( iii) Astigmatism In this defect, a person cannot focus on horizontal and vertical lines at the same distance at the same time.
This defect can be removed by using suitable cylindrical lenses.
( iv) Colour Blindness In this defect, distinguish between few colours. a person is unable to The reason of this defect is the absence few colours. of cone cells sensitive for
This defect cannot be removed.
( v) Cataract In this defect. an opaque white membrane is developed on cornea due to which person lost power of vision partially on completely.
This defect can be removed by removing this membrane through surgery.
Camera
A photograph camera consists of a light proof box, at one end of which a converging lens system is fitted. A light sensitive film is fixed at the other end of the box, opposite to the lens system. A real inverted image of the object is formed on the film by the lens system.
f-Number for a Camera: The f-number represent the size of the aperture. f-number = Focal length of the lens( F) / Diameter of the lens( d) Generally 2, 2.8, 4, 5.6, 8, 11, 22, 32 are f-numbers.
The amount of light( L) entering the camera is directly proportional to the area( A) of the aperture, i. e.,
L ∝A∝ d 2
Brightness of Image ∝( d2 / f2)
where, d = dia meter of the lens and F = focal length of the lens. Exposure time is the time for which light is incident of photographic film.