we can find sin(θᵢ) using the identity sin²θ + cos²θ = 1. $\sin (θ_{i}) = \sqrt{1 - {(\frac{2 \sqrt{3}}{5})}^{2}} = \sqrt{1 - \frac{4 \times 3}{25}} = \sqrt{1 - \frac{12}{25}} = \sqrt{\frac{13}{25}} = \frac{\sqrt{13}}{5}$

Step 3: Apply Snell's Law

Snell's Law states: $n_{1} \sin (θ_{i}) = n_{2} \sin (θ_{r})$

Substituting the known values: $\sqrt{2} \times \frac{\sqrt{13}}{5} = \sqrt{3} \times \sin (θ_{r})$

Now, solve for sin(θᵣ): $\sin (θ_{r}) = \frac{\sqrt{2} \times \sqrt{13}}{5 \sqrt{3}} = \frac{\sqrt{26}}{5 \sqrt{3}}$

Rationalizing the denominator: $\sin (θ_{r}) = \frac{\sqrt{26}}{5 \sqrt{3}} \times \frac{\sqrt{3}}{\sqrt{3}} = \frac{\sqrt{78}}{5 \times 3} = \frac{\sqrt{78}}{15}$

This value does not correspond to a standard angle. Let's re-examine the calculation for the angle of incidence. Notice the z-component of the incident vector is negative, meaning the ray is traveling towards the boundary (negative k̂ direction). The component along the normal (ĵ) is positive. The significant component is along the normal.

Alternate Simpler Approach: Find the angle the vector makes with the normal.

The dot product we calculated was $\hat{a} \cdot \hat{n} = - \frac{2 \sqrt{3}}{5}$ . The magnitude of this dot product is cos(θᵢ). Let's find the numerical value. $\cos (θ_{i}) = \frac{2 \times 1.732}{5} \approx \frac{3.464}{5} \approx 0.6928$

This gives θᵢ ≈ cos⁻¹(0.6928) ≈ 46.1°. Then sin(θᵢ) ≈ sin(46.1°) ≈ 0.721. Let's plug this into Snell's Law.

Snell's Law: $\sqrt{2} \times 0.721 \approx \sqrt{3} \times \sin (θ_{r})$

$1.414 \times 0.721 \approx 1.732 \times \sin (θ_{r})$

$1.019 \approx 1.732 \times \sin (θ_{r})$

$\sin (θ_{r}) \approx 0.588$

θᵣ ≈ sin⁻¹(0.588) ≈ 36°, which is not among the options. There must be a mistake in identifying the normal.

Step 4: Correct Identification of Normal and Angle

The boundary is the x-z plane. The normal to this plane is along the y-axis. Since the ray is coming from Medium 1 (z≥0) and going towards the boundary, the relevant normal is pointing from Medium 1 to Medium 2, i.e., along the negative y-axis. However, let's find the angle the incident ray makes with the vertical (y-axis).

The y-component of the unit vector is $\frac{8 \sqrt{3}}{20} = \frac{2 \sqrt{3}}{5}$ . This is positive, meaning the ray has a component in the +ĵ direction, away from the boundary. This seems counterintuitive for an incident ray. Let's check the z-component, which is -10/20 = -1/2, negative. This means the ray is moving downward (towards negative z), towards the boundary at z=0.

The incident ray vector is $\vec{A} = 6 \sqrt{3} \hat{i} + 8 \sqrt{3} \hat{j} - 10 \hat{k}$ . The large positive j-component suggests the ray is mostly traveling parallel to the boundary rather than towards it. The angle of incidence might be very large.

Perhaps the intended normal is along the z-axis. The boundary is the x-z plane, which has a normal along the y-axis. But if we consider the variation in z, the normal could be considered along the z-axis for a horizontal surface. Let's assume the normal is along the z-axis. Then $\hat{n} = - \hat{k}$ (pointing from Medium 1 to Medium 2).

Now, let's find the angle with this normal. $\hat{a} \cdot \hat{n} = (\frac{3 \sqrt{3}}{10} \hat{i} + \frac{2 \sqrt{3}}{5} \hat{j} - \frac{1}{2} \hat{k}) \cdot (- \hat{k}) = \frac{1}{2}$

Therefore, cos(θᵢ) = 1/2, which means θᵢ = 60°.

This is a standard angle. Now apply Snell's Law.

n₁ sin(θᵢ) = n₂ sin(θᵣ)

$\sqrt{2} \sin (60 °) = \sqrt{3} \sin (θ_{r})$

$\sqrt{2} \times \frac{\sqrt{3}}{2} = \sqrt{3} \sin (θ_{r})$

$\frac{\sqrt{6}}{2} = \sqrt{3} \sin (θ_{r})$

Now, solve for sin(θᵣ): $\sin (θ_{r}) = \frac{\sqrt{6}}{2 \sqrt{3}} = \frac{\sqrt{2}}{2} = \frac{1}{\sqrt{2}}$

Therefore, θᵣ = sin⁻¹(1/√2) = 45°.

Final Answer: 45°

Related Topics and Formulae

Snell's Law: The fundamental law for refraction. $n_{1} \sin (θ_{i}) = n_{2} \sin (θ_{r})$

Refractive Index (n): n = c/v, where c is the speed of light in vacuum and v is the speed of light in the medium.

Angle of Incidence (θᵢ) and Angle of Refraction (θᵣ): These are the angles between the light rays and the normal to the surface.

Vector Geometry: The angle between two vectors A and B is given by cosθ = (A·B) / (|A||B|).

Detailed Solution

Student who searched for similar questions

Questions 1

Questions 2

Questions 3

Questions 4

Questions 5