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Focussing Light onto the retina

Focussing Light onto the retina. WALT How light rays are focussed onto the retina How the cornea and lens refract light rays Accommodation of the eye. Controlling light levels. Your eye are very sensitive and can be damaged by harsh light.

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Focussing Light onto the retina

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  1. Focussing Light onto the retina • WALT • How light rays are focussed onto the retina • How the cornea and lens refract light rays • Accommodation of the eye

  2. Controlling light levels • Your eye are very sensitive and can be damaged by harsh light. • Your iris controls light allowed into the eye by changing the size of the pupil

  3. The colored part of the eye is called the iris.  It controls light levels inside the eye similar to the aperture on a camera.  Radial and circular muscles in the iris control pupil size in response to light The round opening in the center of the iris is called the pupil. 

  4. Control of Pupil Size

  5. Focusing on objects • The lens and cornea focus the light on the retina

  6. Focusing • The lens job is to make the rays hit the same point The red rays will be out of focus

  7. Bending light • Light is refracted and bent to focus it as it passes through the lens • Lens thickness can be changed so the amount of bending is changed

  8. Near vision • The lens needs to bend the light more to focus it • Fat lens needed

  9. Distance vision • Rays enter the eye closer together • Need less bending • Thinner lens needed

  10. Changing lens thickness Ciliary muscles are attached to the lens, when relaxed they pull on the suspensory ligaments causing the lens to be thin When contracted the ciliary muscle cause the suspensory ligaments to slacken and so the lens is fatter

  11. Accommodation • When we open our eyes in the morning they are not focussed on near by objects • At rest the ciliary muscles relax , puling the lens flat • In this state we can focus on distant objects • To focus on something near by the ciliary muscles contract reducing the tension on the suspensory ligaments so the lens gets fatter • This is called accommodation – changing the shape of the lens to focus on near or distant objects

  12. When the eye looks at an object that is far away the ciliary muscle relaxes which pulls on the suspensory ligaments. These pull on the lens and make it flatter (less convex). This brings the rays of light from the object into crisp focus on the retina.

  13. Distant Objects

  14. With an object closer to the eye, the lens needs to be more convex (fatter). To do this, the ciliary muscles contract making the suspensory ligaments slack and to allow the rubbery lens to return to its naturally rounder shape.

  15. Near Objects

  16. Myopia(short sightedness) the eye is too long and so the image focuses short of the retina

  17. Long sightednessthe eye is too short so the imagefocuses past the retina

  18. Correcting short sightednessa concave lens is used.

  19. Correcting long sightedness-a convex lens is used

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