5 refraction, snells law (8.2)

Refraction

8.2 Refraction, Snells Law.

Light travels in a straight path if it is travelling in a uniform medium, but as soon as light enters a different medium its path may be bent.

The light rays from the person’s face passing through the glass of water have been re-directed or bent by the water towards the observer’s eyes.

Does anyone want to try and explain what is happening in the picture?

What is Refraction?

The bending or change of direction of light as it passes from one medium to another is called refraction.

REFRACTION is the bending of the path of light due to a change in speed as it enters a medium of different optical density.

Examples of Refraction

What happens as light passes through a different medium?

Angles

When a wave slows down, it bends closer to the normal.

When a wave speeds up, it bends away from the normal.

n1- from medium

n2 – into medium

Refraction

Speed of Light

v is the speed of light in the new medium

c = 3.0 x 108 m s-1

n is the absolute index of refraction (refractive index). This is a measure of optical density. n is defined as the ratio of the speed of light in a vacuum to the speed of light in a new medium.

As the index, n, increases the speed decreases.

Speed of Light

Refractive Index

Relative Index of Refraction

The medium air has an approximate index of 1. If air is not used, then

= n2 / n1

These values for n refer to the refractive indices of different media.

If < 1 ; light speeds up.

If > 1 ; light slows down.

Refraction an example

Vwater = 2.26 x 108 m s-1

Vglass = 2.00 x 108 m s-1

Calculate the speed of light in water and glass.

n(water) = 1.33 n(glass) = 1.50 n(air) = 1.00

Diverging rays enter your eyes. You “think” in Straight Lines.

A virtual image appears to come from point y.

R – Real Depth

A – Apparent Depth

Apparent Depth

Apparent Depth

If the chest is 20 m below the surface, at what depth will the image appear? Assume nsea water = 1.34

Snell’s law describes the relationship between the angle of incidence, i, and the angle of refraction, r, for a given pair of media:v = velocity

Snell’s Law an example

A monochromatic light ray, f = 5.09 x 1014 Hz, is incident on medium X at 55˚. The absolute index of refraction for material X is n = 1.66

What is the angle of refraction?

What is the speed of light in medium X?