© Boardworks Ltd 2003 Lenses. © Boardworks Ltd 2003 3.2 (c) Thin converging lens Core Describe the action of a thin converging lens on a beam of light.

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Lenses

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3.2 (c) Thin converging lens

• Core• Describe the action of a thin

converging lens on a beam of light• Use the term principal focus and

focal length• Draw ray diagrams to illustrate the

formation of a real image by a single lens

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• Supplement• Draw ray diagrams to illustrate the

formation of a virtual image by a single lens

• Use and describe the use of a single lens as a magnifying glass

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Using refraction : lenses summary

There are two main types of lens:

Convex Concave

Convex lenses work by bending [refracting] rays of light to a principal focus.

The distance from the centre of the lens to the principal focus [F] is called the focal length [ƒ].

The image formed by a convex lens is inverted [back-to-front and upside-down].

The thicker the lens, the shorter the focal length[ƒ].

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A lens can be thought of as a series of prisms.

The lens refracts all the rays to a point called the principal focus [F].

The distance between the centre of the lens and F is called the focal length [].

Imagine parallel rays of light from a distant object hitting the lens.

Draw normal lines [at 90° to the surface] for each ray.

Use the first refraction rule to work out the ray direction.

Draw normal lines where the rays enter the air [at 90º to the surface].

Work out the direction of the refracted rays using the second refraction rule.

When light enters a less dense medium [e.g. air], it bends away from the normal.

Using Refraction : lenses

When light enters a more dense medium [e.g. glass], it bends towards the normal.

F

ƒ

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What do you think happens when…

Parallel light rays strike a convex lens?They pass through the focal point of the lens.

Diverging light rays?Form a parallel beam if they pass though the focal point (F).

F

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Use a ruler to measure the distance between the lens and the screen - this is the focal length [ƒ].

Using Refraction : lenses - finding

ƒ

Chose a distant object [to get parallel rays of light].

Hold a plain white screen in one hand.

Hold the lens in the other hand and move it closer to the screen until a clear image appears.

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Refraction : lenses

1. Find the focal length [ƒ] of your lens.

2. Fix the lens to the centre of a metre rule and mark the distances F and 2F either side of the lens.

2F F F 2F

3. Place the candle >2F away from the lens and move the screen

until an image appears and record observations.

4. Repeat for the candle at 2F, between 2F and F, at F and between F and the lens.

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Results

Object position

Image Position

Real or virtual

Magnified or

diminished

Inverted or erect

>2F

at 2F

between 2F and F

at F

between F and lens

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Refraction : lenses

Object >2F away

O

2F F F 2F

I

The image [ l ] is formed between F and 2F away from the lens, is inverted and diminished.

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Object at 2F

O

2F F F 2F

I

The image [ l ] is formed at 2F away from the lens, is inverted and the same size.

Refraction : lenses

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Object between 2Fand F away

O

2F F F 2F

IThe image [ l ] is formed further than 2F away from the lens, is inverted and magnified.

Refraction : lenses

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Object at F away

O

2F F F 2F

The image [ l ] is formed at infinity - the rays never meet [we use this set-up for searchlights].

Refraction : lenses

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Object between F and lens

O

I

The VIRTUAL image [ l ] is formed on the same side of the lens as the object, is the right way up and magnified.

2F F F 2F

Refraction : A magnifying glass

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Results

Object position

Image Position

Real or virtual

Magnified or

diminished

Inverted or erect

>2F

at 2F

between 2F and F

at F

between F and lens

between F and 2F

at 2F

> 2F

at infinity

same side as object

virtual

real

real

real

magnified

magnified

same size

diminished

erect

inverted

inverted

inverted

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2F F F 2F

Magnification = Distance from lens to image

Distance from object to lens

Refraction : lenses

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To do

• All• P153 Answer all questions• Extended only• Draw an accurate ray diagram to

show a magnifying glass where f=10cm and the object is 5cm from the lens

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Learning check

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Which of the following is the most dense?

A. Air

B. Water

C. Glass

D. Lead

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When light changes direction as it moves from one medium to another we call this effect what?

A. Reflection

B. Refraction

C. Diffraction

D. Total internal reflection

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What happens to the speed of light as it moves from air into glass?

A. Decreases

B. Increases

C. No effect

D. Decreases and increases

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If a ray of light moves from air to glass parallel to the normal what happens?

A. No change in direction

B. It bends away from the normal

C. It bends towards the normal

D. It stops

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If light travelling through a medium has a speed of 150 000 000 m/s. What is the refractive index of the medium?

A. 2.6

B. 0.5

C. 2.0

D. 1.5

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Can you……

Draw ray diagrams depicting the refraction of light by lenses?

Write a sentence using the terms principal focus and focal length

Draw a ray diagrams to show how a lens forms an inverted image

Draw ray diagrams to illustrate the formation of a virtual image by a magnifying glass

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