Chapter 10 · Question 4

Explain the refraction of light through a triangular glass prism. Describe the path of the incident ray, the refracted ray, and the emergent ray, and define the angle of deviation.

Answer Revealed

Direct Answer:

When light passes through a triangular glass prism, it bends twice — towards the normal on entering the prism and away from the normal on exiting. The emergent ray is deviated from the incident ray's direction. The angle between the direction of the incident ray extended and the emergent ray is called the angle of deviation.

Simple Explanation

A prism bends light twice. First, when light enters the prism it bends towards the perpendicular (normal). Then, when it leaves the prism, it bends away from the normal. The ray that comes out points in a different direction from the original ray. The angle between the original direction and the new direction is called the angle of deviation.

Exam-Ready Structure

Refraction of light through a triangular glass prism involves the following concepts and steps: 1. A triangular glass prism has two triangular bases and three rectangular lateral surfaces. The angle between the two refracting surfaces is called the angle of the prism (A). 2. When a ray of light (PE — incident ray) enters the prism at the first face, it bends towards the normal because it travels from air (rarer medium) to glass (denser medium). This produces the refracted ray (EF). 3. At the second face, the refracted ray bends away from the normal as it travels from glass (denser) to air (rarer). This produces the emergent ray (FS). 4. The emergent ray bends more in the direction of the base of the prism. 5. Angle of deviation (D): The angle between the direction of the incident ray extended forward and the direction of the emergent ray. It measures how much the prism has deviated the light from its original path. 6. Key angles: angle of prism (A), angle of incidence (i), angle of refraction (r), angle of emergence (e), and angle of deviation (D).

Key Points

Light bends towards normal on entering prism (air to glass)
Light bends away from normal on leaving prism (glass to air)
Emergent ray deviates towards the base of the prism
Angle of deviation is the angle between extended incident ray and emergent ray
Angle of prism is the angle between the two refracting faces

Common Mistakes

Drawing the emergent ray parallel to the incident ray (that only happens for a glass slab, not a prism)
Forgetting that the second bend is away from the normal

Explain the refraction of light through a triangular glass prism. Describe the path of the incident ray, the refracted ray, and the emergent ray, and define the angle of deviation.