Ray Through Rectangular Glass Slab MCQs Quiz | Class 10

Understanding Refraction Through a Rectangular Glass Slab

The experiment involving a rectangular glass slab is a fundamental concept in optics, demonstrating the phenomenon of refraction of light. When a light ray passes from one transparent medium to another, it bends. This bending is called refraction. A glass slab provides an excellent setup to observe and measure this behavior.

Key Concepts Explained

1. Refraction of Light

Refraction is the change in direction of a wave passing from one medium to another or from a gradual change in the medium. In the context of light, it occurs when light travels from air into glass, and then from glass back into air.

• Air to Glass (Rarer to Denser): The light ray bends towards the normal.
• Glass to Air (Denser to Rarer): The light ray bends away from the normal.

2. Angles Involved

• Angle of Incidence (i): The angle between the incident ray and the normal at the point of incidence on the first surface (air-glass interface).
• Angle of Refraction (r): The angle between the refracted ray (inside the glass) and the normal at the point of incidence on the first surface.
• Angle of Emergence (e): The angle between the emergent ray (leaving the glass) and the normal at the point of emergence on the second surface (glass-air interface).

A crucial observation is that for a rectangular glass slab, the emergent ray is parallel to the incident ray, meaning the angle of incidence ‘i’ is equal to the angle of emergence ‘e’ (i = e).

3. Lateral Displacement

Although the emergent ray is parallel to the incident ray, it is displaced sideways from the original path of the incident ray. This perpendicular distance between the original path of the incident ray and the emergent ray is called lateral displacement. Factors affecting lateral displacement include the thickness of the glass slab, the refractive index of the glass, and the angle of incidence.

4. Experimental Procedure and Interpretation

In the experiment:

1. A rectangular glass slab is placed on a white paper.
2. Its boundary is traced.
3. A normal is drawn at a point on one of the longer sides.
4. Two pins (P1, P2) are fixed to mark the incident ray at a certain angle (i).
5. Looking from the opposite side, two more pins (P3, P4) are fixed such that they appear to be in a straight line with the images of P1 and P2.
6. The pins are removed, and the points are joined to trace the incident, refracted, and emergent rays.
7. Normals are drawn at both points of incidence/emergence.
8. Angles i, r, and e are measured using a protractor.

Interpretation: By comparing ‘i’ and ‘e’, we find they are equal, confirming the emergent ray is parallel to the incident ray. The lateral displacement can be measured. Snell’s Law (sin i / sin r = constant = refractive index of glass relative to air) can also be verified.

Quick Revision Checklist

• Refraction is the bending of light at the interface of two media.
• Light bends towards normal when entering a denser medium (air to glass).
• Light bends away from normal when entering a rarer medium (glass to air).
• Angle of incidence (i) = Angle of emergence (e) for a rectangular slab.
• The emergent ray is parallel to the incident ray but laterally displaced.
• Lateral displacement depends on slab thickness, refractive index, and angle of incidence.
• A protractor is used to measure angles i, r, e.

Practice Questions

1. What is the primary reason for light bending when it enters a glass slab from air?
2. If the angle of incidence is 45 degrees, what would be the angle of emergence for a rectangular glass slab?
3. List two factors that influence the amount of lateral displacement produced by a glass slab.
4. When does a light ray passing through a rectangular glass slab not suffer any deviation or lateral displacement?
5. State the relationship between the angle of incidence and the angle of refraction according to Snell’s Law.