Optics of eye

Optics of eye

1. Optics of the eye By ansiya prasad

2. Objectives : Eye and its components. Axis and visual angles of eye . Describe the properties of light . Differentiate between convex and concave lenses. Describe Accommodation. Describe how the eye focuses light. Differentiate between presbyopia , Myopia , hyperopia and astigmatism.

3. Eye as a camera : Eyelids – shuttter . Cornea – focusing system. Lens – focusing system. Iris – diaphragm . Choroid – dark chamber. Retina – light sensitive film.

4. Cornea : Cornea is a plus lens of +42D Radius of curvature of ant.surface = 7.8mm post.surface = 6.8mm Diameter about 11.5mm and thickness of 0.5 – 0.6mm in centre and 0.6 – 0.8mm periphery. An average horizontal diameter = 11.7 mm vertical diameter = 11mm.

5. Contd... Reasons of refraction : curvature Significant differences in refractive index .

6. Crystalline lens : Lens accounts for about one-third of refraction of eye. accomodation provides a mechanism of focusing at different diatances. Radius of curvature ant.surface = 10mm post.surface = 6mm Refractive index nucleus- 1.41 pole – 1.385 cortex – 1.38 Total power – 15 -18 D.

7. Anterior chamber : Is a cavity between cornea and iris . Filled with aqueous humour . Refractive index of aqueous humour is 1.336. depth of aqueous chamber is 2.5 -4.00mm change in anterior chamber depth , change the total power.

8. Iris and pupil : Regulates the amount of Light entering the eye. at 2.4mm Pupil size, best retinal image obtained, as abberation and diffraction are balanced. pupil size varies 2-4mm in diameter in bright light 4-8mm in diameter in dim light .

9. Vitreous humour : Is a transparent ,colorless ,gelatinous mass that fills the space in between lens and the retina . is 8-9mm in diameter. Refractive index of vitreous humour is 1.337.

10. Retina : Maximum resolving power is at fovea light sensitive film. Concave spherical surface with radius of 12mm. 22mm in diameter . contains about 7 million cones and 75- 150 million rods. Advantages of curvature of retina is that Curved images formed by the optical system Is brought in the right order. A much wider field of view Is covered by the steeply curved retina .

11. Axis and visual angles of eye : Optical axis – the line passing through the centre of cornea and centre of lens and meets the retina on the nasal side of the fovea. Visual axis – is the line joining the fixation point , nodal point and the fovea.

12. Axis and visual angles of eye : Fixation axis – is the straight line that joins centre of rotation of eyeball with fixation point .

13. Visual angles : Angle alpha – angle formed between visual axis and optical axis at nodal point. Angle gamma – angle between optic axis and fixation axis . Angle kappa – angle formed between pupillary line and visual axis .

14. Properties of light Electromagnetic radiation Visible light Wavelength of 400- 700nm Violet- short wavelength , highest energy . Red- long wavelength , lowest energy.

15. Properties of light Reflection Light blocked by non- transparent object and bounce off a surface . Refraction Bending of light as it passes from one transparent medium to another with a different density .

16. Focusing light on the retina : Light is bent 3 times Enters the cornea Enters the lens Leaving the lens Only can adjust the amount of refraction through lens.

17. Convex And concave lenses : Convex lenses Light converges as it passes through. Converge to single point = focal point A thick lens causes more refraction and therefore shorter focal point . A thin lenses causes less refraction and therefore longer focal point.. Results in image being upside down and reversed at focal point.

18. Convex and concave lenses Concave lenses Diverge ( spreads out the light rays that have been refracted through it )Light .

19. Changing the thickness of lens : Ciliary muscles

20. Emmetropic eye Distance Light coming to eye as parallel rays Dont need to bend light as much Ciliary muscles are relaxed

21. Emmetropic eye Near vision Light coming in as a divergent rays . Need to thicken the lens . ciliary muscles contracted.

22. Focusing for close vision Accommodation Of lenses its the ability of eye to change its focus from distant to near objects , this process is achieved by the lens changing its shape . Ciliary muscles contract so lens can thicken. Far point of vision = Farthest point till where we can see. That distance in which no accommodation Is needed ( 6m Or 20ft ). near point of vision= the nearest point Closest point on which we can focus clearly ( lens cant get any fatter )

23. Contd ... Constriction of pupils Prevents the most divergent light rays from entering the eye . convergence of eyeballs Keeps object focus on fovea . getting cross eyed.

24. Myopia : Near sightedness parallel rays of light are focused in front of retina when accommodation is at rest . occurs when eyeball is too long or cornea becomes too steeply curved . Can correct through concave lenses.

25. Hyperopia: Long sightedness. parallel rays of light coming from infinity are focused behind the retina with accommodation being at rest. Occur if eye is too short can correct through convex lens .

26. Presbyopia :

27. Astigmatism :

28. Thank you