THIN LENSES

Question 1

1. What can be said about Converging Lenses?

Answer 1

• these lenses are positive
• the lenses are thicker at the centre than at the rim
• these lenses converge a beam of parallel light to a
  real focus

Question 2

2. What can be said about Diverging Lenses?

Answer 2

• these lenses are negative
• they are thinner at the centre than at the rim
• these lenses diverge a beam of parallel light from a
  virtual focus

Question 3

3. What is the Focal Point of a thin lens with spherical surfaces?

Answer 3

• this is the point where the rays parallel to and near the
  central axis are brought to a focus
• a central axis is also known as an Optical axis
• this point is represented by the symbol f

Question 4

4. What can be said about the focus for converging and diverging lenses?

Answer 4

CONVERGING LENSES:
- the focus is real

DIVERGING LENSES:
- the focus is virtual

Question 5

5. What is the Focal Length?

Answer 5

• it is represented by a symbol f
• it is the distance of the principal focus from the lens

Question 6

6. What is the result of each lens being reversed without altering the rays?

Answer 6

• two symmetric focal points will exist for each lens

Question 7

7. What do all the rays that pass through the lens form?

Answer 7

• they form an image of a positive distance (d2) to the
  right of the lens

Question 8

8. What is the chemical equation to work out d2?

Answer 8

• it is given by the Lens Equation
• d1= distance the object has to the lens
• d2= distance the lens has to the real image
• f = focal length

Question 9

9. Where does a real image form?

Answer 9

• a real image forms at d2
• it does this when d1 is greater than f

WE WILL SEE AN IMAGE AT D2:
- if you place a screen at that point
- such as a piece of paper

Question 10

10. What happens when an image is inverted?

Answer 10

• its size will be magnified
• its size is represented by the symbol y2

Question 11

11. How do we calculate Magnification?

Answer 11

M = magnification
di,0,1,2 = distances
y1,y2= vertical heights of the object and the image

Question 12

12. What can be said about the 2 images?

Answer 12

THE 2ND IMAGE:
- is converging
- this is a real image
- this is the creation of a real inverted image

THE 1ST IMAGE:
- it is located outside of the focus

Question 13

13. What do Converging lenses form?

Answer 13

• inverted real images of objects
• they are located outside the principal focus

Question 14

14. What happens when the object is between the principal focus and the lens?

Answer 14

• the image is then virtual
• it is on the same side of the lens as the object is
• the image is erect and enlarged

A VIRTUAL IMAGE IS:
- up right image
- it has a + magnification

Question 15

15. What happens in Diverging lenses when it comes to magnification?

Answer 15

• the diverging lenses produce only virtual images
• these are erect
• they are smaller images of real objects

Question 16

16. When it comes to the drawing of all figures, where is light initially travelling from?

Answer 16

• it travels from left to right
• the object is to the left of the lens originally

Question 17

17. What can be said about the distance form the object to a lens?

Answer 17

• it is positive

THE DISTANCE FROM A LENS TO A REAL IMAGE:
- this is positive
- the real image is located to the right of the lens

THE DISTANCE FROM A LENS TO A VIRTUAL IMAGE:
- this is negative
- the image is located to the left of the lens

Question 18

18. What can be said about f (Focal Point) for a converging (convex) lens?

Answer 18

• f is positive

Question 19

19. What can be said about f (Focal Point) for a concave (diverging) lens?

Answer 19

• f is negative

Question 20

20. What are the positive lenses of the eye?

Answer 20

• the cornea
• the crystalline lens
• they form real images
• they form them on the Retina

Question 21

21. What can be said about Corrective lenses?

Answer 21

• eyeglass lenses and contact lenses are examples of
  corrective lenses
• they can have positive and negative focal lengths
• this depends on the necessary correction needed

Question 22

22. Which kind of corrective lens is used to treat Presbyopia?

Answer 22

• a lens with a positive focal length

PRESBYOPIA= gradual loss of your eyes’ ability to focus
on nearby objects

Question 23

23. What kind of corrective lens is used to treat Myopia?

Answer 23

• a lens with a negative focal length

MYOPIA = a common vision condition in which near
objects appear clear, but objects farther away
look blurry

Question 24

24. What are focal lengths expressed as?

Answer 24

• they are expressed as distances
• they are expressed in cm or in m

Question 25

25. How would we calculate the power of a lens?

Answer 25

• this is the inverse of the focal length calculation
• P = 1 / f
  P= power of the lens
  f= focal length (cm or m)

LENS POWER IS MEASURED IN:
- Diopters (D)
- 1D = 1/m

Question 26

26. What happens when there are two refracting interfaces in succession?

Answer 26

• we use the Lensmaker’s Equation
• n = refractive index of the lens material
• r1= radii of the curvature of lens surface one
• r2= radii of the curvature of lens surface two
• this equation holds for all the types of thin lenses

Question 27

27. When is the radius of the curvature (r) positive?

Answer 27

• it is positive when its centre of curvature lies to the
  right of the surface

Question 28

28. When is the radius of the curvature (r) negative?

Answer 28

• this is when the centre of the curvature lies to left of
  the surface

Question 29

29. Does this make sense?

Answer 29

• yes

Question 30

30. Can lenses have a range of shapes?

Answer 30

• yes they can
• even if they have the same focal length

Question 31

31. What are the shapes of the lenses when they have positive focal lengths?

Answer 31

• they can be biconvex
• they can be planoconvex (one side flat)
• they can be positive meniscus

Question 32

32. What are the shapes of the lenses when they have negative focal lengths?

Answer 32

• they can be biconcave
• they can be planoconcave (one side flat)
• they can be negative meniscus

Question 33

33. What can be said about the shape of the isolated cornea lens?

Answer 33

• it is a negative meniscus lens
• it has a negative focal length

Question 34

34. What can be said about the shape of the Crystalline lens?

Answer 34

• it is an asymmetric biconvex lens
• it has a positive focal length

Question 35

35. How do we work out the focal length (f) for a combination of two thin lenses?

Answer 35

• lens 1 has a focal length of f1
• lens 2 has a focal length of f2

Question 36

36. Answer this question.

Answer 36

• B

Question 37

37. Answer this question.

Answer 37

• B
• length that is propagated into empty space
• s = d / t
  = 80 / 67 x 10⎺³
  = 2.0 x 107 m

Question 38

38. Answer this question.

Answer 38

• D

Question 39

39. Read through Summary 1.
    Does everything make sense?

Answer 39

• yes

Question 40

40. Read through Summary 2.
    Does everything make sense?

Answer 40

• yes

Question 41

41. Read through Summary 3.
    Does everything make sense?

Answer 41

• yes

Question 42

42. Read through Summary 4.
    Does everything make sense?

Answer 42

• yes

Question 43

43. Read through Summary 5.
    Does everything make sense?

Answer 43

• yes

Question 44

44. Read through Summary 6.
    Does everything make sense?

Answer 44

• yes

Question 45

45. Read through Summary 7.
    Does everything make sense?

Answer 45

• yes

Question 46

46. Read through Summary 8.
    Does everything make sense?

Answer 46

• yes