Definitions (Light)

LIGHT

Lens: a lens is a piece of transparent glass bounded by two refracting surfaces which are usually spherical.

Convex Lens: a lens that is thicker at its center than at edges is called convex lens.

Concave lens: The lens which is thicker at its edges than at the middle is called concave lens.

Center of curvature: The center of the spherical surface whose parts forms the lens is called center of curvature.

Radius of curvature: The radius of a sphere whose surface forms the lens is called radius of curvature.

Principal axis: It is the straight line passing through the optical centre and the centres of curvature of the lens.

Optical center: The geometrical center of a lens is called its optical center.

Principal focus: It is the point on the principal axis where parallel rays of light meet or seem to meet after refraction through the lens.

Focal length: It is the distance between the optical centre and the principal focus of a lens.

Real image: The image which can be obtained on the screen is called real image.

Virtual image: Those images which cannot be obtained on the screen are called virtual image.

Magnification: The magnification of a lens is defined as the ratio of height of the image to the height of the object.

Power of lens: It is defined as the reciprocal of the focal length of a lens expressed in metres (P = 1/f).

One Dioptre: A lens is said to be of 1 dioptre if its focal length is 1 m.

Optical instruments: Instruments that use mirror, lens and prism.

Simple camera: A Simple camera is an optical instrument that is used for taking photographs of object and people.

Eye: Eyes are natural optical instrument which are concerned with the sense of vision.

Near point: The nearest point, up to which an object can be seen clearly by a human eye, is called near point.

Far point: The farthest point, up to which an object can be seen clearly by human eye is called far point.

Power of Accommodation: The ability of an eye to change the focal length of the eye lens so as to obtain the image on the retina is called power of accommodation.

Persistence of vision: it is effect of vision due to which the impression of an image last on the retina even after the removal of the object.

Defect of vision: An eye which does not form the image of an object lying between the near point and far point on the retina is said to suffer from defect of vision.

Long-sightedness (Hypermetropia): it is the defect of vision in which the eye cannot see the nearby object clearly but can see distant object clearly.

Blind spot: The region of the retina where the optic nerve leaves the eye (it has no rods or cones) is called blind spot.

Yellow spot: The region in the center of the retina which has maximum concentration of cones is called yellow spot.

Short-sightedness (Myopia): It is the defect of vision in which the eye can see near object clearly but cannot see distant object is called Short-sightedness.

Microscope: It is an optical instrument which is used to see the magnified image of a small object placed very close to it.

Telescope: It is an optical instrument which is used to see the distant objects clearly.

Aperture: Aperture of a lens is the maximum portion of the spherical surface through which refraction takes place.

Focusing; Focusing is the process of adjusting the distance between the lens and the screen to produce sharp and clear image.

Lens formula: A formula which gives the relationship between the focal length, object distance and image distance is called lens formula. The reciprocal of focal length of lens is equal to the sum of reciprocal of object distance and image distance, is lens formula.

Photographic camera: It is an optical instrument which is used to record a permanent image of an object on a photographic film.