5
The ray of light entering water undergoes a deviation at the point on the surface where the media get separated.
Speed of light and optical density:
Medium
– speed of light (m/s)
Vacuum
– 3×108 m/s
Water
-2.25×108 m/s
Glass
– 2×108 m/s
Diamond
– 1.25×108 m/s
• The characteristics of cach medium influences the speed of light that passes
through the respective medium. Optical density is a measure that shows how a medium influences the speed
of light passing through it. • Optical density increases, the speed of light decreases.
Refraction of Light:
It is the difference in the optical densities that causes the deviation.
When a ray of light entering obliquely from one transparent medium to another, its path undergoes a deviation at the surface of separation. This is refraction.
Refraction in different media:
• While entering from air to glass (from a medium of lower optical density
to that of a greater one) the refracted ray deviates towards the normal.
• While entering from glass to air (from a medium of greater optical density
to that of a lower one) the refracted ray deviates away from the normal. • The angle of incidence, angle of refraction and the normal at the point of
incidence are in the same plane. No deviation takes place in the case of a light ray falling normally on a
medium.
Note:
• When light passes obliquely from a medium of higher optical density to a
medium of lower optical density, the refracted ray deviates away from the normal.
When light is incident obliquely, from a medium of lower optical density
to a medium of greater optical density, the refracted ray deviates towards
the normal.
Laws of Refraction
•
The angle of incidence, the angle of refraction and the normal at the point of incidence on the surface of separation of the two media will always be in the same plane.
• The ratio of the sine of the angle of incidence to the sine of the angle of
refraction
sin i
will always be a constant. This is known as Snell’s law. sinr
•
The constant from Snell’s law is known as refractive index (n).
Speed of light in media and refractive index
•
The refractive index of one medium with respect to another is called relative refractive index.
The refractive index of a medium with respect to vacuum is called absolute
refractive index.
Total Internal Reflection:
When a ray of light passes from a medium of greater optical density to that of lower optical density, the angle of incidence at which the angle of refraction becomes 90° is the critical angle.
The critical angle in water is 48.6°. When a ray of light passes from a medium of higher optical density to a medium of lower optical density at an angle of incidence greater than the critical angle, the ray is reflected back to the same medium without undergoing refraction. This phenomenon is known as total internal reflection.
Practical applications of total internal reflection:
•
It is used in endoscopy It is used in optical communication.
• It is used in many optical instruments like telescopes, microscopes,
binoculars etc.
The brilliance of diamond is due to total internal reflection. • The phenomenon of mirage.
OFC:
Total internal reflection is made use of in optical fibre cables. Through optical
ores, thousands of signals of different frequencies can be sent to distant places simultancously, making use of total internal rcflcction of light, without losing the intensity
Lens:
A lens is a transparent medium having spherical surfaces.
Optic centre – Optic centre is the midpoint of a lens (P).
Centre of curvature – A lens has two spherical surfaces as parts of the lens.
Centre of curvature (C) is the centre of the imaginary spheres of which the sides
of the lens are parts.
Principal axis – Principal axis is the imaginary line that passes through the optic centre joining the two centres of curvature.
The principal focus:
• Light rays incident parallel and close to the principal axis after refraction
converges to a point on the principal axis of a convex lens. This point is the principal focus of a convex lens.
Light rays incident parallel and close to the principal axis diverge from one
another after refraction. These rays appear to originate from a point on the same side. This point is the principal focus of a concave lens.
It is impossible to produce real convergence of light using a concave lens. Therefore, the principal focus of a concave lens is virtual.
Focal length
• Focal length is the distance from the optic centre to the principal focus.
This is denoted by the letter f.
Formation of image using a lens:
Image formed by concave lens:
Position(object) – Position(image) – Image Size – Nature (image)
At infinity – At F – Highly diminished point-sized – Real and inverted
Beyond 2F – Between F and 2F – Diminished Real and inverted
28/45
At 2F – At 2F – Same size – Real and inverted
Between 2F and F – Beyond 2F – Enlarged – Real and inverted
At F – At infinity – Highly enlarged – Real and inverted
Between 0 and F – On the same side of the lens as object – Enlarged – Virtual and erect
Image formed by concave lens:
• Virtual image. •Images are always smaller than object. • Upright image.
Magnification:
• Magnification is the ratio of the height of the image to the height of the
object.
hu
• Magnification shows how many times the image is larger than the object. • If magnification is negative, the image will be real and inverted, • If magnification is positive, the image will be virtual and erect.
Power:
• Power is a term related to the focal length of a lens. • Power of a lens is the reciprocal of focal length expressed in metres. • Unit of power is dioptre. It is represented by D.
The power of a convex lens is positive and that of a concave lens is negative.
Atmospheric Refraction
When light passes through media of different optical densities it undergoes successive refractions. Hence the source of light appears like twinkling,
Light coming from distant stars passes through different layers of air. Each layer differs from the other in their optical densities. Hence light undergoes successive refraction. Since stars at a greater distance they appear like a point source. The rays of light appear to come from different points on reaching the eye after refraction. This is the reason for the twinkling of
stars.
