Visible Light

Reflection

Reflection is the process by which light waves bounce of a surface. In most situations, this is a disorganised process and results in the scattering of the light. In this situation, no image can be formed as the light has been dispersed in lots of different directions. This is known as__ diffuse reflection__.

Shiney mirror surfaces, however, reflect light in an organised manner and we are able to form an image or focus the light rays. This is known as specular reflection.

Such mirrors are used for looking at our own reflection to shave or do our make-up etc and are known as vanity mirrors. Some mirrors are used for focusing light in telescopes too.

Law of Reflection

For a plane mirror (flat mirror) the angle that the light hits the mirror (angle of incidence) will equal the angle of reflection. This is measured from an imaginary line at 90° to the mirror called the normal.

Θ_r_ = θi

This law holds true for all mirrors at the point of incidence of the ray of light. It is easy to see this in a plane mirror, but it is also true for concave and convex mirrors.

Refraction

Refraction is the bending of light as it moves from one transparent material to another. The differences in density of the materials affect the speed of the light. It slows down in denser materials. If the ray hits the surface at 90°, the ray slows down together and is not refracted. If it hits the surface at an angle then it slows down at different rates across its wavefront, thus it bends.

As the angle of incidence the degree of bend increases. At a specific angle, the ray of light will be refracted along the boundary between the materials. This is known as the critical angle. At angles less than the critical angle the light will leave the material and enter the second material. This angle is different for different combinations of materials.

At angles greater than the critical angle the light will be bent back inside the original material. This is known as Total Internal Reflection. This should not be confused with specular reflection which requires a shiny surface.

Try this: Half fill a glass with water and place a pencil into the glass, observe how the reflection of the light leaving the water makes the pencil look like it is bent in the middle.

This process also makes the bottom of swimming pools look closer than they really are when you are standing on the side. Have you ever jumped into a pool on holiday expecting to be deep enough to stand in, only disappear below the surface, well that’s thanks to refraction.

The image is not manipulated, it is due to refraction.

Colour

The colour of light depends upon its wavelength, blue light has a wavelength between 490 - 520 nm, yellow is 590 - 560 nm, for example.

The properties of materials can affect the way different wavelengths of the same wave behave when they interact with the material.

White light is made up of all the colours of the rainbow, but if it has to pass through a coloured filter then all but one part of the spectrum is absorbed, only a limited range of wavelengths is transmitted.

This filter is transparent to light waves in the 490 to 520 nm range but absorbs all other wavelengths.

The apparent colour of an object is partly determined by the range of wavelengths that are reflected by the material. If all are reflected then the object appears to be white, none and it looks black. This differential reflection helps to create our colour visual perception.

Visible light and the colours are just one small part of the electromagnetic spectrum. All the ideas about reflection and refraction that apply to visible light also apply to all the invisible part of the spectrum too. We think of visible light and colours being different only because they are the parts of the spectrum we can see.

Types of Lens

A lens is a shaped piece of transparent material that exploits the idea of refraction to bend light to perform various functions. This allows us to focus light to make glasses, contact lenses, cameras and cinema projectors to name a few simple examples.

There are two types of lens, converging and diverging. This describes the action they have on rays of light.

Converging (or convex) Lens

These lenses bring the rays of light together to a focus and form real images. A real image is one that can be projected onto a screen.

The degree of curve to the lens and its thickness affect the power of the lens. A more powerful lens will bend the light more and have a shorter focal length.

The material the lens is made from also affects the power of the lens.

Diverging (or concave) Lens

This type of lens makes the rays of light spread out, the move away from each other. This means that they do not come to a focus as they do in a converging lens.

The image the is formed is a virtual image. It is virtual because it cannot be projected onto a screen, but you can see the image on the other side of the lens if you look through the lens.

Looking at the ray diagrams for a lens, explain why the ray passing directly through the middle of the lens does not change direction.

has an angle of incidence with the lens of 90°
Explanation: Rays of light change direction when passing through a lens due to refraction. A ray of light striking the lens in the middle has an angle of incidence with the lens of 90°. Light is not refracted when it has this angle of incidence.

What sort of lens would you choose to us in a home TV projecting system, explain your answer?

converging
Explanation: A converging lens, because it will bring the image to a focus and the image will be real so it can be seen on a screen.

What happens to light passing from glass to air if the angle of incidence is greater than the critical angle, suggest an application for this.

Your answer should include: bent / back
Explanation: The light is bent back into the glass by total internal reflection. This process is used in the transmission of information in fibre optic cables.