LensesA quick recap of some of the major points from last lesson

Incident rays(parallel)

F

Principal focus

f(focal length)

When parallel rays of light fall on a converging lens 1) They all meet at the Principal focus2) The ray passing through the centre of the lens (C) does not

change direction

These two simple ideas allow us to predict where an image will be formed

C

LensesRule 1: Parallel rays of light are refracted through the principal focus F

F F 2F2F

Central axis of lens

In Ray Diagrams and only in Ray Diagrams we don’t draw the rays bending on entering and leaving then lens. We draw on change of direction when the ray reaches the central axis of the lens.

Ray Diagrams for convex lenses

Lenses Ray Diagrams for convex lenses

Rule 2: Rays of light passing through the centre of the lens travel straight on

F F 2F2F

The centre of the lens

Central axis

It is best to draw ray diagrams on graph paper.

Ray diagrams can be drawn to scale with 1cm on the graph paper representing a much larger distance.

Lenses Ray Diagrams for convex lenses

Lets get drawingYou will need

1. A 30 cm ruler2. A sharp pencil3. A4 graph paper

A4 Graph paper

Draw a line along the centre of the graph paper

Lenses

You are going to draw a ray diagram to show the position and size of the image of an object 4cm tall that is placed 30 cm from a lens which has a focal length of 10cm.

First you need to write down all the relevant information Click on reveal when you have done so.

Reveal

Lenses

You are going to draw a ray diagram to show the position and size of the image of an object 10cm tall that is placed 30 cm from a lens which has a focal length of 10cm.

First you need to write down all the relevant information Click on reveal when you have done so.

Focal length of lens = 10cmObject height = 10cmObject to lens distance = 30cm

This is the information we need to draw the ray diagram.Our scale will let 2 cm on the graph paper represent 5cm.

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Draw in the central axis of the lens And mark on either side of the lens the postion of F and 2F

Graph paperCentral axis

FF 2F2F

4cmon graph paperrepresents 10 cm

4cm 4cm4cm

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Now draw in our object to scale. 30cm from the lens is represented by 12cm on our scale10cm height is represented by 4cm on the graph paper

Graph paperCentral axis

FF 2F2F

Object

4cm

12cm on graph paperrepresents 30 cm

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Now follow rule 1.Draw in the path of a parallel ray from the top of the object

Graph paperCentral axis

FF 2F2F

Object

The parallel ray must pass through F on leaving the lens

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Now draw in a ray from the top of the object that passes through the centre of the lens Remember this doesn’t change direction.

Graph paperCentral axis

FF 2F2F

Object

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The image will form where the two rays meet.The size of the object and its distance from the lens can be measured on the graph paper.

Graph paperCentral axis

FF 2F2F

Object

The image

The image is inverted(upside down) anddiminshed

LensesOn the graph paper measure from the centre of the lens to the image. Remembering the scale we used predict what the real distance would be. Write it down

Now measure the height of the object and calculate the magnification

Magnification = height of image

height of object

Write this down.Your teacher will have set up this experiment for you. Measure the actual lens to image distance and the magnification.How close were your predictions?

Lenses

If you have time you can draw some more diagrams.Try them with the object 20cm, 15cm and 5cm away from the lens.

Now test your predictions by setting out the experiment as demonstrated by your teacher.Remember all distances are measured to or from the lens.

Some real examples of ray tracing

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Object further than 2F away from lens

Image is real, inverted and diminished

Uses: in a camerain your eye – yes the image on your retina is upside down but your

brain corrects for this!!

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Object placed at exactly 2F

Image is real, inverted but the same size as the object

LensesObject placed between F and 2F from the lens

Image is real, inverted and larger than the object (magnified)

Uses: projectors in cinemas and in the classroom

LensesThe object is placed between the lens and F.

The image is virtual (can not be put on a screen, forms on the same

side of the lens as the object), it is upright and magnified.

Uses: Magnifying glass

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