Ray Through Glass Prism MCQs Quiz | Class 10

Understanding Ray Through Glass Prism

A glass prism is a transparent optical element with flat, polished surfaces that refract light. When a light ray passes through a prism, it undergoes two refractions – one when it enters the prism and another when it exits. This phenomenon leads to the deviation of the light ray from its original path.

Key Concepts of Light Refraction Through a Prism

• Refraction: The bending of light as it passes from one medium (air) to another (glass prism) and then back to the first medium (air).
• Angle of Incidence (i): The angle between the incident ray and the normal at the first refracting surface.
• Angle of Refraction (r1, r2): The angles between the refracted ray and the normal inside the prism.
• Angle of Emergence (e): The angle between the emergent ray and the normal at the second refracting surface.
• Angle of Prism (A): The angle between the two refracting surfaces of the prism.
• Angle of Deviation (d): The angle by which the incident ray deviates from its original path after passing through the prism. It is the angle between the incident ray produced forward and the emergent ray produced backward. For a monochromatic light ray, the angle of deviation is always towards the base of the prism.

Tracing the Path of a Ray

When a monochromatic light ray (single color) enters a prism, it bends towards the normal. Inside the prism, it travels in a straight line until it hits the second surface. Upon exiting, it bends away from the normal. The overall effect is that the ray deviates from its initial direction. Practical experiments involve setting up a prism, marking the incident and emergent rays using pins, and then connecting them to trace the path and measure various angles, especially the angle of deviation.

Observation of Deviation

The angle of deviation depends on several factors:

• Angle of Incidence: As the angle of incidence increases, the angle of deviation first decreases, reaches a minimum value (angle of minimum deviation), and then increases.
• Material of the Prism (Refractive Index): A higher refractive index leads to greater deviation.
• Angle of the Prism: A larger prism angle generally results in a larger angle of deviation.
• Wavelength of Light: Different colors (wavelengths) of light deviate by different amounts, leading to dispersion (splitting of white light into its constituent colors). For monochromatic light, this factor is constant.

Basic Interpretation and Applications

The deviation of light by a prism is a fundamental concept in optics. While this topic focuses on a single ray, understanding this behavior is crucial for comprehending phenomena like the formation of rainbows (due to dispersion by water droplets acting as tiny prisms) and the working of spectrographs. For monochromatic light, the key interpretation is the consistent bending towards the base, a principle used in various optical instruments to redirect light.

Quick Revision Points

• A prism causes light to refract twice and deviate.
• The emergent ray is deviated towards the base of the prism.
• Angle of deviation is the angle between incident and emergent rays.
• Factors affecting deviation: angle of incidence, refractive index, prism angle.
• There is a unique angle of minimum deviation for a given prism and wavelength.

Practice Questions

1. Define the angle of deviation for a ray of light passing through a triangular glass prism.
2. State two factors on which the angle of deviation produced by a prism depends.
3. Draw a neat labeled diagram to show the path of a ray of light passing through a glass prism.
4. How does the angle of deviation change when the angle of incidence is increased from a small value to a large value?
5. Why does a prism deviate light towards its base?