Opticsor

The Physics of forming images with a Convex Lens

A convex lens is like a series of prisms that refract the light by different amounts

The quantity which dictates how much the light is converged is the focal length of the lens.

This is a rough method of measuring the focal length of a convex lens

Measure the distance from the lens to the image (wall or screen) to obtain a rough value of the focal length of the lens.

The distance between the lens and screen varies with focal length, i.e. the thicker the lens the shorter the focal length.

There are 3 rules for constructing ray diagrams:rule 1

Rule 2

Rule 3

Object at infinity

This is how the focal length is found

•Real image

•Upside down

Camera object further than 2 F away

•Real image

•Upside down

•Reduced in size

Projector object between F and 2F

•Real image

•Upside down

•enlarged

Magnifying Glass – Object within focal length

•Virtual image

•Right way up

•enlarged

The images produced by cameras and projectors need to be focussed onto a screen or film etc. To do this it is necessary to move the lens.

For example, if the object is moved towards the camera the lens would need to move away from the film.

NB•Virtual images cannot be projected onto a screen but are the right way up.•Real images can be projected onto a screen and are inverted.

Magnification

magnification = image size/object size

Example:The magnifying glass is used to view a jewel of size 3mm. The image size observed is 1.2cm. Calculate the magnification.

Mag = 1.2/0.3 = x4

Calculate the magnification for the Camera, Projector & Magnifying Glass using the sizes from your diagrams.

Draw out a ray diagram (on graph paper) for a projector with the following dimensions:

Focal length = 4cm

Distance of object from lens = 7cm

Height of the Object arrow = 3cm

Deduce the size of the image and hence the magnification of the projector.

Image =

Magnification = x