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 A convex or converging lens. Notice the way the rays are brought to a common focus. |
 A concave or diverging lens. Notice the way the rays appear to meet behind the lens. |
 A ray through the optic centre passes straight through the lens. |
 A ray parallel to the principal axis passes through the focus. |
 A ray through the focus refracts parallel to the principal axis. |
Real Image (MIX)
A real image is an image formed by the actual intersection of light rays.
Such an image can be located on a screen or by the method of no parallax.
Virtual Image
(MIX)
A virtual image is formed by the apparent intersection of rays.
Such an image can never be formed on a screen. It can be located by the method
of no parallax
For a Convex Lens (MIX)
If the object is outside the focus the image is real and
located at the opposite side of the lens to the object.
The image is inverted.
If the object is inside the focus the image is virtual
and
is located at the same side of the lens as the object.
The image is upright (erect).
| The Natures of the Images in a Convex Lens | |
 Real Inverted Diminished |
 Real Inverted Same Size |
 Real Inverted Magnified |
 Infinity |
 Virtual Erect Diminished |
Remember: An image in a convex lens can have any of five natures: RID, RIS, RIM, Infinity, VEM. |
Summary of Natures
of Images in a Convex Lens |
|||||
| Position of Object | Outside 2f |
At 2f |
Between 2f and f |
At f |
Inside f |
| Nature of Image | RID |
RIS |
RIM |
Infinity |
VEM |
where f is the focal length,
u is the distance between the object and the centre
of the lens,
and v is the distance between the image and the centre of
the lens.
and 
where m is the magnification,
u is the distance between the object and the centre of the
lens,
and v is the distance between the image and the centre of the lensr.
Note: if the image is virtual, v is negative.
| Example |
| An object is placed 40 cm from a convex lens. A real image is found 60 cm from the lens. (i) Calculate the focal length of the lens. (ii) Calculate the magnification of the image. |
Answer |
| Example |
| An object is placed 16 cm from a convex lens of focal length 20 cm. (i) Find the position and nature of the image. (ii) Calculate the magnification of the image. |
Answer i.e. the image is Virtual, Erect and Magnified. It
is located 80 cm from the lens, on the same side as the light source. Note: The minus indicates that the image is virtual. |
| Example |
A convex lens has a focal length of 15 cm. |
Answer
|
Use the lens to focus the image of a distant object on a sheet of paper.
The distance between the sheet and the centre of the lens is approximately one
focal length.
 A ray through the optic centre passes straight through the lens. |
 A ray heading for the focus refracts parallel to the principal axis. |
 A ray parallel to the principal axis refracts as if it came from the focus. |
The only type of image formed is VIRTUAL, ERECT and DIMINISHED.
where f is the focal length,
u is the distance between the object and the centre
of the lens,
and v is the distance between the image and the centre of
the lens.
REMEMBER: v and f
are negative since they are both on the same side
of the lens as the source of light.
and
where m is the magnification,
u is the distance between the object and the centre of the
lens,
and v is the distance between the image and the centre of the lensr.
Note: if the image is virtual, v is negative.
| Example |
An object 4 cm high is placed vertically on the principal axis 30 cm from a concave lens of focal length 20 cm. Calculate (i) the position, (ii) the height, of the image. |
Answer (ii) |
  |
The unit of the power of a lens is  |
For a CONVEX lens P
is POSITIVE. |
 |
| Example |
Find the power of |
Answer (ii) |
| Example |
A convex lens of focal length 8 cm is placed in contact with a concave
lens of focal length 40 cm. |
Answer |
Most of the focusing occurs at the cornea, a transparent window at the front
of the eye.
The lens further focuses images. The lens can change shape thanks to the action
of the ciliary muscles.
This is the ability of the eye to focus on objects at different distances.
This is the smallest distance between an object and the eye for which that object can be seen clearly without eye strain.
A short-sighted person can see
nearby objects clearly. They cannot see distant object clearly.
This can be corrected with a concave
(diverging) lens. Such a lens has a negative power.
A long-sighted person can see
distant objects clearly. The cannot see nearby objects clearly.
This can be corrected with a convex
(converging) lens. Such a lens has a positive power.
