06-07 CE Physics/Ch 12 Lenses
Chapter 12 Lenses
12.1 Convex 凸透鏡 and concave lenses 凹透鏡
 Terminology 術語 for lenses
12.2 Construction rules for image formed by lenses
 Construction rules for image formed by convex lens
 Construction rules for image formed by concave lens
12.3 Image formation by lenses
 Locating images by graphical method
 Nature of image formed by lenses
 Finding focal length 焦距 of convex lens
1
06-07 CE Physics/Ch 12 Lenses
12.1 Convex and concave lenses (p. 141)
Activity 1 Spectacles and magnifying glass (p. 141)
1. Lenses 透鏡(p. 143) Fig. 12.1 (p. 143)
(a) Made of:
Transparent materials (e.g. glass, perspex 透明
膠)
(b) When light rays pass through it, they are
refracted according to the laws of refraction.
(c) Shapes of lenses: Fig. 12.1 (p. 143)
(i) Cylindrical lenses 柱面透鏡:
The lenses with cylindrical surfaces.
(ii) Spherical lenses 球面透鏡:
The lenses with spherical surfaces.
(d) Types of lenses: Fig. 12.2 (p. 143)
(i) Convex lens:
Thicker at its centre than its edges.
(ii) Concave lens:
Thinner at its centre than its edges.
2. Convex and concave lenses (p. 143)
(a) Convex lens 凸透鏡: Fig. 12.3 (p. 143)
(i) When parallel light rays pass through it, the
refracted rays converge 會聚 to a point after
refraction.
(ii) It is called a converging lens 會聚透鏡.
(b) Concave lens 凹透鏡: Fig. 12.4 (p. 144)
(i) When parallel light rays pass through it, the
refracted rays diverge 發散 after refraction.
(ii) It is called a diverging lens 發散透鏡.
2
06-07 CE Physics/Ch 12 Lenses
3. Terminology for lenses (p. 144)
(a) Optical centre 光心(C): Fig. 12.5 (p. 144)
(i) The geometric centre of the lens.
(ii) The point through which light rays pass
without deviation 偏折.
(b) Principal axis 主軸: (p. 145) Fig. 12.5 (p. 144)
(i) The line passing through C.
(ii) It is perpendicular to the lens.
(c) Principal focus or focus 主焦點或焦點(F): Fig.
12.6 (p. 145)
For parallel incident rays that are parallel to the
principal axis:
(i) Convex lens:
The point which the refracted light rays
converge on the principal axis is the focus.
(ii) Concave lens:
The point which the refracted light rays are
extended backwards and meet on the principal
axis is the focus.
(iii)Since light rays can be directed towards a
lens from either side, a lens has two foci.
(d) Focal length 焦距(f): Fig. 12.6 (p. 145)
(i) The distance between F and C.
(ii) It depends on: Fig. 12.7 (p. 146)
- refractive index of the material.
- thickness (curvature 曲率) of the lens.
(iii)A thicker lens converges light rays more.
- The focal length of a thicker lens
< the focal length of a thinner lens.
(e) Focal plane 焦平面: Fig. 12.8 (p. 146)
(i) - A plane which cuts F.
- It is perpendicular to the principal axis.
(ii) For parallel incident rays that are not parallel
to the principal axis:
Convex lens:
The refracted rays converge to a point on the
focal plane.
Concave lens:
The refracted rays appear to diverge from a
point on the focal plane.
3
06-07 CE Physics/Ch 12 Lenses
12.2 Construction rules for image formed
by lenses (p. 147)
4. Graphical symbols of lenses (p. 147)
Fig. 12.9 (p. 147)
Use the graphical symbols to represent a convex
lens and a concave lens.
5. Construction rules for image formed by convex lens
(p. 147) Table 12.1 (p. 147)
Experiment 12A Refraction of light by convex
and concave lenses
(A) Refraction of light by convex lens (p. 147)
(a) Experimental procedures:
(i) Pass three light rays towards a convex lens.
(ii) Observe the refracted ray in each case.
(b) Result and conclusion:
(i) Ray 1: parallel to the principal axis
- The refracted ray passes through F on the
opposite side of the incident ray.
(ii) Ray 2: towards F
- The refracted ray is parallel to the principal
axis.
(iii)Ray 3: towards C
- The refracted ray passes through C without
deviation.
(c) (i) Ray 2 is the reverse of ray 1. This is an
example of reversibility of light 光的可逆性.
(ii) The principle of reversibility of light:
When a light ray is reversed in direction, it
retraces its original path.
Example 1 (p. 148), Class Practice 1 (p. 149)
6. Construction rules for image formed by concave
lens (p. 150) Table 12.2 (p. 150)
Experiment 12A Refraction of light by convex
and concave lenses
(B) Refraction of light by concave lens (p. 150)
(a) Experimental procedures:
(i) Pass three light rays towards a concave lens.
(ii) Observe the refracted ray in each case.
(b) Result and conclusion:
(i) Ray 1: parallel to the principal axis
- The refracted ray appears to come from F on
the same side of the incident ray.
4
06-07 CE Physics/Ch 12 Lenses
(ii) Ray 2: towards F
- The refracted ray is parallel to the principal
axis.
(iii)Ray 3: towards C
- The refracted ray passes through C without
deviation.
Example 2 (p. 151), Class Practice 2 (p. 152)
12.3 Image formation by lenses (p. 152)
7. Image formation by lenses (p. 152)
Experiment 12B Formation of images by convex
and concave lenses (p. 152)
(a) Experimental procedures: Fig. 12.10 (p. 152)
(i) The illuminated letter “P” placed in front of
the ray box acts as an object.
(ii) For convex lens:
Move the screen to and fro along the metre ruler
until a sharp image is formed on the screen.
(iii)For concave lens:
Remove the screen and look at the image
through the lens directly.
(b) Result and conclusion:
(i) Convex lens:
The nature of the image formed depends on the
object position.
(ii) Concave lens:
The nature of image must be:
virtual, erect, diminished and formed between F
and C.
5
06-07 CE Physics/Ch 12 Lenses
8. Locating images by graphical method (p. 153)
Fig. 12.11 (p. 153)
The construction rules:
(i) Use an arrow to represent the object (O),
with its “foot” on the principal axis.
(ii) Direct any two special light rays from the
“tip” of the object towards the lens.
(iii)
- For convex lens:
Draw the refracted rays to meet at a point.
- For concave lens:
Extend the refracted ray backwards to meet at a
point.
- The point of intersection of the rays is the
position of the “tip” of the image.
(iv) Draw an arrow from the “tip” of the image
to the principal axis. It represents the whole
image (I ).
9. Magnification 放大率(p. 154) Fig. 12.12 (p. 154)
(a) Definition:
(i) The ratio of the height of a image (hi) to the
height of an object (ho).
(ii) The ratio of the image distance (v) to the
object distance (u).
(b) Mathematically:
Magnification (m) = hi = v
u
ho
(c) Example:
(i) hi < ho and v < u in the figure
(ii) Magnification (m) < 1. The images are
diminished.
Example 3 (p. 154), Class Practice 3 (p. 155)
6
06-07 CE Physics/Ch 12 Lenses
10. Real 實像 and virtual images 虛像 (p. 156) Fig.
12.13 (p. 156)
(a) Real image:
(i) The refracted light rays really meet at a
certain position.
(ii) The image can be formed on a screen.
(b) Virtual image:
(i) The image is located by extending the
refracted rays backwards.
(ii) The refracted light rays do not meet actually.
They only appear to be emitted from the image.
(iii)The image cannot be formed on a screen 屏
幕.
(iv)We can view it directly through the lens.
11. Nature of image formed by lenses (p. 157)
Activity 2 Image formed by lens (p. 157)
7
06-07 CE Physics/Ch 12 Lenses
Nature of image formed by a convex lens:
(a) The nature of the image formed changes with
(i) the position of the object.
(ii) the focal length of the lens.
(b) Nature of image (p. 158) Table 12.3 (p. 158)
(i) O between C and F:
- I on the same side as the object
Nature of image:
- virtual
- erect
- magnified (m > 1)
(ii) O at F:
- I at infinity 無限遠處
Nature of image:
- cannot be determined
(iii) O between F and 2F:
- I beyond 2F on the opposite side of the lens
Nature of image:
- real
- inverted
- magnified (m > 1)
(iv) O at 2F:
- I at 2F on the opposite side of the lens
Nature of image:
- real
- inverted
- same size as the object (m = 1)
(v) O beyond 2F:
- I between F and 2F on the opposite side of the
lens
Nature of image:
- real
- inverted
- diminished (m < 1)
(vi) O at infinity:
- I on the focal plane on the opposite side of the
lens
Nature of image:
- real
- inverted
- diminished (m < 1)
Example 4 (p. 159), Class Practice 4 (p. 160),
Class Practice 5 (p. 161)
8
06-07 CE Physics/Ch 12 Lenses
12. Nature of image formed by a concave lens (p. 162)
(a) The image formed by a concave lens is seen
through the lens.
Fig. 12.14 (p. 162)
(b) If the object is moved towards the concave lens,
(i) the image size increases.
(ii) it is still smaller than the object, i.e. m < 1.
Fig. 12.15 (p. 162)
(c) The image is always:
(i) virtual
(ii) erect
(iii)diminished (m < 1)
(iv) between C and F on the same side as the
object
(d) When the object is at infinity, the image is
formed on the focal plane.
Class Practice 6, Class Practice 7 (p. 163),
Class Practice 8 (p. 164)
13. Finding focal length of convex lens (p. 164)
Experiment 12C Finding the focal length of a convex lens
(p. 164)
(a) Experimental procedures: Fig. 12.16(a) (p. 164)
(i) Place a convex lens in front of a screen.
(ii) Parallel light rays from a distant object are
incident on the convex lens.
(iii)Move the screen to and fro along the metre
ruler until a sharp image is formed.
(b) Result and conclusion: Fig. 12.16(b) (p. 164)
(i) Parallel light rays from distant objects
converge and meet at a point on the focal plane
of convex lens.
(ii)The focal length of the convex lens (f) is the
distance between the lens and the image.
9
06-07 CE Physics/Ch 12 Lenses
14. Alternative method to find the focal length of
convex lens (p. 165)
(a) Experimental procedures: Fig. 12.17(a) (p. 165)
(i) Place a screen side by side with an object,
and a plane mirror behind a convex lens.
(ii) The lens-mirror combination is moved to
and fro in front of the object until a sharp image
is formed on the screen.
(b) Explanation: Fig. 12.17(b) (p. 165)
(i) When the object is placed at the focus of the
convex lens, the refracted light rays from it
become parallel.
(ii) Upon hitting the plane mirror behind the
lens, the light rays are reflected back along their
original paths.
(iii)Therefore, an image is formed alongside the
object. It has the same size as the object.
(c) Result and conclusion:
The focal length of the convex lens is the
distance between the object and the convex lens.
STS Corner 1 Optical instruments (p. 167)
STS Corner 2 Defects of vision (p. 170)
10