Refraction through a glass slab: refractive index

Objectives

• Measure angles of incidence and refraction.
• Verify Snell’s law.
• Find the refractive index of glass.

Theory

Light slows down on entering glass and bends towards the normal. Snell’s law: for a given pair of media, sin i ÷ sin r is constant. That constant is the refractive index n of the glass (relative to air). A graph of sin i against sin r is a straight line through the origin with gradient n. On leaving the slab the ray bends away from the normal by the same amount, so the emergent ray is parallel to the incident ray but shifted sideways.

n = sin i ÷ sin r

Procedure

1. Measure the angle of refraction for one angle of incidence.
2. Repeat for at least five different angles of incidence.
3. Plot sin i against sin r and note the gradient.

Precautions

• Measure angles from the normal, not from the surface.
• Use a sharp pencil and keep the slab on its outline.
• Avoid very small angles, where the percentage error is large.

Subject

Physics

Grade

9–11

Chapter

Light (Nepal National Curriculum (CDC), grade 10)

Difficulty

Medium

Duration

20 min

Safety level

Low

Version

1.0

Apparatus

• Rectangular glass slab
• Ray box
• Drawing board
• Paper
• Protractor
• Pins