Refraction by spherical lenses
Refraction by spherical lenses
Refraction means to state that the change of the direction
of the parallel beam of ray of lights when it passes through one medium to
another medium.
The image formed by the convex lens is enlarged. And the
image formed by the concave lens is diminished.
Lens :--
A lens
is a transparent material bounded by two surfaces of which one or both surfaces
are spherical. It means to say that a lens is bounded by at least one spherical
surface. And other surface is plane surface.
Or we can say that lens is a transparent material or glass bounded by the two spherical surfaces
and is used for magnifying the objects.
The working of lens is mainly based on the refraction of
ray of lights when they passes through it.
There are two types of lens. A concave lens and convex
lens.
Convex lens :--
A convex lens is the spherical lens having two spherical surfaces
bulging outwards. It is also called as double convex lens.
It is thicker at the
middle as compared to the edges of the lens.
It is also called as converging lens. It converges the
light rays when ray of light passes
through the convex lens.
Take a paper and hold the convex lens in the position that
beam of light falls on the paper through the lens. Observe the paper. The paper
starts burning and it produces smoke. And after some time it may catches a
fire. The light from the sun constitutes parallel ray of lights. These parallel
light converged by the convex lens at
the sharp bright spot formed on the paper. and the sharp bright spot is the
real image of the sun. the concentration of the parallel sharp bright spot
light rays generates the heat. And these heat energy focused sunlight has
burned a hole in the paper where the image of the sun is made.
Concave lens:--
A concave lens is the spherical lens that have two spherical surfaces which curved inwards. It is also called as
double concave lens.
It is thicker at the
edges as compared to the middle of the lens.
It is also called as diverging lenses because the refracted
ray diverge from the focus of the lens. Concave lens is opposite to the convex
lens. And the parallel ray of light is diverged after passing through the concave
lens. Concave lens has two principal foci. And they are represented by the
letter F1 and F2. And in the concave lens , a parallel ray of light is passing through
the concave lens from the left side then they diverge towards left side only. And
if the parallel ray of light is passing through the concave lens from the right
side then they get diverge towards the right side only.
Centre of curvature of lens:--
A lens (concave lens or convex lens) has two spherical surfaces. And
each part of these surfaces forms a sphere. The centre of these sphere is
called as centre of curvature of the lens.
The centre of curvature of lens is denoted by the letter C.
since there are two centre of curvature of lens and they represents as C1 and
C2.
Optical centre:--
The centre
point of the lens is called as optical
centre. It is denoted by the letter O. a beam of light ray is passing through
optical centre goes straight. And it does not get deviated.
Principal axis:--
An imaginary straight line passing through the
two centres of the curvature of a lens is called as principal axis.
Aperture of lens :--
Aperture of
lens is the effective diameter of the circular outline of the spherical lens is
called as aperture of lens.
Thin lens:--
Such type of lens whose aperture is much less than the its radius of curvature. And the two centre of curvature
is equidistance from the optical centre O. such type of lens is called as thin
lenses with small aperture.
Principal focus of concave lens :--
Several light
rays which are parallel to the principal axis falling on the concave lens. After
refraction , All these ray of light after passing through the concave lens are appearing diverge from a point on the
principal axis. This point is called as principal focus of the concave lens. It
is represented by the letter F.
Principal focus of a convex lens:--
Several light rays which are parallel to the principal axis falling on
the convex lens. After refraction , All these ray of light after passing
through the concave lens are appear converging
to a point on the principal axis. This point
is called as principal focus of the convex lens. It is represented by the
letter F.
Focal length of a convex lens:--
The distance of the principal focus from the optical centre of a lens is
called as focal length of a lens.
A lens has two principal foci. They are represented by the
letter F1 and F2.
Focal length of a lens depends on the:--
Refractive index of the glass
Its curvature
If the refractive index is higher then the focal length is
shorter
If the curvature of the lens is more then the focal length
is short.
What happens when parallel beam of light are
incident on a lens? And why?
Take a paper and hold the lens in the position that beam of
light falls on the paper through the lens. Observe the paper. The paper starts
burning and it produces smoke. And after some time it may catches a fire. The
light from the sun constitutes parallel ray of lights. These parallel light
converged by the lens at the sharp bright spot formed on the paper. and
these sharp bright spot is the real
image of the sun. the concentration of the parallel sharp bright spot light
rays generates the heat. And this may causes the paper to burn.
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