2. Lenses are mainly made of glass clear
plastics. They are widely used in
spectacles, cameras, telescopes and
many other optical instruments.
Our human eye has two crystalline
lenses which enable us to form
images.

3. Lenses
 There are two main types of lenses:
• convex lenses—these curve outwards and are
fatter in the middle
• concave lenses—these curve inwards (a little
like a cave) and are thinner in the middle.
Convex lenses Concave Lenses

4. Lenses – An application of
refraction
 There are 2 basic types of lenses
A converging lens (Convex) A diverging lens (concave) takes
takes light rays and bring light rays and spreads them
them to a point. outward.

5. Convex Mirrors
 Curves outward
 Reduces images
 Virtual images
Use: Rear view mirrors, store
security…
CAUTION! Objects are closer than they appear!

6. Thicker in the center
than edges.
 Lens that converges
(brings together) light
The Magnifier
rays.
 Forms real images and
virtual images
depending on position of
the object

7. A ray thru the
Center of the
lenses Parallel ray
Passes thru
The focus
2f focus (f) focus (f) 2f
x x x x
Remains
unbent
Refracts parallel
To principal axis

8. MOVEMENT OF LIGHT THROUGH LENSES
CONVEX LENS
 In a convex lens, an incoming ray parallel to the principal
axis is refracted through the principal focus (F).

9. Movement of Light through a
Lens
 The distance from the centre line (plane) of
the lens to the principal focus is called the
focal length of the lens.
 A ray passing through the centre of either
type of lens is unaffected.
 As with all images, rays of light that come
from a part of the object come together again
at that same part of the image.

10. Finding the focal length
 Rays coming into a lens from a distant object
are almost parallel and form an image very
close to the focus.
 We can then measure the distance from lens
to image to determine the focal length of
the lens.

11.  The greater the curvature of a lens, the more it bends
light and hence the shorter the focal length.

12.  Convex lenses produce two different types of
images, depending on where the object is
located.
 If the object is at a distance greater than the
focal length of the lens, a real image is
formed.

13.  If the object is at a distance less than the
focal length of the lens, a virtual image is
formed.

14. REAL AND VIRTUAL IMAGES
Real Image –
Image is made from “real” light rays
that converge at a real focal point so
the image is REAL
Can be projected onto a screen
because light actually passes
through the point where the image
appears
Always inverted
Virtual Image–
“Not Real” because it cannot be

15. Image is:
Real
Inverted
Reduced
Parallel ray Appears between
f and 2f
2f focus (f) focus (f) 2f
x x x x
Object beyond 2f

16. Image is:
Real
Inverted
Same size
Appears between
f and 2f
Parallel ray
2f focus (f) focus (f) 2f
x x x x
Object at 2f

17. Image is:
Real
Inverted
Enlarged
Appears beyond 2f
Parallel ray
2f focus (f) focus (f) 2f
x x x x
Object between
f and 2f

18. Image is:
Virtual
Apparent Erect
Convergence Enlarged
Of rays Appears on same
Side as Object
2f focus (f) focus (f) 2f
x x x x
Object
Inside
focus

19. APPLICATIOSNS

21. OPTICAL INSTRUMENTS
 Telescopes
 Telescopes make small, far objects appear larger.
 Two lenses are used.
 The objective lens produces a real, inverted image
just inside the focus of a second lens, called the
eyepiece lens. The image produced by the first
lens now acts as the object for the second lens.
Because the first image is inside the focus of the
second lens, the second image (the one seen by
the telescope user) is virtual and enlarged
compared to the first one

22. CAMERAS, IN BRIEF
object pinhole
image at
film plane
In a pinhole camera, the hole is so small that light hitting any particular point
on the film plane must have come from a particular direction outside the camera
object image at
film plane
lens
In a camera with a lens, the same applies: that a point on the film plane
more-or-less corresponds to a direction outside the camera. Lenses have
the important advantage of collecting more light than the pinhole admits

23. JAZAKALLAH!
PREPARED BY:
CLASS:XI-O