1. What have these all got in common?

2. Learning Goals:
To be able to compare the path of light when it enters a thin converging or a thin diverging lens.
To be able to draw simple ray diagrams to show the principle axis, focal point, focal length and focus.
To be able to explain the effect of increasing the thickness of a lens determines its strength.

3. Diverging Converging Causes rays to disperse from a central focus.
Brings parallel rays into focus (causes them to converge to a point)

4. Types of lenses Convex (Converging) lens
The principal focus is where rays ___________ to the principal axis ___________ (meet). We can find the principal focus by finding the _____________ of the ___________ of a distant object on a screen from the optical centre of the lens.
The _________ __________ is the distance from the centre of the lens to the principal ________ along the principal axis.
parallel
converge
distance
image
Focal
length
focus

5. Types of lenses Concave (Diverging) lens
The light __________ (spreads out) as if it has come from the principal focus.
diverges

6. The wider the lens the shorter the focal length

7. What effect does a concave lens have on parallel rays of light?
Principal Axis F
Causes them to disperse, so that they will never converge.
Thus when we talk about the focal point for a diverging lens it is a virtual image and must be found by extending the diverging rays backwards.

8. Real
Virtual
Which type of image do you need in a projector or camera?

9. Experiment: Finding the focal length of a lens
Experiment: Finding the focal length of a lens. Method Find the focal length (F) of each of the lenses by focussing a distant object ‘at infinity’ onto the white screen. The distance between the midpoint of the lens and the screen is the focal length.

10. Experiment: Investigating the image size and position with a convex lens.
Method: Using the lens with focal length F=15cm investigate the image formed to complete the table. Use the words, enlarged, same size, diminished, upright or inverted.
Object position
Type of image
Real/Virtual
Inverted/ Non-inverted
Position of image
Further than 2F
Smaller/ Diminished
Real
Inverted
Between 2F and F
At 2F
At F
Closer than F

11. Object position
Type of image
Real/Virtual
Inverted/ Non-inverted
Position of image
Further than 2F
Smaller / Diminished
Real
Inverted
Between 2F and F
At 2F
Same sized
Larger / Enlarged
At F
More than 7F
Closer than F
Larger/ Enlarged
Virtual
Non-inverted/ Upright
On the same side as the object

12. Extension
Using p152/3 (Complete Physics) draw a ray diagram to show how a convex lens produces an image if the object is at 2F and as the object moves closer or further away from the lens.

13. Diverging or Converging Lens

14. Learning Goals
By the end of this lesson you should be able to…
To be able to draw complex ray diagrams to compare the image formed with the object seen.
To be able to compare the formation of real and virtual images.
To be able to explain the path of light in a magnifying glass and a telescope.

15. Name the feature... E A C A = B = C = D = E = Focal Length
Divergent Lens
Focus
Principle Axis
Ray

16. Drawing Ray Diagrams
Diagrams should be drawn to scale
Draw object as arrow pointing upwards
Construct diagram using two rays

19. Drawing Ray Diagrams 6 Steps... Draw your lens and a principle axis.
Mark your Focal point (F) on both sides of the lens using information given.
Draw your image as an arrow up from the principle axis.
Draw one ray going through the centre of the lens, undeflected.
Draw a second ray parallel to the principle axis, deflecting through the Focus once it has passed through the lens.
Look for where the 2 rays meet!

20. object F F
Focal Length = 9cm

21. Drawing Ray Diagrams 6 Steps... Draw your lens and a principle axis.
Mark your Focal point (F) on both sides of the lens using information given.
Draw your image as an arrow up from the principle axis.
Draw one ray going through the centre of the lens, undeflected.
Draw a second ray parallel to the principle axis, deflecting through the Focus once it has passed through the lens.
Look for where the 2 rays meet!

22. How can we describe the image formed?
object F
image F
How can we describe the image formed?

23. How do we describe our image?
Magnified, same size or reduced
Upright or inverted
Real or virtual

24. Focal length of lens = 4cm Object height = 2cm
Drawing Ray Diagrams
Focal length of lens = 4cm
Object height = 2cm
Draw accurate ray diagrams on graph paper to show & describe the image formed in these situations.
Object distance (from lens) = 7cm
Object distance (from lens) = 2cm

25. This image is magnified, erect and virtual
eye
This image is magnified, erect and virtual

26. The magnification of an image is given by:
Magnification = Image size
Object size

27. Work through Uses of lenses sheet using p 156 – 157 of the text book.
STICK into your book

28. Focussing a Camera

29. Real or Virtual? Mirror Projector Binoculars Magnifying glass
Microscope
Camera
Eye
Remember with a virtual image the rays will never really converge but it will appear that the image you are seeing converges behind the lens.