Light is a form of electromagnetic energy that can be detected by the human eye, that's the key part of the definition.
Light travels in the form of a travsverse wave and as such may be defined in terms of a wave function; the simplest being f(
) = A sin. Phase sifts will be ignored.
Rectilinear propagation of light describes the fact that light travels in straight lines and you should remember this fact from studying the pin-hole camera.
What's the electromagnetic spectrum? Well here's a diagram to show you
But this still doesn't tell you how a ray or beam of light is produced. Light beams come in bundles of energy called photons and how are these produced?
Objects that produce light are called luminous objects and those that don't are non-luminous. Non-lumonous objects reflect light, that is why they are visible. Since visible light comes in many different frequencies or colours, and non-lumonous objects absorb some of these frequencies, most objects are seen in thir reflective colour. A green leaf will absorb all the colours except green. This frequency is reflected and detected by your eye and your brain says "green".
Transparent objects allow light to pass directly through the material and as such are studied.
Translucent materials scramble the light beams as they pass through and essentailly are not studied.
Opaque objects do not allow light beams to pass through instead reflecting lightwaves per colour of the object.
The quantum model of light is somewhat of a combination of the particle model and the wave model of light, since these two explinations or models cannot explain all of the phenomenae of light.
| Reflected Ray |  |
Incident Ray |
i always equals the reflected angle ri and r are measure from the normal
Key words and terms that you must be familiar with:
ray beam parallel beam converging beam diverging beam transparent translucent opaque
Have you ever noticed that when you put a spoon or straw in water that it seems split into two sections at the surface of the water? This phenomena is called refraction.
The speed of light is a function of the medium through which it travels. It has its greatest velocity in vaccuum 2.9979 x 108 m/s or 3.00 x 108 m/s (three digit accuracy). As the medium becomes more dense the speed of light decreases. See table 10.1 in your text book for exact values. The actual velocities are not gnerally used, however the ratio of speed of light in vacuum to speed of light in a particular medium is much more useful. Because the speed of light in a vacuum and air are almost the same, the term index of refraction will be defined as the ratio of the speed on light in air to the speed of light in the medium under study.
Because it is a ratio the index of refraction has no units. Usually a table of values is provided in your text book for index of refraction values.
As you should know light is made up of different colours, which all travel at more or less the same speed in vacuum and air. The important thing to note is that different colurs travel at different speeds in a material medium. The index of refraction for a specific colour is a constant for a specific medium but changes from colour to colour. This is why this happens.
and each colour bends at a different angle.
A ray of light passing from one medium to another will be bent on passing into the second medium.
| Incident Ray travelling in a less dense medium |
 |
Refracted Ray, bent towards the normal, passing into a more dense medium |
1. You can also switch between a ray and a beam and view reflected rays also.
Click Here
The relationship between the angle of incidence and the angle of refraction is called Snell's Law. The equations may be stated as follows:
The second equation is valid when you are passing from air to another medium because n1 is approximately equal to 1.00 and the n value quoted will be the index of refraction of the medium being studied. This concept is used in the lab exercise.
Everything remain the same when passing from a more dense material to a less dense one. As you can see in the diagram n1 is > than n2.
This situation will occur only when a beam or ray of light is trying to pass from a more dense medium into a less dense medium. Because the angle of refraction is bending away from the normal and as the angle of incidence increases, eventual the angle of refraction will become 90o. This incident angle is called the critical angle and the refracted beam travels parallel to the surface of the two mediums. As the incident angle is increased more the beam or ray becomes reflected inside the medium. This is called total internal reflection
To view this phenomena & see two uses just Click Here
Uses of total internal reflection are binoculars, periscopes fiber optic cable. Your text book has pictures of each.
For a more complete note of Fiber Optics
Click Here
Apparent depth is a phenomena based on light rays travelling from an object in one medium being refracted on passing into the second medium , causing what appears to be a bending of the object. See the above diagram of the straw in water. This causes the actual depth (d) to be viewed at a different position than where it actually is. The position of this image or apparent depth (d') is in the ratio of the two refractive indices of the two mediums. Because of the complexity of the math, only those situations where the observer and object are on the normal, will be computed.
The formula is d' = d (n2/n1) where n2 is the refracrtive index of the medium in which the observer is located and n1 is the refractive index of the medium in which the object is located.
More often than not this is a air to water situation.
Curved lenses are used to control and focus the bending of a beam of light.
There are two main classification of lenses: convex & concave.
Lens terminology that must be known: angle of deviation converging lens diverging lens principle axis axis of symetry focal length radius of curvature focal plane virtual image real image
It will be expected that you familiarize yourself with terms.
Without further adieu check these two out
Another applet Diverging Lens with Sliding Object
| object position | image position |
| at infinity | a single point at F |
| beyond 2F | real, inverted, smaller, between F and 2F |
| at 2F | real, inverted, same size, at 2F |
| between 2F and F | real, inverted, larger, beyond 2F |
| at F | no image; parallel rays |
| close than F | virtual, erect, larger, on same side of lens |
In this diagram parallel rays from infinity pass through a convex lens and are focused to a point. This point is the focal point. This technique can be used to experimentally determine the focal point of a convex lens.
The following table summarizes diverging lenses.
| object position | image position |
| at infinity | a single point at F on same side of lens |
| closer than infinity | virtual, erect, smaller, on same side of lens |
In this diagram parallel rays from infinity diverge and if traced backwards, thsy will meat at the focal point of this lens.
You must be able to draw ray diagrams given various sets of information and be able to deduce the unknown quanties. Taught item in class so follow closely and for review just go to 
On viewing you will see that this site shows you step by step how to draw a ray diagram for both a convex and a concave lens. Where the three rays intersect is where the image is formed. (Ignore the mirror drawings)
Thin lens equations are based on the following diagram
Working with and using these equation will be a taught item.
You must be aware of the sign convention used in lenses;
Instruments, devices that will be looked at are as follows:
A diagram of these devises may be found by Clicking Here
