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Physics for Secondary Schools
line is the vertical axis of the lens.
Task 5.4 The point of intersection between
the vertical axis and the principal
On a sunny day, arrange a convex lens axis is called the optical centre of
and a sheet of paper such that the sunlight the lens.
is directed through the convex lens and
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converges to a bright spot on the sheet of Light rays incident to a lens and
paper. Hold the lens at the same position travelling parallel to the principal
for some minutes. Observe what happens. axis will either converge or
Discuss with your classmates about your diverge. In a convex lens, the
observation.
light rays converge to to a point
called the focal point or principal
Caution: Avoid looking at the Sun directly or focus. In a concave lens, the light
even into a refracted sunlight. rays diverge, but when traced
backwards, they appear to come
A concave lens is a lens that diverges rays of from a common point, also called
light that pass through it. Concave lenses are a focal point. . In both cases, the
relatively thinner across their middle and thicker focal point is denoted by the
at their edges. At least one side of a concave lens letter F. The distance between the
curves inwards. Types of concave lenses include focal point and the optical centre
biconcave lens, plano-concave lens and the is the focal length, denoted by
concave meniscus. Figure 5.32 shows the types the letter, f.
of concave lenses.
Note that, unlike mirrors,
lenses allow light to pass
through either face. Therefore,
a lens has two focal points on
each side of the lens. A lens also
Biconcave Plano-concave Diverging meniscus has an imaginary point called
Figure 5.32: Type of diverging lenses the centre of curvature, which
is the centre of the sphere of
Terminologies related to lenses which the surface of a lens is
The discussion of refraction of light rays and a part. The centre of curvature
the formation of images by lenses requires the lies on the principal axis and its
use of a variety of terms. If one face of a lens distance from the optical centre
is thought of as being a portion of a spherical is called the radius of curvature
surface, then the imaginary line passing through R. The lens has two centres of
the centre of the sphere and the exact centre of curvature, on each side of the
the lens is called the principal axis of the lens. lens. Note that, the focal point,
A lens also has an imaginary vertical line that F is real for a convex lens and
bisects it symmetrically into two halves. This virtual for a concave lens.
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Student’s Book Form Two
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