L2 & 3 - Refraction

objectives

Core

• Interpret and Describe an Experimental Demonstration of the Refraction of Light
• Use the terminology of the angle of incidence (i) and the angle of refraction (r) and describe the passage of light through parallel-sided transparent material

Extended

• Recall and use the definition of refractive index (n) in terms of speed
• Recall and use the equation for refractive index sin i/ sin r = n

What is Refraction?

The straw appears to be bent in the liquid. What is causing this effect?

As the light crosses the boundary between fluid and glass, it is bent, producing a distorted image.

This known as refraction.

How does light refract?

If an incident ray enters glass at an angle, then it is refracted, and bends towards the normal.

The angle of incidence (i) is larger than the angle of refraction (r).

When the light leaves the glass, the opposite happens: it bends away from the normal.

A material which light passes through, such as glass or air, is known as a medium.

This material has to be transparent - i.e a medium that allows light to pass through and is not absorbed

Light in different mediums

As different transparent mediums have different densities Light travels through them at different speeds.

The Speed of Light in a vacuum is 300,000,000 metres per second - but this changes if light enters a different medium.

The below table shows how light's speed changes in different media:

As the speed of light varies depending on the medium, different materials refract light by different amounts.

The Refractive Index

Refractive index is a measure of how much a substance slows down light. The higher its value, the more a medium slows light. The more the light is slowed, the more it bends towards the normal.

Refractive index is calculated by comparing speed of light in a vacuum to that in a given medium:

A worked example:

• The speed of light in a vacuum is 300,000,000 m/s, and the speed of light in water is 225,000,000 m/s. What is the refractive index of water?

refractive index = 300,000,000 = 1.33

225,000,000

Snells Law - An alternative way to calculate refractive index

The refractive index can also be calculated using Snell’s Law, which uses the angle of incidence (i) and angle of

refraction (r) to establish how much a medium slows light.

Worked Example:

• If a Ray of light enters a medium at an angle of incidence of 45 degrees and has an angle of refraction of 28 degrees what is the refractive index?

refractive index = sin 45°

sin 28°

refractive index = 1.5

More examples of Snells Law calculations below: