PHYSICS 3PHYSICS 3

Convex LensesConvex LensesLesson objectivesLesson objectives

►Understand what effect Understand what effect convex lenses have on light convex lenses have on light rays.rays.

►Be able to draw ray Be able to draw ray diagrams for convex lenses.diagrams for convex lenses.

Saturday 22 April 2023Saturday 22 April 2023

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

Biconvex LensesBiconvex LensesBiconvex lenses are converging lenses.

When incoming parallel light rays are incident upon a biconvex lens...

The rays are focussed to the principle focus

f – focal length, distance from F to lens

F – principle focus

F

Drawing Ray Diagrams (1)Drawing Ray Diagrams (1)► Draw one ray from the top of the object Draw one ray from the top of the object

parallel to the centre axis. This is parallel to the centre axis. This is refracted through the principal focus.refracted through the principal focus.

► Draw a second ray through the centre Draw a second ray through the centre of the lens. This passes straight on – it of the lens. This passes straight on – it is not refracted at all!is not refracted at all!

► Where the rays meet the image is Where the rays meet the image is formed.formed.

Drawing Ray Diagrams (2)Drawing Ray Diagrams (2)

Two light rays leave the object O and pass through Two light rays leave the object O and pass through the lens. Where they meet an image I is produced. the lens. Where they meet an image I is produced. This is a REAL, DIMINISHED and INVERTED image.This is a REAL, DIMINISHED and INVERTED image.

Lens

Your Ray Diagram 1Your Ray Diagram 1A convex lens has a focal length of A convex lens has a focal length of

4 cm and an object 3 cm high is 4 cm and an object 3 cm high is placed 8 cm in front of the lens placed 8 cm in front of the lens (i.e. at 2F).(i.e. at 2F).

Find the position, size and nature of Find the position, size and nature of the image formed.the image formed.

Convex Lens Ray Diagrams – Object at Convex Lens Ray Diagrams – Object at 2F2F

Image is real, inverted and the same size as the object. So its 8 cm from the lens and 3 cm high.

Your Ray Diagram 2Your Ray Diagram 2A convex lens has a focal length of A convex lens has a focal length of

4 cm and an object 2 cm high is 4 cm and an object 2 cm high is placed 6 cm in front of it.placed 6 cm in front of it.

Find the position, size and nature of Find the position, size and nature of the image formed.the image formed.

Convex Lens Ray Diagrams – Convex Lens Ray Diagrams – Object Between 2F and FObject Between 2F and F

The image is 12 cm from the lens, 4 cm high and is real, inverted and magnified.

Your Ray Diagram 3Your Ray Diagram 3A convex lens has a focal length of A convex lens has a focal length of

6 cm and an object 1 cm high is 6 cm and an object 1 cm high is placed 4 cm in front of it.placed 4 cm in front of it.

Find the position, size and nature of Find the position, size and nature of the image.the image.

Convex Lens Ray Diagrams – Convex Lens Ray Diagrams – Object Between F and CObject Between F and C

The image is 12 cm in front of the lens, 3 cm high and is virtual, upright and magnified.

1. Which statement is true?

A. Virtual images can be projected onto screens

B. Erect images are upside-down

C. Concave mirrors are diverging

D. Biconcave lenses are diverging

Optics TestOptics Test

2. Which statement is true?

A. Diminished images are smaller than the object

B. Convex mirrors are converging

C. Biconvex lenses are diverging

D. Real images can not be projected onto

screens

Optics TestOptics Test

Optics TestOptics Test3. What optical device is shown?

A. Biconvex lens

B. Biconcave lens

C. Convex mirror D. Concave mirror

4. What do you think happens when…4. 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?Emerge as a parallel beam if they pass though the focal point (F).

F

F

SummarySummary

Object Object positionposition

Image Image PositionPosition

Real or Real or virtualvirtual

Magnified Magnified or or

diminisheddiminished

Inverted or Inverted or erecterect

>2F>2F

at 2Fat 2F

between 2F between 2F and Fand F

at Fat F

between F between F and lensand lens

between F and 2F

at 2F

> 2F

at infinity

same side as object

virtual

real

real

real

magnified

magnified

same size

diminished

upright

inverted

inverted

inverted

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.

Refraction : lenses1. 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.

ResultsResults

Object Object positionposition

Image Image PositionPosition

Real or Real or virtualvirtual

Magnified Magnified or or

diminisheddiminished

Inverted or Inverted or erecterect

>2F>2F

at 2Fat 2F

between 2F between 2F and Fand F

at Fat F

between F between F and lensand lens

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.

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

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

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

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 : lenses

2F F F 2F

Magnification = Distance from lens to image

Distance from object to lens

Refraction : lenses

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].

Convex Lens Ray Diagrams – Convex Lens Ray Diagrams – Object Beyond 2FObject Beyond 2F

Image is real, inverted and diminished

Your Ray DiagramsYour Ray DiagramsDraw ray diagrams with your object Draw ray diagrams with your object

positioned:positioned:a)a) At 2F – describe the image,At 2F – describe the image,b)b) Between F and 2F – describe the Between F and 2F – describe the

image,image,c)c) Between the lens and F – describe Between the lens and F – describe

the image.the image.

Optics TestOptics Test3. What optical device is shown?

A. Biconvex lens

B. Biconcave lens

C. Plane mirror D. Concave mirror