images formed by lenses. convex (converging) lenses, f>0
Post on 03-Jan-2016
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Images formed by lenses
Convex (converging) lenses, f>0.
Three light rays Parallel in, focus out; Focus in, parallel out; Centre in, centre out.
F
f < do <2 f
F
F
f > do
F
Properties of the Image (convex lens)
do is the distance between the object and the lens
f is the focal length.
Distance to the mirror Properties of the image
do >2 f Inverted, diminished, real
do = 2 f Inverted, same size, real
f < do <2 f Inverted, magnified, real
do= f No image
do< f upright, magnified, virtual
Images formed by a convex mirror
The image is upright, diminished, and virtual, behind the mirror and between the mirror and the principal focus.
F
Concave (diverging) lenses, f <0
Descartes’ formula
oi ddf
111
f: focal length, negative when the lens is concave.
di: distance the image is in front of mirror, negative when image is virtual (behind lens).
do: distance the object is in front of lens.
Magnification
o
i
o
i
d
d
h
hm
hi: height of image, negative when image is virtual. ho: height of object.
eg1.An object 2cm high is placed 10cm in front of a concave lens of focal length 3.5cm. Find the height and position of the image.
Ans. do = 10cm, ho=2cm, f=-3.5cm. di =?, hi =?
hi=-0.26ho=-0.26x2 = -0.52cm
10
11
5.3
1
id
26.010
6.2
o
i
o
i
d
d
h
hm
io ddf
111
3857.010
1
5.3
11
id
6.23857.0
1
id
What does the negative sign stand for?
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