Concepts of Physics

IIT JEE Physics (1978-2018: 41 Years) Topic-wise Complete Solutions

Rising of the Coin due to Refraction

Objective

To understand rising of coin due to refraction.

Introduction

When light travels from one medium to another medium, it bends at the interface between two media according to Snell's Law. This shows up in various ways in daily life. This demo is an interesting way to demonstrate refraction of light.

Materials

glass, coin, pencil

Procedure

Put a coin in an opaque vessel placed on a table. Looking at the coin, move back your head till the coin just disappear from the view. Then ask someone to pour water into the vessel gently, without displacing the coin. As the vessel fills with water, the coin will rise into view.

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Variant: Place a pencil in glass of water. Why does the pencil look bend? Drawing rays, explain why the pencil appears more sharply bent when viewed at an angle? Because of refraction parts of the pencil in water appear to rise up. When viewed from a side, apparent rise of the bottom of the pencil is more and hence the pencil looks more bent.

Discussion

Initially, the rays emerging from the coin do not fall on the eye. However, when water is poured into the glass, the rays bend at the surface of the water and reach the eye. As the rays from the coin comes to water surface, it bends away from the normal. Thus light which was earlier going from above the eye now reaches the eye after refraction. This makes the coin visible.