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When a ray of light is incident on a boundary separating the two media having different densities:
- A part of the light gets reflected.
- Rest of the light changes its direction as it enters the second medium.
The change of direction suffered by a ray of light as it passes obliquely from one medium to another is known as refraction
Incident Ray
The ray of light striking the surface of separation of the media through which it is travelling is known as the incident ray.
Point of Incidence
The point at which the incident ray strikes the surface of separation is called the point of incidence.
Normal
The perpendicular drawn to the surface of separation at the point of incidence is called the normal.
Refracted Ray
The ray of light that has changed its direction at the surface of separation when travelling from one medium to another is called the refracted ray.
Angle of Incidence
The angle, which the incident ray makes with the normal at the point of incidence, is called the angle of incidence.
Angle of Refraction
The angle, which the refracted ray makes with the normal at the point of incidence, is called the angle of refraction.
Laws of Refraction
The laws of refraction are:
The incident ray, the refracted ray and the normal to the surface at the point of incidence all lie in one plane.
For any two given pair of media, the ratio of the sine of the angle of incidence to the sine of the angle of refraction is a constant.
The second law is called Snell's law after the scientist Willebrod Snell who first formulated it
where nm is the refractive index of the second medium with respect to the first medium.
Refractive Index
Refractive index or absolute refractive index of a medium is equal to the ratio of the speed of light in vacuum or air to the speed of light in that particular medium.
This ratio is always a constant for a given pair of media.
Lens
A lens is a transparent medium that has at least one curved surface.
Convex Lens
A convex lens is thicker in the middle and thinner at its edges.
Concave Lens
A concave lens is thicker at the edges and thinner in the middle.
Optic Centre
The centre of a lens is called the optical centre or optic centre. It is denoted by the letter O.
Centre of Curvature
The centres of the spherical surfaces forming a lens are known as the centres of curvature of the lens. The letter C is used to denote centre of curvature.
Principal Axis
The line passing through the centres of curvature C1 and C2 of two spherical surfaces of the lens is called the principal axis of the lens.
Principal Focus of a Convex Lens
The principal focus F of a convex lens is a point on the principal axis to which rays parallel to the principal axis converge after refraction.
Principal Focus of a Concave Lens
The principal focus F of a concave lens is a point on the principal axis from which rays parallel to the principal axis appear to diverge after refraction.
Focal Length
The distance between the optic centre and the focus is the focal length f of the lens.
Rays that are generally considered while constructing ray diagrams:
Any ray of light, travelling parallel to the principal axis, after refraction through the lens, passes through the principal focus of the lens.
Any ray of light, which first passes through principal focus or which appears to meet at the focus after refraction, always travels parallel to the principal axis.
Any ray of light which passes through optical centre of lens does not deviate form its path.
The image is formed at the point of intersection of these refracted rays.
Sign Convention for Lenses
All distances on the principal axis are measured from the optical centre.
The distances measured in the direction of incident rays are positive and all the distances measured in the direction opposite to that of the incident rays are negative.
All distances measured perpendicular to and above the principal axis are positive. Thus, height of an object and that of an erect image are positive
And all distances measured perpendicular to and below the principal axis are negative.
Lens Formula
Where u is the object distance or distance of the object from the lens, v is the image distance or the distance of the image from the lens and f is the focal length or the distance of the principal focus from the lens.
Magnification
It is the ratio of the height of the image to the height of the object. It is represented by the letter m
Magnification produced by a lens is also related to the object distance u, and the image distance v.
Power of a Lens
Power of a lens is defined as the reciprocal of its focal length in metres.
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