Refraction

Refraction

Refraction is the process by which a ray of light changes direction (refracts) when passing between two different mediums (materials). This direction change is caused by the light slowing down as it passes into the denser medium.

The following video shows a brief introduction to Refraction.

How to label Optics diagrams

The following diagram shows how to label all optics diagrams.

The most important line to draw on these diagrams in the Normal, a line drawn at right angles to the surface, which all angles are measured from.

If we look closely at the transition from air to glass, we can see that the angle of incidence is greater than the angle of refraction.

This is because:-

1. When the wave enters the more dense medium it slows down and refracts towards the normal.

2. When the wave enters the less dense medium it speeds up and it refracts away from the normal.


Rectangular Block

The above diagram shows how the path of light is refracted by the rectangular block.

The diagram shows:-

1. Angle b is equal to angle c (They form a geometrical "Z angle")

2. Angle a is equal to angle d (as long as only two media are involved)


Semi-circular Block

In the rectangular block, refraction occurs twice, but in a semi-circular block refraction occurs only once. This is because the light entering the curved side of the block enters along the normal, so no refraction occurs. Refraction only occurs at the plane glass boundary.

Triangular Prism

When a ray of white light passes through a triangular prism, the ray is refracted twice. The overall effect of these refractions is to cause the white light to be dispersed. This causes a spectrum to be observed.

Convex Lenses

Convex lenses are thicker in the middle than at the edges.

Convex lenses make the rays converge. The more curved the lens, the more refraction it causes.


Concave Lenses

Concave lenses are thinner in the middle than at the edges.

Concave lenses make the rays diverge. The more curved the lens, the more refraction it causes.


Power of a Lens

The the curvature of the lens affects the size of the focal length. The more curved the lens, the shorter the focal length will be. This is used to great benefit within Optics, especially as this allows for the correction of sight defects.

The power of a lens is used as a measure of how quickly the lens brings the light to a focus, and is described by the formula :-

P = 1 / f

Where :-

P - Power of a lens (Dioptres)

f - focal length (meters)

Note - In questions using this formula, it is possible to have negative power for a lens and therefore a negative focal length. Obviously a negative distance is not possible, the symbol is simply used to show lens type :-

Positive Power/Focal length - Convex Lens

Negative Power/Focal length - Concave Lens


Eye Defects

The diagram below shows the inside of a human eye, showing the major parts that allow vision:-

The key sections that allow vision are :-


Cornea - Transparent tough coat, most of refraction takes place here

Pupil - Gap at centre of Iris which allows light to enter the eye

Iris - Can alter the size of the Pupil to control how much light enters the eye

Lens - Flexible, has its shape altered by the ciliary muscles to focus light

Retina - Lining of the back of the eye, covered in light sensitive cells

Optic Nerve - Carries signals from the retina to the brain


In order for clear, crisp vision, light must be focussed correctly such that the light is brought to a focus point on the retina, as shown in the diagram below:-

In modern society, the wearing of glasses is commonplace. Their invention seems to have occurred during the late 1200s, but no one is really sure by whom.

There is, however, anecdotal evidence written by Pliny the Elder in 79CE that the Emperor Nero used an Emerald to watch gladiatorial contests, possible to correct for short sightedness.


Short Sight (Myopia)

The diagram above shows the effect of short sight on the rays of light entering the eye. Uncorrected, the lens is too powerful and the light is brought to a focus short of the retina, giving a blurry image.

By using a concave lens to diverge the light before it enters the eye, the light can be made to focus correctly on the retina.


Long Sight ( Hypermetropia )

The diagram above shows the effect of long sight on the rays of light entering the eye. Uncorrected, the lens is too weak and the light is brought to a focus long of the retina, giving a blurry image.

By using a convex lens to converge the light before it enters the eye, the light can be made to focus correctly on the retina.