When a ray of light enters perpendicular to the glass medium from the air medium How will be the refracted ray travel?

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Snell's Law Lenses

At first we will look at a simple refraction, one that happens at a flat boundary. To the left is a picture of such a situation. Here a single ray of light strikes a boundary between two mediums and is refracted.

Note that the angle of refraction is smaller than the angle of incidence.

Here are descriptions for the terms in the above diagram:

• The ray of light which travels through the incident, or first, medium and strikes the boundary, or interface, is called the incident ray.
• The ray of light which travels into the refracted, or second, medium and leaves the interface is called the reflected ray.
• A line perpendicular to the surface is imagined at the point of refraction. This line is called a normal. In this context the word normal means perpendicular. In the above diagram the normal is colored blue.
• The angle between the incident ray and the normal is called the angle of incidence, or the incident angle.
• The angle between the refracted ray and the normal is called the angle of refraction, or the refracted angle.

The above picture demonstrates the general behavior of a light ray as it travels from air into some transparent medium such as water or glass.

We will often describe the light beam as bending toward the normal or away from the normal. The following picture, much like the one directly above, shows light bending toward the normal. Again, this is the general behavior for light going from air into water or glass.

Here light bends toward the normal. The red arrow is meant to show what we mean by the phrase 'bending toward the normal.'

Notice that in these conditions the angle of refraction is smaller than the angle of incidence.

This next picture shows light bending away from the normal. This would be the general behavior for light going from water or glass into air.

Here light bends away from the normal. The red arrow is meant to show what we mean by the phrase 'bending away from the normal.'

Notice that in these conditions the angle of refraction is larger than the angle of incidence.

The situations where light bends away from the normal can become more complicated than those where light bends toward the normal. This is because the angle of incidence can become large enough so that the angle of refraction, being even larger, can reach 90 degrees. This eliminates the possibility that the ray will enter the refracted medium. These conditions demonstrate what is known as a critical angle.

Almost all of the time when light changes mediums, it changes direction. However, if it strikes the interface with an angle of incidence of 0 degrees, then no bending will occur. The picture at the left shows this

Also, there are a few conditions where light travels from one medium into another and does not bend for any angle of incidence. This happens when the two mediums are different substances, but have the same optical density. This situation will be covered in the explanation of Snell's law, which has its own section.

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When light passes from a less dense to a more dense substance, (for example passing from air into water), the light is refracted (or bent) towards the normal.

The normal is a line perpendicular (forming a 90 degree angle) to the boundary between the two substances. The bending occurs because light travels more slowly in a denser medium.

A demonstration of refraction can be conducted at home in a dark room. All that is needed is a flashlight, a clear glass filled with water and a small mirror.

• Figure (a): Shine the light directly into the glass. If the light strikes the water straight on (or parallel to the normal), no bending occurs and it simply passes directly into the water undisturbed, leaving only a straight beam of light all the way to the bottom of the glass.

• Figure (b): Shine the light into the glass at an angle. As the light enters the water, it is refracted. Since the light is passing from air (less dense) into water (more dense), it is bent towards the normal. The beam of light would appear to bend at the surface of the water.

• Figure (c): Place a mirror at the bottom of the glass of water and again shine the light into the glass of water at an angle. As light initially enters the water, it is refracted as in figure (b) and then reflected off the mirror (at the bottom of the glass). Upon exiting the water, the light is bent away from the normal as it passes from water (more dense) and into air (less dense). The light would leave the flashlight, bend at the surface of the water, reflect off the mirror at the bottom of the glass and move towards the surface, where it would bend outward at the same angle it bent in on the way in.

Refraction is the bending of light (it also happens with sound, water and other waves) as it passes from one transparent substance into another.

This bending by refraction makes it possible for us to have lenses, magnifying glasses, prisms and rainbows. Even our eyes depend upon this bending of light. Without refraction, we wouldn’t be able to focus light onto our retina.

Change of speed causes change of direction

Light refracts whenever it travels at an angle into a substance with a different refractive index (optical density).

This change of direction is caused by a change in speed. For example, when light travels from air into water, it slows down, causing it to continue to travel at a different angle or direction.

How much does light bend?

The amount of bending depends on two things:

• Change in speed – if a substance causes the light to speed up or slow down more, it will refract (bend) more.
• Angle of the incident ray – if the light is entering the substance at a greater angle, the amount of refraction will also be more noticeable. On the other hand, if the light is entering the new substance from straight on (at 90° to the surface), the light will still slow down, but it won’t change direction at all.

Refractive index of some transparent substances

All angles are measured from an imaginary line drawn at 90° to the surface of the two substances This line is drawn as a dotted line and is called the normal.

If light enters any substance with a higher refractive index (such as from air into glass) it slows down. The light bends towards the normal line.

If light travels enters into a substance with a lower refractive index (such as from water into air) it speeds up. The light bends away from the normal line.

A higher refractive index shows that light will slow down and change direction more as it enters the substance.

Lenses

A lens is simply a curved block of glass or plastic. There are two kinds of lens.

A biconvex lens is thicker at the middle than it is at the edges. This is the kind of lens used for a magnifying glass. Parallel rays of light can be focused in to a focal point. A biconvex lens is called a converging lens.

A biconcave lens curves is thinner at the middle than it is at the edges. Light rays refract outwards (spread apart) as they enter the lens and again as they leave.

Refraction can create a spectrum

Isaac Newton performed a famous experiment using a triangular block of glass called a prism. He used sunlight shining in through his window to create a spectrum of colours on the opposite side of his room.

This experiment showed that white light is actually made of all the colours of the rainbow. These seven colours are remembered by the acronym ROY G BIV – red, orange, yellow, green, blue, indigo and violet.

Newton showed that each of these colours cannot be turned into other colours. He also showed that they can be recombined to make white light again.

The explanation for the colours separating out is that the light is made of waves. Red light has a longer wavelength than violet light. The refractive index for red light in glass is slightly different than for violet light. Violet light slows down even more than red light, so it is refracted at a slightly greater angle.

The refractive index of red light in glass is 1.513. The refractive index of violet light is 1.532. This slight difference is enough for the shorter wavelengths of light to be refracted more.

Rainbows

A rainbow is caused because each colour refracts at slightly different angles as it enters, reflects off the inside and then leaves each tiny drop of rain.

A rainbow is easy to create using a spray bottle and the sunshine. The centre of the circle of the rainbow will always be the shadow of your head on the ground.

The secondary rainbow that can sometimes be seen is caused by each ray of light reflecting twice on the inside of each droplet before it leaves. This second reflection causes the colours on the secondary rainbow to be reversed. Red is at the top for the primary rainbow, but in the secondary rainbow, red is at the bottom.

Use these activities with your students to explore refration further:

• Investigating refraction and spearfishing – students aim spears at a model of a fish in a container of water. When they move their spears towards the fish, they miss!
• Angle of refraction calculator challenge – students choose two types of transparent substance. They then enter the angle of the incident ray in the spreadsheet calculator, and the angle of the refracted ray is calculated for them.
• Light and sight: true or false? – students participate in an interactive ‘true or false’ activity that highlights common alternative conceptions about light and sight. This activity can be done individually, in pairs or as a whole class.

Learn more about different types of rainbows, how they are made and other atmospheric optical phenomena with this MetService blog and Science Kids post.

Learn more about human lenses, optics, photoreceptors and neural pathways that enable vision through this tutorial from Biology Online.