It is the coloured part of the eye. It may be blue, brown or green in colour. Every person has a unique colour, pattern and texture. Iris controls amount of light entering into the pupil like camera aperture.

(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) 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) 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) 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.

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 red colour has longer wavelength.
(ii) So, it can travel for longer distance and will be seen clearly. So red colour is used in traffic signals.

The ability of the eye lens to focus nearby as well as the distant objects is called power of accommodation of the eye.

Snell's law state that, the ratio of the sine of the angle of incidence and sine of the angle of refraction is equal to the ratio of refractive indices of the two media.
\(\frac{\sin i}{\sin r}=\frac{\mu_2}{\mu_1}\)