A wave transfers energy from on place to another without transferring matter in the process. For example, when a pebble is dropped into a pond, circular ripples move outward on the surface of the water. Any object on the surface of the water (e.g. a leaf) would only bob up and down, not moved from its position (because water waves are transverse waves).

Wave motion is the transfer of energy from one point to another. We can demonstrate this by hanging an object on a stretched string and then sending a pulse down the string (by moving it up and down quickly like a wave). When the pulse meets the suspended object, the object bobs up and down for a moment.

A wavefront is a surface over which an optical wave has a constant phase. The wave front at any instant is defined as the locus of all the particles of the medium which are in the same state of vibration.

- The top most parts of a wave are called crests and the bottom are called troughs.

- The amplitude of a wave is its maximum distance from the equilibrium.

- The wavelength of a wave is the distance between a point on one wave and the same point on the next wave

- This can be from one crest to the next crest or from one trough to the next trough. It is symbolised by the Greek alphabet lambda, λ.

- The frequency of a wave is the number of waves produced each second. It is denoted by the letter f and is measured in hertz (Hz).

For transverse waves, the displacement of the medium is perpendicular to the direction of propagation (pulsing) of the wave i.e. the displacement and propagation are at right angles. A ripple on a pond and a wave on a string are examples of this.

In longitudinal waves, the displacement of the medium is parallel to the propagation of the wave i.e. the displacement and propagation are in the same direct. Sound waves are an example.

When a wave hits a smooth plane barrier, it is reflected so that its angle of incidence equals the angle of reflection.

When a wave hits a rough plane barrier, it is mostly refracted; the wave bends and travels through the plane barrier instead of reflecting.

However, the wave's speed, direction and wavelength will change. The frequency of the wave will remain constant.

The speed of a wave (v) is its frequency (f) multiplied by its wave length (λ).

When waves select off a plane barrier, there is no travelling through a different medium, so the density of the medium is the same, which means that the speed and wavelength of the waves remain the same. The angle between the incident wave and the normal will be equal to the angle between the reflected wave and the normal.

A wave refracts because the medium it is now travelling through has a different density, which causes its speed to change. When this speed changes, the wavelength charges and consequently its direction changes too.

For example: when waves hit water, which has an uneven surface, it will refract. The water is denser than air (water molecules are more closely packed together than air molecules), so the speed of the wave decreases, and so does its direction.

The same thing happens to waves as they travel through small holes, or push past obstacles. As the water waves go through the gap they spread out, this is called diffraction. The longer the wavelength of the wave the larger the amount of diffraction.

Diffraction is often demonstrated with water waves in a ripple tank. A set of straight waves pass through a gap in a barrier. Curved waves come out the other side.

The effect is greater when the wavelength of the waves is similar in size to the gap or object size.

Notice that the wavelength is the same on both sides - spreading out does not alter the speed of the wave, so the wavelength is unchanged.

It is due to reflection (of light). Light rays will strike the mirror and reflect of it into our eye.

The optical image formed will be:

- the same size as the object

- upright

- virtual: a real image is formed on a screen when all of the rays from a single point on an object strike a single point on a screen. A virtual image is produced when rays of light come into our eyes and appear to com from an object, when in reality, that object is not present at the apparent position of the source. So, due to the direction the light rays come from, our brain makes us think that the object is in one place when in reality, it is in another. The most common example of virtual images are reflections in plane mirrors.

The law of reflection states that the angle of incidence is always equal to the angle of reflection.

The light ray that hits the mirror from the object is called the incident ray (ray of incidence) and the ray that is reflected off the mirror into our eyes is called the reflected ray (ray of reflection).

The normal is the line that runs perpendicular to the mirror between the two rays. The angle of incidence is the angle between the normal and incident ray, and the angle of reflection is the angle between the normal and reflected ray.

When you put a pencil in a full glass of water, the submerged pencil, when viewed from the side, seems to be bent in the water. This is because the light rays bend when they enter a different medium to the one they were originally travelling in - in this example, they enter the air from the water. This phenomenon is called refraction.

Light changes speed as it passes from one medium to another. This is called refraction.

The frequency of light does not change as it refracts.

The refractive index of a material is a measure of the change in the speed of light as it passes from a vacuum (or air as an approximation) into the material.

U2

In the equation above:

- U2 is the refractive index of the material

- U2 is the speed of light in a vacuum

- U2 is the speed of light in a material

The bigger the refractive index, U2, the slower the light travels in that material, i.e. the smaller the U2 is.

When light travels from a less dense material to a more dense material, the light ray bends towards the normal. That is, the angle of incidence > angle of refraction.

When light travels from a more dense material to a less dense material, the light ray bends away from the normal. That is, the angle of refraction > angle of incidence.

- V2 is the refractive index of the material

- V2 is the speed of light in a vacuum

- V2 is the speed of light in a material.

When the angle of incidence is equal to or more the critical angle, total internal reflection is achieved and the ray of light is completely internally reflected.

Once the angle of incidence reaches the critical angle value, total internal reflection occurs. All of the light rays are reflected back into the original medium.

The critical angle is the angle of incidence beyond which rays of light passing through a denser medium to the surface of a less dense medium are no longer refracted but totally internally reflected.

In other words, it is the angle between the incident ray and normal, when the refracted ray is parallel to the medium's surface.

Optical fibres are cables made from high quality glass or plastic. When light enters one end of an optical fibre cable, it undergoes total internal reflection until it reaches the end of the cable.

Digital signals are emitted as pulses of light and reflected along the cable until they reach their destination. This method allows the transmission of information at high speeds.

When infrared is used, the cables are lined with glass. When visible light is used, the cables are lined with plastic.

The visible light and plastic combo is cheaper but lower quality - the different colours in visible light will have slightly different critical angles, so the signal becomes more distorted the longer it travels. Also, at a molecular level, plastic will have more irregularities than glass, which affects the direction that the rays will be reflected.

This is why the glass and IR combo is always used when the information has to be transmitted over long distances.

A lens is a transparent object that causes the light that passes through it to refract. A converging lens that is curved on both sides (there are two types of converging lens - concave and convex).

A converging lens causes the light rays that are travelling parallel to its principal axis to refract and cross the principal axis at a fixed point called the focal point.

It should also be noted that converge is a word that describes the tendency of two lines to meet.

Principal focus or the focal point is the point where rays of light travelling parallel to the principal axis intersect the principal axis and converge.

The focal length is the distance between the centre of the lens and the focal point.

Based on where the object is placed, the image formed could be inverted/upright, real/virtual and reduced/enlarged.

Inverted means that the image is upside down and upright obviously means that the image is the right way up.

Real images are formed on a screen (or another detector, like your eyes) when all of the light rays from a single point on an object hits a single point on the screen. In virtual images, on the other hand, are produced when light enters our eyes that appear to come from a real object when in reality, there is no object at the apparent source.

Reduced means that the image is smaller than the object and enlarged means that the image is larger than the object.

The position of the object is described in terms of the number of focal lengths between the optic centre and the object.

There are also some important things you should note when drawing ray diagrams:

Light rays are always drawn as straight lines with arrows along them. Virtual light rays are drawn the same way, but instead of a solid line, the line is dashed. The arrows show us which direction the rays travel in.

You will also see some diagrams where they simply draw a line to represent the lens, however, how the light rays are refracted will change according to whether the lens is convex or concave.

This is why it is important you always draw the shape of the lens. You should also draw a dashed vertical line up through the centre of the lens.

It should also be noted that the arrow that represents a virtual image must be made of a dashed line.

In addition, a distance of one focal length from the optic centre is written as F, two focal lengths is 2F, three is 3F, etc. This distance is always measured along the principal axis.

When the object is more than 2 focal lengths away from the optic centre, the image formed will be inverted, reduced and real.

Note that the lens is called biconvex because it is convex on both sides.

When the object is less than 1 focal length away from the optic centre, the image formed will be upright, enlarged and virtual.

Here, our brains trick us into thinking that the rays came from a single point, the object - it creates a fake source point and thus a virtual image is formed.

This is how magnifying glasses work. The image is on the same side as the object (there is no mirror image) and it is enlarged and upright.

- When the image distance is positive, the image is on the same side of the mirror as the object, and it is real and inverted. When the image distance is negative, the image is behind the mirror, so the image is virtual and upright. A negative means that the image is inverted.

- Positive means an upright image.