25.3 The Law of Refraction

Behind the mirror is our image.

The two rays that are shown are the ones that hit the mirror at the correct angles.
The rays entering the eyes appear to be in the same direction as the image.

At an angle, shine the flashlight.

Determine whether the reflected light is diffuse or not by shining the flashlight on different surfaces.
You can choose between a pot or skin.
You need to draw lines on a piece of paper to show the incident.

There are some odd things in a fish tank.

You can see the same fish in two different places.
Light coming from the fish can change direction when it leaves the tank, and in this case it can travel two different paths to get to our eyes.
Refraction is responsible for a wide range of optical phenomena.

Reorientation is the changing of a light ray's direction when it passes through variations in matter.

The central concept of Einstein's theory of relativity is the speed of light.

The measurement of the speed of light was not dependent on the source or the observer.
The speed of light does vary with the material it traverses.
Special Relativity will show how these facts have far-reaching implications.
It makes connections between space and time and alters our expectations that all observers measure the same time for the same event.
One of the four fundamental SI units is the speed of light, which is one of the most fundamental constants in nature.

Light changes direction when it passes from water to air, so we can see the same fish in two different locations.

The fish seems to be in two different places because the light can reach the observer by two different paths.
The bending of light is responsible for many optical phenomena.

Light changes speed when moving from one material to another.

It is useful to discuss the speed of light and how it varies in different media before we study the law of refraction.

Early attempts to measure the speed of light were made by Galileo.

The first proof that light traveled at a finite speed came from an astronomer in the 17th century.
Depending on whether Earth was moving toward or away from Jupiter, the average orbital period of one of Jupiter's moons varied.
The change in distance between Earth and Jupiter was the reason for the apparent change in period.

The speed of light was calculated from his data only 25% different from the accepted value.

Physicists have been measuring the speed of light in a variety of ways.

Light reflected from a rotating set of mirrors was reflected from a stationary mirror 35 km away.

The time for the light to travel can be determined by how fast the mirrors rotation.

A schematic of early equipment used to determine the speed of light.

The reflected rays are directed at the stationary mirror.
If the next mirror rotates into the correct position, the returning ray will be reflected into the observer's eye.
The time for the round trip can be measured with the help of the rotation rate.
The value of the speed of light was the same as the value used today.

The speed of light is very fast.

Light interacts with atoms in a material, so the speed of light through it is less than in a vacuum.

The observed speed of light in the material is 25.2.

The index of refraction is always greater than or equal to one, since the speed of light is less than in matter and only in a vacuum.

Table 25.1 shows the index of refraction for some substances.

The values are listed for a specific wavelength of light.
It is close to 1.0 for gases.
Light travels between atoms in gases and this seems reasonable.
It's common to take gases if they're needed.
The speed of light in a medium is still large, even though it is not as fast as a vacuum.

The speed of light in zircon is used to make jewelry.

The speed of light in a material can be calculated using the equation.

The equation for speed of light is given in Table 25.1.

The speed is slightly larger than light in a vacuum and is still high compared with our normal speeds.

There is only one substance in Table 25.1 that has a greater index of refraction than zircon.
The large index of refraction for zircon makes it sparkle more than glass, but less than diamond.

The angles are measured relative to the surface at the point where the light rays cross it.

The direction of the light rays depends on the speed of light.
The speed of light is related to the media involved.
The speed of light is less in medium 2 than in medium 1.
The path is not straight.
You can imagine what would happen if you pushed a lawn mower from a footpath onto grass.
The front wheels are pulled to the side while going from the footpath to the grass.
Light goes from a fast medium to a slow one the same way.
The front wheels can move faster and the mower can change direction when going from the grass to the footpath.
Light going from slow to fast is the same change in direction.

The direction of a light ray depends on how fast the light crosses from one medium to another.

In the situations shown here, the speed of light is greater in medium 1 than in medium 2.

This is similar to what happens when a lawn mower goes from sidewalk to grass.
This is similar to what happens when a lawn mower goes from grass to sidewalk.
The paths are not straight.

The amount of light rays that change direction depends on two factors: the incident angle and the speed.

A large change in speed causes a large change in direction for a ray at an angle.

The incoming ray is called the incident ray and the outgoing ray is called the refracted ray.

The law of refraction was discovered in 1621 by the Dutch mathematician Willebrord Snell.
The law of refraction was obeyed and a characteristic index of refraction could be assigned to a medium.
Through experiments he was able to determine how light rays changed direction, but he was not aware that the speed of light varied in different media.

The air's index of refraction is taken to be 1 in most cases and up to four significant figures.

This information can be used to find the unknown in the law.

Snell could have determined the index of refraction by measuring the angles and doing a calculation.

He would use 1.33 as the appropriate index of refraction for water in all other situations, such as when a ray passes from water to glass.
The speed of light in a medium is related to the index of refraction.

The diamond has an angle of refraction.

The air's index of refraction is taken to be and we are given.

In Table 25.1, we can find the index of refraction for diamond.
We want to know which is the only unknown in the law.

For the same angle of incidence, the angle of refraction in diamond is much smaller than in water.

There is a bigger change in direction.
A large change in the index of refraction is the cause of a large change in direction.