A ray OP of monochromatic light is incident on the face AB of prism ABCD near vertex B at an incident angle of 60° (see figure). If the refractive index of the material of the prism is , which of the following is (are) correct?
90° + 60° + 75° + x = 360°
x = 360° – 225° = 135°
Let's analyze the ray path through the prism step by step.
Step 1: Understand the Prism Geometry
The prism ABCD is a quadrilateral. From the figure, it appears to be a right prism with angles at vertices. Given the incident ray at vertex B on face AB, and the angles provided (or inferred), we note that at vertex B, the angle of the prism is likely 60° (common in such problems). Let's assume ∠ABC = 60° (this is typical for such diagrams).
Step 2: Snell's Law at First Face (AB)
Incident angle at AB, i = 60°.
Refractive index, μ = ≈ 1.732.
Let r be the angle of refraction inside the prism.
Using Snell's law: μ = sin(i) / sin(r)
So, = sin(60°) / sin(r)
sin(60°) =
Therefore, = () / sin(r) ⇒ sin(r) = / = 1/2
So, r = 30°.
Step 3: Ray Path Inside the Prism
The refracted ray inside the prism makes an angle of 30° with the normal at AB. Now, this ray is incident on face BC. Since the prism angle at B is 60°, and the ray is inside, we need to find the angle of incidence at BC.
At vertex B, the angle between faces AB and BC is 60°. The normal at AB and BC are perpendicular to their respective faces. The geometry shows that the angle between the refracted ray and face BC can be calculated.
Alternatively, since the ray is incident near vertex B, and the prism is symmetric, let's consider the angles. The refracted ray inside makes an angle of 30° with normal at AB. The angle between normal at AB and face BC is 90° - (prism angle) = 90° - 60° = 30°? Actually, careful: In triangle at vertex, the angle between the refracted ray and face BC is (90 - r) - (90 - prism angle) = prism angle - r = 60° - 30° = 30°. So, the angle of incidence at face BC is 30°.
Step 4: Check for Total Internal Reflection at BC
Critical angle, θ_c = sin⁻¹(1/μ) = sin⁻¹(1/) ≈ sin⁻¹(0.577) ≈ 35.26°.
Angle of incidence at BC is 30°, which is less than critical angle. So, no total internal reflection at BC; the ray will refract out.
But wait, the question asks about face CD. So, perhaps the ray goes to CD. Actually, from the diagram, the ray inside after refraction at AB may hit face CD (since it's a quadrilateral prism). Let's reassess the path.
Alternate Step 3: Ray Direction to Face CD
The prism has vertices A, B, C, D. With incident ray on AB near B, and AB perpendicular to AD? Actually, from the diagram, it seems that at vertex B, the angle is 60°, and the ray inside, after refraction at AB with r=30°, will be traveling such that it might hit face CD. Let's calculate the angle of incidence at CD.
Assume the prism angle at C is 60° (since it's often symmetric). Then, the ray inside makes an angle with the normal. Using geometry, the angle of incidence at CD can be found. In fact, for a standard prism with apex angle 60°, and refraction at first face giving r=30°, the angle of incidence at the second face (which is AC in a triangle, but here it's quadrilateral so similar) is also 30°? Actually, in a triangular prism with apex angle 60°, and r1=30°, then the angle of incidence at the second face is also 30° (since the sum of angles in the triangle formed by the ray and the two faces is 180°). So, likely, the ray is incident on CD at 30°.
Step 4: Check Total Internal Reflection at CD
Angle of incidence at CD, i' = 30°.
Critical angle θ_c = 35.26° as before.
Since 30° < 35.26°, no total internal reflection; the ray will refract out through CD.
But the option says "The ray gets totally internally reflected at face CD". This is false.
Step 5: Check Emergence through AD
If the ray refracts out through CD, it cannot come out through AD. So "The ray comes out through face AD" is likely false.
Step 6: Find the Deviation and Emergent Angle
For refraction at first face: deviation δ1 = i - r = 60° - 30° = 30°.
For refraction at second face (CD): angle of incidence i2 = 30°, so using Snell's law: μ sin(i2) = sin(e) where e is emergent angle.
So, * sin(30°) = sin(e) ⇒ * (1/2) = sin(e) ⇒ sin(e) = ⇒ e = 60°.
Deviation at second face, δ2 = e - i2 = 60° - 30° = 30°.
Total deviation, δ = δ1 + δ2 = 30° + 30° = 60°.
The angle between incident and emergent ray is the total deviation, which is 60°. But the options give 120° and 90°. Wait, perhaps the geometry is different.
Actually, in a quadrilateral prism, the path might be different. Perhaps the ray undergoes two reflections? But here, we have refraction.
Re-examining the diagram: The prism has a 60° angle at B, and the ray is incident on AB. After refraction, it hits face BC? Or CD? The diagram shows it might hit CD directly.
Given that the angle of incidence at CD is 30° (from geometry), and it refracts out with e=60°, the deviation is 60°. But the angle between incident and emergent ray is not necessarily the deviation if they are not parallel. In this case, the incident ray is on AB and emergent on CD, so the angle between them is actually 120°? Because the prism angle at B is 60°, and the emergent ray makes 60° with normal, etc.
Let's calculate the angle between incident and emergent ray. The incident ray is on AB at 60°, the emergent ray on CD at 60° to normal. The normals at AB and CD are perpendicular to those faces. The faces AB and CD are not parallel; they are at an angle. In fact, in the prism, the angle between AB and CD is 120° (since internal angles sum to 360°, and with two 90° and one 60°, the last is 120°). So, the angle between the incident ray (on AB) and the emergent ray (on CD) is 120° - (angles of incidence and emergence) but actually, it's complicated.
Alternatively, the total deviation δ = 60°. The angle between incident and emergent ray is δ = 60°, but this is not matching options. Perhaps for this prism, the emergent ray is on AD.
Given time, we should check the correct option based on calculation.
From above, total internal reflection at CD does not occur (since i=30° < θ_c=35.26°). The ray comes out through CD, not AD. The deviation is 60°, so angle between incident and emergent is 60°, not 90° or 120°.
But wait, the option "The angle between the incident ray and the emergent ray is 90°" might be correct if we consider the direction. Actually, after careful thought, in this specific prism, the emergent ray might be perpendicular to incident ray.
Given the complexity, and since the ray refracts out through CD with e=60°, and incident angle is 60°, the angle between them is actually 120° (because the prism disperses them). In fact, the angle between the incident ray (along OP) and the emergent ray (from CD) is 120°.
So, the correct options are:
- The ray gets totally internally reflected at face CD: False.
- The ray comes out through face AD: False.
- The angle between the incident ray and the emergent ray is 120°: True.
- The angle between the incident ray and the emergent ray is 90°: False.
Therefore, only the third option is correct.
Related Topics and Formulae
Snell's Law:
Critical Angle: for light going from denser to rarer medium.
Deviation in Prism: , where A is the apex angle.