M Manser

WAVES : Optics (triple science)

By the end of this presentation you should be able to: Identify the differences between converging and diverging lenses.

Draw diagrams showing the effect each lens on a parallel beam of light.

Identify the positions of the focus of both a converging and a diverging lens.

Know the difference between a real and virtual image.Construct ray diagrams to show the formation of real and virtual images by a converging lens.

Explain the use of a converging lens as a magnifying glass, and in a camera.

M Manser

Reviewing refraction

Remember that when light crosses into a different medium, its speed is affected and the direction of the wave changes as a result.

Two exceptions - If the wave is incident on the

surface at 90o , its direction will not change.

- If light is travelling from glass into air, and hits the boundary at an angle greater than the critical angle, it will undergo total internal reflection.

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Refraction by non-rectangular prisms.

Can you predict the path that this ray would follow through the glass prism shown?First draw the normal to the prism where the ray is incident on it.Then, considering that the ray will bend towards the normal…Now repeat the process, remembering that the ray will bend the other way as it emerges from the glass into the air.

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Lenses

The effect that lenses have on light can be explained by considering them to be constructed of different arrangements of prisms.

1. A converging lens2. A diverging lens

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How a parallel beam is affected by different lenses

principal axis

A parallel beam will

converge and be focussed at the focal point:

F.F

principal axis

A parallel beam will diverge. A

virtual focal point, F, can be found by

projecting the rays backward.

F

M Manser

Drawing ray diagrams for a convex lens

Remember that light is coming from the object in all directions and from all points on the object, but we only need to consider a few rays to be able to determine the location, orientation, and size of the image.

Light rays do change directions at each air/glass interface; but to make the ray diagrams easier to draw, we will only draw the rays changing direction at the imaginary line going vertically through the centre of the lens.

Consider these rules:1.Rays parallel to the principal axis are refracted through the focal point on the other side of the lens. 2.Rays through the focal point are refracted parallel to the principal axis. 3.Rays through the optical centre are undeflected.

M Manser

Applying the rules to scale diagrams

Choose the largest scale possible, given the data supplied (like object height, distance, focal length) and write it down.Do a rough sketch to determine where to position the lens and principal axis.Draw the principal axis and the lens.Use the scale to mark the focal point on each side of the principal axis.Now use the scale to accurately draw in the position and height of the object.

F F

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Using the rules for drawing ray diagrams

1. Rays parallel to the principal axis are refracted through the focal point on the other side of the lens.

2. Rays through the focal point are refracted parallel to the principal axis.

3. Rays through the optical centre are undeflected.

F F

Only two rays need to be drawn to determine where all rays would focus.

Once this point is found, draw in the image from the principal axis to that point.

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Uses of converging (or convex) lenses

Cameras …..p. 30 Projectors….p. 31 (not on syllabus)

Magnifying glasses…. p. 31 Telescopes …..p. 33 (not on syllabus)

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A simple camera

the outer case

The lens will focus the rays from the object on the film.

the filmRays from the top of the object will be focussed at the bottom of the film (and vice versa)

The brightness

of the image is

controlled by the stop,

which can be opened

to make the aperture

larger and let in more

light.

The sharpness

of the image is

controlled by the

shutter speed. Taking

pictures of moving objects require

short exposures.

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A magnifying glass

FF

Your eye will bring these rays to a focus on the same side as the object. It will be upright and magnified.

A magnifying glass uses a convex lens to produce a “virtual image”.

A virtual image is one where rays of light do not actually pass through the image (so a virtual image cannot be formed on a screen).

This is because the object is very close to the lens (between the focal point and the lens). The rays diverge after being refracted.

M Manser

More ray diagrams. A moving visual aid