Human Eye

Human Eye Light enters eye through a thin membrane called cornea. Iris behind the cornea is a dark muscular diaphragm which controls size of pupil. Pupil regulates & controls the amount of light entering the eye. Eyelens forms an inverted real image on retina. Retina is a light sensitive scr...

Human Eye Light enters eye through a thin membrane called cornea. Iris behind the cornea is a dark muscular diaphragm which controls size of pupil. Pupil regulates & controls the amount of light entering the eye. Eyelens forms an inverted real image on retina. Retina is a light sensitive screen How do we see Retina haslight sensitive cells. The cells get activated upon illumination & generate electric signals. These signals are then sent to brain via optic nerves. In bright light, iris contracts the pupil to allow less light to enter the eye. In dim light, iris expands the pupil to allow more light to enter. Thus, pupil opens completely through relocation of iris. How do we see distant objects Ciliary muscles are relaxed. Lens becomes thin. Focal length increases We can see distant objects How do we see close objects Ciliary muscles contract. Lens becomes thick Focal length decreases We can see nearby objects Ability of eye to adjust its focal length is called accomodation. Near point: 25 cm The minimum distance at which objects can be seen without strain. Far point: Infinity Cataract is when crystalline lens of eye becomes millky & cloudy. Eye lens becomes opaque We have two eyes as Detecting faint objects in better with 2. Wider field of view. To see world in 3D. One pair of eyes gives vision to 4 corneal blind people. Wavelength of red light is 1.8x of blue light. Stoplights are red as: Red light is least scattered by fog or smoke. Can be seen from distance. Sky appears dark to people at high altitude as scattering is not prominent at such heights. What should be kept in mind when donating eyes? Eyes must be recovered within 4-6 hours of death. Eye removal takes only 10-15 minutes. Person infected with diabetes, asthma can still donate eyes. Person with AIDS, Hepatitis, rabies shouldn't donate eyes. Myopia Near sightedness Cant see distant object clearly Image formed before retina Due to Excessive curvature of lens Elongation of eyeball Concave lens can fix. Hypermetropia Far sightedness Cant see near objects clearly Image formed after retina Due to Focal length is too long. Eyeball is too small Convex lens can fix Presbyopia: Defect due to gradual weakening of the ciliary muscles and diminishing ability of the eye lens. Cured by Bi-Focal lens. Prism has 2 triangular bases and 3 rectangular bases Band of coloured components of a light beam is called is spectrum. Splitting of light into its component colours is called dispersion.  Different colours of light bend through different angles with respect to the incident ray, as they pass through a prism. Red light bends the least Violet light bends the most Isaac newton was the first to use a glass prism to obtain the spectrum of sunlight. A rainbow is a natural spectrum appearing in the sky after a rain shower. It is caused by dispersion of sunlight by tiny water droplets. A rainbow is always formed in the direction opposite to sun. Water droplets act like small prism. Refraction, Internet Reflection and then Refraction happens. Atmospheric refraction in fire Air just above fire becomes hotter than air above it. Therefore the refractive index is lower than cooler air. So, objects seen through the hot air are on a different position from their apparent position Apparent position of stars Due to atmospheric refraction, atmosphere bends starlight towards the normal Thus, apparent position of star is different from actual position. It appears above its actual position. Rapid refraction also causes the starlight entering eye to flicker, thus we see stars twinkling. Planets Twinkle? Planets don't twinkle because. Much closer than stars Seen as an extended source of light. Atmospheric refraction is also responsible for seeing sun rising early and setting late. Rapid refraction through our atmosphere makes the sun rise 2 minutes early and set 2 minutes late. Flattening of sun also happens because of this. Scattering of light by colloidal particles gives rise to Tyndall effect. Seen when sunlight enters a dark room. - Fine particles scatter light with shorter wavelength. - Large particles scatter light with longer wavelength. Why is sky blue Molecules of air have size smaller than wavelength of visible light. They are more comfortable in scattering blue light than red  Thus, when sunlight enters out environment, blue light is scattered more than red light. And thus the atmosphere appears blue. At noon, the sun appears as white because all the colours of the white light are scattered away. Most refraction in the eye occurs when light enters through the outer surface of Cornea. Different coloured lights have different wavelength. Thus they bend at diferent anges of deviation through prism and emerge as a band of colours.