1. Vision LO: To know how lenses can be used to correct problems with vision Can you come up with any conditions that can affect the eye or your vision?

2. How the lens works:

3. Short and long sightedness Short sight occurs where the eye cannot focus on distant objects. The uncorrected image is formed in front of the retina. The muscles cannot make the lens thin enough to focus the image. – What type of lens is used to correct short sight? Long sight occurs when the eye cannot focus on nearby objects. The uncorrected image is formed behind the retina. – What type of lens is used to correct long sight?

4. 06/12/2015 Using lenses to correct vision ShortLong

5. Comparing the eye to a camera The EyeThe Camera Type of LensVariable focus converging lens Fixed focus converging lens Focussing AdjustmentCiliary muscle alters the lens thickness Adjustment of the lens position ImageReal, inverted, magnification less than 1 Image DetectionLight sensitive cells on the retina Photographic film or CCD sensors (digital) Brightness ControlIris controls width of pupilAdjustment of aperture ‘stop’

6. Lens power The power of a lens is defined as: 1 focal length in meters The unit of power is the dioptre (D) The type of lens is indicated by: -a positive value for a converging lens -a negative value for a diverging lens What is the power of a converging lens of focal length 0.05m?

7. Spec Savers Opticians are continually trying to design glasses and contacts to be as thin as possible. What factors do you think will affect the thickness of glasses lenses? – The refractive index and the curvature of the lens The greater the refractive index or the greater the curvature of the lens surface the greater the power of the lens. For a lens of a given focal length, the greater the refractive index of the lens material, the flatter and thinner the lens can be manufactured. This is because the lens surface would be less curved.

8. Photographic evidence The camera never lies… Is this statement true? Can you identify whether these images are real or fake: RealFake Real

9. Task and Extension How can lenses be used to correct long and short sightedness? Use diagrams and your knowledge of lenses to explain how laser eye surgery works to correct vision defects?