Its now time to see the light….

A lens is a curved transparent material that is smooth and regularly shaped so that when light strikes it, the light refracts in a predictable and useful way.

Made of transparent glass or very hard plastic

Light undergoes two refractions: the first on entering the lens and the second on leaving the lens

The index of refraction of a lens is greater than that of air

When light passes from air into the lens, the light ray refracts away from the lens surface and towards the normal

When the light passes out of the lens at a an angle, the light rays refract again, this time bending away from the normal

Converging Lens (convex)

Diverging Lens (concave)

Axis of Symmetry: an imaginary vertical line drawn through the optical centre of a lens

Principle Axis: an imaginary line drawn through the optical centre, perpendicular to the axis of symmetry

F and F’: Both kinds of lenses have two principal focuses. The focal point where the light either comes to a focus or appears to diverge from a focus is given the symbol F, while that on the opposite side is represented by F’

Focal length (f): the distance from the axis of symmetry to the principal focus measured along the principal axis. There are two equal focal lengths since light behaves the same when travelling in either direction through a lens.

Secondary Pricipal focus (F’)

Optical Centre

Thicker at the centre of the lens than at the edges

Convex lenses are able to form a real image on a screen

A common example is the magnifying glass

1. The first ray of a convex lens ray diagram travels from the tip of the object parallel to the principal axis. When it emerges from the lens it passes through the principal focus.

2. The second ray travels from the tip of the object through the optical centre of the lens and is not refracted.

3. Draw the real image where the rays appear to intersect.

Thinner in the centre than at the edges The image formed is always upright,

smaller than the object, and virtual Image is always located on the object

side of the lens

1. The first ray of a concave lens ray diagram travels from the tip of the object parallel to the principal axis. When it emerges from the lens, it appears to come from the principal focus.

2. The second ray travels from the tip of the object through the optical centre of the lens, and is not refracted.

3. Draw the virtual image where the rays appear to intersect.

Ray diagrams may help one determine the approximate location and size of the image but will not provide numerical information about image distance and image size

To obtain this type of numerical information, it is necessary to use the Lens Equation and the Magnification Equation.

Both equations usethe variables illustratedin this diagram

A thin lens is a lens that has a thickness that is slight compared to its focal length. You can assume that all refraction takes place at the axis of symmetry for ray diagrams.

The thin lens equation expresses the quantitative relationship between the object distance (do), the image distance (di), and the focal length (f). The equation is stated as follows:

Keep in mind when working with this equation: A concave lens has a negative (-) focal point

and a negative (-) distance to the image A convex lens has a positive focal point (+)

and A positive distance to the image, if the image is

real A negative distance to the image, if the image is

virtual Convex and concave lens’ have positive

distances to the object

The magnification equation relates the ratio of the image distance and object distance to the ratio of the image height (hi) and object height (ho). The magnification equation is stated as follows:

Depending on how far away an object is the focus point behind the lens will change.

The lens must be moved toward or away from the camera image sensor/film so the image is clear

Interactive Ray Diagram Applet

More detailed explanations of concepts and practice questions