Cartesian sign conventions are used for measuring the various distances in the ray diagrams of spherical lenses. According to the Cartesian sign convention.
i) The object is always placed on the left side of the lens.
ii) All the distances are measured from the optical center of the lens. 
iii) The distances measured in the same direction as that of incident light are taken as positive.
iv) The distances measured against the direction of incident light are taken as negative.
v) The distances measured upward and perpendicular to the principal axis are taken as positive.
vi) The distances measured downward and perpendicular to the principal axis are taken as negative.

i) Object at infinity:
1.  When an object is placed at infinity, p real image is formed at the principal focus.
2. The size of the image is much smaller than that of the object.

ii) Object placed beyond C (>2F):
1. When an object is placed behind the center of curvature (beyond C), a real and inverted image is formed between the center of curvature and the principal focus.
2. The size of the image is the same as that of the object.

iii) Object placed at C:
1. When an object is placed at the center of curvature, a real and inverted image is formed at the other center of curvature.
2. The size of the image is the same as that of the object.

iv) Object placed between F and C:
1. When an object is placed in between the center of curvature and principal focus, a real and inverted image is formed behind the center of curvature.
2. The size of the image is bigger than that of the object.

v) Object placed at the principal focus F:
1. When an object is placed at the focus, a real image is formed at infinity.
2. The size of the image is much larger than that of the object.

(i) The ability of the eye lens to focus nearby as well as the distant objects is called power of accommodation of the eye.
(ii) This is achieved by changing the focal length of the eye lens with the help of ciliary muscles.
(iii) Eye lens is made of a flexible, jelly-like material.
(iv) By relaxing and contracting the ciliary muscle, the curvature and hence the focal length of he eye lens can be altered.
(v) When we see distant objects the ciliary muscle relaxes and makes the eye lens thinner
(vi) This increases the focal length of the eye lens. Hence, the distant object can be clearly seen.
(vii) On the other hand, when we look at a closer object, the focal length of the eye lens is decreased by the contraction of ciliary muscle.
(viii) Thus, the image of the closer object is clearly formed on the retina.

Advantages :
(i) Elaborate view of the Galaxies, Planets, stars and other heavenly bodies is possible.
(ii) Camera can be attached for taking photograph for the celestial objects.
(iii) Telescope can be viewed even with the low intensity of light.
Disadvantages :
(i) Frequent maintenance needed.
(ii) It is not easily portable one.

Telescope is an optical instrument to see the distant object.
Types of telescope: According to optical property, it is classified into 2 groups (i) Refracting telescope (ii) Reflecting telescope.
Refracting Telescope :
Lenses are used. Galilean telescope, Keplerian telescope, Achromatic refractors, are some refracting telescopes.
Reflecting Telescope :
Parabolic mirrors are used Gregorian, Newtonian, Cassegrain telescope are some Reflecting telescopes. According to the things which are observed, Astronomical Telescope and Terrestrial Telescopes are the two major types of telescope.
Astronomical Telescope :
An astronomical telescope is used to view heavenly bodies like stars, planets galaxies and satellites.
Terrestrial Telescope :
The image in an astronomical telescope is inverted. So, it is not suitable for viewing objects on the surface of the Earth. Therefore, a terrestrial telescope is used. It provides an erect image. The major difference between astronomical and terrestrial telescope is erecting the final image with respect to the object with the help of correcting lens.

The human eyes are most valuable and sensitive organs responsible for vision. They are the gateway to the wonderful world.
(i) Structure of the eye:
(i) The eye ball is approximately spherical in shape with a diameter of about 2.3 cm. It consists of a tough membrane called sclera, which protects the internal parts of the eye.
(ii) Cornea:
(i) This is the thin and transparent layer on the front surface of the eyeball.
(ii) It is the main refracting surface.
(iii) When light enters through the cornea, it refracts or bends the light on to the lens
(iii) Iris:
(i) It is the coloured part of the eye.
(ii) It may be blue, brown or green in colour.
(iii) Every person has a unique colour, pattern and texture. 
(iv) Iris controls amount of light entering into the pupil like camera aperture.
(iv) Pupil: 
(i) It is the centre part of the Iris. It is the pathway for the light to retina.
(v) Retina:
(i) This is the back surface of the eye.
(ii) It is the most sensitive part of human eye, on which real and inverted image of objects is formed.
(v) Ciliary muscles :
(i) Eye lens is fixed between the ciliary muscles    
(ii) It helps to change the focal length of the eye lens according to the position of the object.
(vi) Eye lens:
(i) It is the important part of human eye.
(ii) It is convex in nature.

Cartesian sign conversions are used for measuring the various distances in the ray diagrams of spherical lenses. According to Cartesian sign convention,
(i) The object is always placed on the left side of the lens.
(ii) All the distances are measured from the optical centre of the lens.
(iii) The distances measured in the same direction as that of incident light are taken as positive.
(iv) The distances measured against the direction of incident light are taken as negative.
(v) The distances measured upward and perpendicular to the principal axis is taken as positive.
(vi) The distances measured downward and perpendicular to the principal axis is taken as negative.

(i) Object at infinity.
(ii) Object anywhere on the principal axis at a finite distance.
(iii) Position and size of image with object distance.
(i) Object at infinity:
When an object is placed at infinity, a virtual image is formed at the focus. The size of the image is much smaller than that of the object.

(ii) Object at infinity: 
When an object is placed at a finite distance from the lens, a virtual image is formed between optical center and focus of the concave lens. The size of the image is smaller than that of the object.

(iii) Object at infinity : 
As the distance between the object and the lens is decreased, the distance between the image and the lens also keeps decreasing. Further, the size of the image formed increases as the distance between the object and the lens is decreased.

(i) Object at infinity.
(ii) Object placed beyond C (>2F).
(iii) Object placed at C.
(iv) Object placed between F and C.
(v) Object placed at the principal forces F.
(vi) Object placed between the principal forces F and optical centre O.
(i) Object at infinity : 
When an object is placed at infinity, a real image is formed at the principal focus. The size of the image is much smaller than that of the object.

(ii) Object place beyond C (>2F): 
When an object is placed behind the center of curvature (beyond C) a real and inverted image is formed between the center of curvature and the principal focus. The size of the image is same as that of the object.

(iii) Object place at C:
When an object is placed at the center of curvature, a real and inverted image is formed at the other center of curvature, the size of the image is the same as that of the object.

(iv) Object place between F and C:
When an object is placed in between the center of curvature and principal focus, a real and inverted image is formed behind the center of curvature. The size of the image is bigger than that of the object.

(v) Object placed at the principal focus F: 
When an object is placed at the focus, a real image is formed at infinity. The size of the image is much larger than that of the object.

(vi) Object placed between the principal focus F and optical centre O:
When an object is placed in between principal focus and optical centre, a virtual image is formed. The size of the image is larger than that of the object.

Plano — convex lens :
If one of the faces of a bi-convex lens is plane, it is known as a Plano—convex lens.
Plano — concave lens :
If one of the faces of a bi-concave lens is plane, it is known as Plano—concave lens.

A lens is an optically transparent medium bounded by two spherical refracting surfaces or one plane and one spherical surface.
Lens is basically into 2 types. They are Convex lens and Concave lens.
i) Convex (or) bi—convex lens: 
It is a lens bounded by two spherical surfaces such that it is thicker at the centre than at the edges. A beam of light passing through it, is converged to a point. So a convex lens is also called as Converging lens.
ii) Concave (or) bi—concave lens:
It is a lens bounded by two spherical surfaces such that it is thinner at the centre than at the edges. A parallel beam of light passing through it, is diverged or spread out. So a concave lens is also called as Diverging lens.

(i) When a parallel beam of monochromatic light passes through a gas (or) liquid or transparent solid, a part of light rays are scattered.
(ii) The scattered light contains some additional frequencies other than that of incident frequency. This is known as Raman scattering (or) Raman effect.
(iii) Raman scattering is defined as the interaction of light ray with the particles of pure liquids or transparent solids, which leads to a change in wave length or frequency.
(iv) The spectral lines having frequency equal to the incident ray frequency is called Rayleigh line and the spectral lines which are having frequencies other than the incident ray frequency are called Raman lines.
(v) The lines having frequencies lower than the incident frequency is called Stokes lines and the lines having frequencies higher than the incident frequency are called Antistokes lines.

When a beam of sunlight, enters into a dusty room through a window, then its path becomes visible to us. This is because, the tiny dust particles present in the air of the room scatter the beam of light. This is an example of Tyndall scattering.
The scattering of light rays by the colloidal particles in the colloidal solution is called Tyndall scattering (or) Tyndall effect.

(i) Rayleigh's scattering law states that "the amount of scattering of light is inversely proportional to the fourth power of its wave length".
(ii) Amount of scattering 'S' \(\alpha \frac{1}{{\lambda }^{4}}\).
(iii) According to this law, the shorter wave length colours are scattered much more than the longer wave length colours.

(iii) When the sunlight passes through the atmosphere, the blue colour is scattered to a greater extent than the red colour (long wave length). This scattering cause the sky to appear in blue colour.
(iv) At sunrise and sunset, the light rays from the sun have to travel a larger distance in the atmosphere than at noon. Hence, most of the blue lights are scattered away and only the red light which gets least scattered reaches us. Therefore, the colour of the Sun is red at sunrise and sunset.