(i) This is the thin and transparent layer on the front surface of the eyeball. It is the main refracting surface.
(ii) When light enters through the cornea, it refracts or bends the light on to the lens.

(i) A lens is an optically transparent medium bounded by two spherical refracting surfaces or one plane and one spherical surface.
(ii) Lens is basically classified into two types. They are: (i) Convex Lens (ii) Concave Lens.

(i) The rays coming from the objects which are at a distance less than 25 cm are not properly a focused on retina therefore ciliary muscles cannot be contracted beyond this limit.
(ii) The focal length of the eye cannot be reduced below 25 cm.

(i) At sunrise and sunset, the light rays from the sun have to travel a larger distance in the atmosphere than at noon.
(ii) Hence, most of the blue lights are scattered away and only the red light which gets least scattered reaches us.
(iii) Therefore, the colour of the sun is red at sunrise and sunset.

(i) Due to ageing, ciliary muscles becoming weak and the eye-lens becoming rigid (inflexible), the eye loses its power of accommodation.
(ii) Because of this, an aged person cannot see the nearby objects clearly. So, it is also called as 'old age hypermetropia'.

(i) Near point : The minimum distance required to see the objects distinctly without strain is called least distance of distinct vision. It is 25 cm for normal human eye.
(ii) Far point: The maximum distance up to which the eye can see objects clearly is called as far point of the eye. It is infinity for normal eye.

The other types and uses of microscopes are
(i) Travelling microscope: It is for measuring very small length with high degree of an accuracy at the order of 0.01 mm.
(ii) Electron Microscope: It is much higher magnifications and has a greater resolving power to see much smaller objects in finer detail.
(iii) Transmission Electron Microscope: It is capable of resolving to better than nanometre (10-9m). It is also capable of 3-Dimensional tomography.

(i) To see distant objects, the ciliary muscle relaxes and makes the eye lens thinner. This increases the focal length of eye lens.
(ii) Hence, the distant object can be clearly seen. When we look at a closer object, the focal length of the eye lens is decreased by the contraction of ciliary muscle.
(iii) Thus, the image of the closer object is clearly formed on the retina.

(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 anywhere on the principal axis at a finite distance: When an object is placed at a finite distance from the lens, a virtual image is formed between pole and focus of the convex lens. The size of the image is smaller than that of the object.

(i) Convex lenses are used as camera lenses.
(ii) Convex lenses are used as magnifying lenses.
(iii) Convex lenses are used in making microscope, telescope and slide projectors.
(iv) Convex lenses are used to correct the defect of vision called hypermetropia.

(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. It is a converging lens. So, 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. It is a diverging lens. So, a parallel beam of light passing through it is diverged or spread out. So, a concave lens is also called as diverging lens.

(i) Mie scattering takes place when the diameter of the scatterer is similar to or larger than the wavelength of the incident light. It is also an elastic scattering. The amount of scattering is independent of wave length.
(ii) Mie scattering is caused by pollen, dust, smoke, water droplets, and other particles in the lower portion of the atmosphere.

Based on initial and final energy of the light beam, scattering can be classified as, (i) Elastic scattering, (ii) Inelastic scattering.
(i) Elastic scattering : If the energy of the incident beam of light and the scattered beam of light are same, then it is called as 'elastic scattering'
(ii) Inelastic scattering : If the energy of the incident beam of light and the scattered beam of light are not same, then it is called as 'inelastic scattering:

(i) Tiny water droplets are formed in the atmosphere due to rainfall. The rainbow is formed when sun light falling on these tiny water droplets undergoes dispersion.
(ii) When the sun light is incident on them, it is first refracted and dispersed at the front surface. When the refracted light falls on the back surface of the water droplet, it undergoes total internal reflection.
(iii) This reflected beam of coloured lights are refracted at the front surface of the water droplet. Thus, the rainbows can always be seen opposite to the sun.

(i) The speed of light in a medium is low if the refractive index of the medium is high and vice versa.
(ii) When light travels from a denser medium into a rarer medium, the refracted ray is bent away from the normal drawn to the interface.
(iii) When light travels from a rarer medium into a denser medium, the refracted ray is bent towards the normal drawn to the interface.