Light (science)

Lens: A lens is a transparent medium bounded by two spherical surfaces 

·         Centre of curvature:

The centre of the curved surface of a lens is called centre of curvature. A lens has two centers of curvatures, which are denoted by c₁ and c₂ as shown in figure (i).

·         Principles axis:

The line passing through the two centers of curvature of a lens is known as Principle axis as shown in figure (i).

·         Optical centre:

The geometrical centre of a lens is called its optical centre. A ray of light passing through it does not bend. It is denoted by ‘o’.

·         Principle Focus:

When narrow parallel rays of light converge to a point or appear to come from a point after refraction. The point is known as Principle focus or focus. It is denoted by F.

·         Focal length:

The distance between principle focus (F) and optical centre (O) is known as focal length. It is denoted by f. It is taken as +ve for convex lens and +ve, for concave lens.

Image formed by a convex lens:

SN	Position of		Nature & size
	Object	Image
1	At infinity	At focus F	Real, inverted and highly diminished
2	Beyond 2F	Between F & 2F	Real, inverted and diminished.
3	At 2F	At 2F	Real, inverted and of the same size.
4	Between F & 2F	Beyond 2F	Real, inverted and magnified.
5	At F	At infinity	Real, inverted and highly magnified.
6	Between F & Optical center	On the same side of lens	Virtual, erect and magnified.

·      ·         In concave lens the image formed is virtual, erect ad diminished. When the object is at infinity the image will be formed at focus. When the object is in between optical centre and infinity, the image will be formed in between focus and optical centre on the same side of the lens as shown in the figure.

Magnification:

The process of variation of the size of image produced by a lens, of position of object is called magnification. Mathematically, it is equal to the ratio of height of the image to the height of the object. It is denoted by ‘m’

The power of lens:  

The power of lens is the ability to converge or to diverge a beam of light. Mathematically, it is equal to the reciprocal of focal length (in meter) of the lens. It is denoted by P. It is unit is Dioptre (D).

Optical Instrument:  

Optical instrument is that instrument in which mirrors, prisms, lens etc. are used. It is of two types.

                                                              i.      Real image forming: Camera, film projector, slide projector, eye, etc.

                                                            ii.      Virtual image forming: Binoculars, microscope, telescope, etc.

Camera:   

It is an optical instrument used for taking photographs.

·         The inner surface of the camera is made black in order to absorb maximum light which passes through the lens.

·         The main parts of the human eyes are (i) cornea (ii) iris (iii) lens (iv) ciliary muscle (v) optic nerve (vi) retina, etc.

·         Light coming from an object enters in the lens through the cornea. After refraction through the lens, it is focused on the retina, where inverted image of the object is formed. The op tic nerves transmit the image impulse (making it erect) to the brain. 

The nearest point

The nearest point, up to which an object can be seen clearly, by an eye, is known as near point.  (25cm) of that eye.

The farthest point

The farthest point, up to which objects can be seen clearly, by an eye, is known as far point. (Infinity) of that eye.

Accommodation:

 The ability of the eye lens to focus objects at various positions on the retina is known as accommodation of the eye. Ciliary muscles conduct it by its contraction and relaxation.

·         The size of the pupil becomes small in bright light.

·         The size of the pupil becomes big in dim light.

The ordinary sun glass should not be used for a long time even in sunny days. The sunglass makes shade to the eyes due to which the size of the pupil increases eve in bright light. The ultra- violet rays passing through the glass falls on the retina. It may damage the retina.

Defect of vision:

It is a defect due to which the image of distance or nearer object is not formed at retina. Hence the object appears unclear.

Short sightedness (myopia):  

The defect of vision, in which a person cannot see the distant objects, clearly, is known as short sightedness. The image of distant objects is formed in front of retina as shown in figure (i).  It is caused by elongation of the eye ball or shortening of focal length of eye lens. The removal of the defect is to use concave lens as shown in figure (ii). A person suffering from this defect holds the newspaper at proper distance from his/her eyes, while reading without using lens.

Long Sightedness (hypermetropia):

The defect of vision which a person cannot see the near object clearly, is known as long sightedness.  The image of near objects is formed behind the retina as shown in figure (i). It is caused by shortening of the eye ball or increasing focal length of the eye lens. The removal of the defect is to use concave lens as shown in figure (ii). A person suffering from this defect holds the newspaper a little bit away from his/ her eyes, while reading without using lens.

 Differences between eye lens and camera.

Eyes	Camera
Its focal length is variable.	Its focal length is fixed for a camera.
The image is formed on the retina.	The image is formed on the film.
Retina of an eye is used again and again.	A photographic film cannot be reused.
It forms temporary image.	It forms permanent image.

Similarities between the camera and the eyes are:

                                                              i.        Both of them have a convex lens and form a real and inverted image.

                                                            ii.      The diaphragm of camera does the same work of pupil of eye.

                                                          iii.      The shutter of camera does the same work of eyelids of eye.

Differences between a compound microscope and a telescope:

Compound Microscope	Telescope
It is used for viewing microscopic objects clearly.	It is used for viewing heavenly bodies clearly.
It is used by pathologists or scientists.	It is used by astronomers.
The focal length of objective lens is shorter than that of eye lens.	The focal length of objective lens is longer than that of eye lens.
The image formed by the objective lens lies within the focus of the eyepiece lens.	The image formed by the objective lens lies exactly at focus of the eyepiece lens.
The final image is formed beyond the objective lens.	The final image is formed at infinity.