LENSES – PURE PHYSICS
Types of lenses:
Convex (converging) lenses.
Concave (diverging) lenses.
ACTION OF LENSES ON A PARALLEL BEAM OF LIGHT.
(a) ACTION OF CONVEX LENSES
A convex lens converges a parallel beam of light to a point called the principal focus or the focal point
after refraction.
Illustration:
principal focus
principal focus (focal
(focal point)
point)
focal length
Note: A convex lens converges heat rays to the focal point resulting in intense heating thus a fire. (This explains
why a bottle left lying in the bush can set the bush on fire)
(b) ACTION OF THE CONCAVE LENS ON A PARALLEL BEAMOF LIGHT:
A concave lens diverges a parallel beam of light. The divergence appears to come from the principal focus.
Illustration:
concave lens
focal point (principal focus)
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TERMS USED TO DESCRIBE LENSES:
Illustration:
X X’
F1 O F2
Optical centre, O: Is the centre of the lens.
Principal axis, XX’: Is the line passing through the optical centre.
Principal focus (focal point), F1, F2: Is a point to which a parallel beam of light incident on a convex lens is
converged to.
Focal length, f: Is the distance between the optical centre and the principal focus.
IMAGE FORMATION IN CONVEX LENSES:
A convex lens forms images due to refraction of light. It forms both virtual and real images.
A virtual image is that formed by apparent intersection of rays on it i.e. it cannot be formed on the screen.
A real image is that formed by actual intersection of rays on it i.e. it can be formed on the screen.
RAY DIAGRAMS SHOWING IMAGE FORMATION INCONVEX LENSES:
CHIEF (PRINCIPAL) RAYS USED IN RAY DIAGRAMS:
1. A ray of light parallel to the principal axis, on refraction, passes through the focal point.
2. A ray of light through the optical centre, is unaffected i.e. there is no change in direction.
3. A ray of light through the focal point, on refraction, is parallel to the principal axis.
Illustration:
1
2
3
F1 F2
L
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RAY DIAGRAM SHOWING IMAGE OF AN OBJECT BETWEEN THE FOCAL POINT AND THE LENS:
I F1 O F2
L eye
PROPERTIES OF THE IMAGE FORMED:
Image formed is virtual, upright (erect) and magnified. It is also on the same side as the image.
RAY DIAGRAM FOR IMAGE OF AN OBJECT AT THE FOCAL POINT:
O
F1 F2
L
∞ infinity
PROPERTIES OF IMAGE FORMED:
Image formed is real, inverted and magnified. It is formed at infinity i.e. it is a distant image.
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RAY DIAGRAM FOR IMAGE OF AN OBJECT BETWEEN F AND 2F:
I
2F1 O F1 F2 2F2
PROPERTIES OF IMAGE FORMED:
Image formed is real, inverted and magnified. It is formed beyond 2F on the other side of the lens.
RAY DIAGRAM FOR IMAGE OF AN OBJECT AT 2F:
O I
2F1 F1 F2 2F2
PROPERTIES OF IMAGE FORMED:
Image formed is real, inverted and same size as the object. It is formed at 2F on the other side of the lens.
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RAY DIAGRAM FOR IMAGE OF AN OBJECT BEYOND 2F:
I
O 2F1 F1 F2 2F2
PROPERTIES OF IMAGE FORMED:
Image formed is real, inverted and diminished (smaller than the object). It is formed between F and 2F on the
other side of the lens.
Note: Any ray of light from the head of the object, on refraction, will pass through the head of the image.
RAY DIAGRAM FOR IMAGE OF AN OBJECT AT INFINITY (DISTANT OBJECT):
infinity
∞
F2
F1
I
PROPERTIES OF THE IMAGE:
Image formed is real, inverted and diminished. It is formed at the principal focus (focal point) on the other side of
the lens.
MAGNIFICATION (m):
Magnification = image size (height) = image distance
object size (height) object distance
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USES OF A SINGLE CONVEX LENS:
1. USE OF A LENS AS A MAGNIFYING GLASS:
The magnifying glass forms a virtual, upright and magnified image.
For the image to be virtual and magnified, the object should be between the lens and the focal point.
(Refer to the ray diagram for image of an object between the lens and the focal point)
2. USE OF A LENS IN A CAMERA:
The camera forms a real, inverted and diminished image.
For the image to have the properties mentioned, the object should be beyond 2F.
(Refer to the ray diagram for image of an object beyond 2F)
Note: The camera can be focused by adjusting the distance between the lens and the film or by changing
the distance between the camera and the object.
3. USE OF A LENS IN A PROJECTOR (SLIDE, CINE, PHOTOGRAPHIC ENLARGER):
The slide projector forms real, inverted and magnified images.
The object i.e. the slide or the film should be placed between F and 2F.
In order for the image to be upright on the screen, the object i.e. the slide or the film should be upside
down.
(Refer to ray diagram for image of an object between F and 2F.)
Note: The slide projector is focused by adjusting the distance between the lens and the slide/film or the
lens and the screen.
