Lenses

Lenses bend light to form images.

There are 2 types of lenses: Converging and Diverginglenses.

CONVEX LENS

A converging lens is also known as a convex lens

It is thick on the middle and thin on the edges

DIVERGING LENS

A diverging lens is also known as a concave lens

It is thin on the middle and thick on the edges

Ray diagrams

Parts of a ray diagramOptical centre: is the center of the lens.

Principal axis: is a straight line passing through the optical centre. It is not refracted.

Optical axis: is the line perpendicular to the principal axis and passing through the optical centre. It is used to show the position of the lens.

Principal focus: it is the point at which the rays of light meet (converge) after passing through the lens. It is also called the Focal point. Since the lens can converge light on both sides, it has focal points on either sides.

Focal length: is the distance between the optical center and the focal point.

Images An image that is:

smaller than the object is diminished

larger than the object is magnified

Upside down is inverted

Standing upright is upright

On the same side of the lens as the object is virtual

On the other side of the lens is real

Magnification The magnification of a lens is calculated as:

Magnification = height or size of image (I)

height or size of object (O)

Exercise Use a hand lens to draw an image of the given spacemen. Calculate the magnification.

Drawing ray diagramsObject located beyond 2F1. Draw the principal and optical axes.2. Locate the first and second focal points: F and 2F on either

sides of the optical axes.3. Draw you object beyond 2F and label it ‘O’4. Draw a ray from the top of the object and passing through the

optical center. Label it R1.5. Draw a second ray, R2, parallel to the principal axis up to the

optical axis and then passing through F.6. Draw a third ray, R3, passing through F up to the optical axis

and then parallel to the principal axis.7. All the three rays meet at the image location.8. Measure the height of the object and image.9. Calculate magnification.10. What are the features of the image formed?

Prep workDraw ray diagrams for the object located on the

following positions;

1. At 2F

2. Between 2F and F

3. At F

4. Between F and optical centre.

State the properties of the images formed and calculate magnification.

Thank you!