Physics of Vision

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The Eye as a Lens

The eye acts as a lens and focuses light so we can see.

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The structure of the eye

  • The cornea and lens in the eye form a system that acts as a single thin lens.
  • For clear vision, the image must be projected onto the retina, which is a fixed distance from the lens.
  • The lens of the eye adjusts its power to produce an image on the retina for objects at different distances.
  • The pupil changes its size depending on the intensity of light so the retina isn't damaged.
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Image formation

  • The cornea provides about two-thirds of the power of the eye, the lens provides the remaining power needed to produce an image on the retina. -The cornea and lens can be treated as a single thin lens.
  • The image is inverted on the retina but the brain flips the image so it is upright.

Sensitivity of the Eye

The eye is sensitive to wavelengths in the range 380 to 760nm approximately.

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Rods and cones

  • The retina contains two types of light sensitive cells, rods and cones.
  • Rods can detect low intensity light on a grey scale, cones detect high intensity light and different wavelengths or colours.
  • Rods and cones are photo detectors because changes in intensity and colour are converted by them into electrical signals which are then interpreted by the brain.

How the eye resolves

The eye receives many different colours and intensity of light from many distances. It has several mechanisms in place to resolve all of these.

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  • When our eyes receive pure-wavelength light, we tend to see only a few colours. These are the rainbow of colours produced when white light is dispersed according to different wavelengths.
  • The two major types of light-sensing cells (photoreceptors) in the retina are rods and cones.
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  • Rods are more sensitive than cones by a factor of about 1000.
  • Rods are responsible for peripheral vision as well as vision in very dark environments.
  • There are about 120 million rods in the human retina.
  • Rods do not give colour information. You may notice that you lose colour vision when it is very dark, but you see different shades of grey.
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  • Cones are responsible for central vision.
  • The cones work best in bright light and are responsible for high-resolution vision.
  • There are about 6 million cones in the human retina.
  • There are three types of cones, and each type is sensitive to different ranges of wavelengths.
    • These wavelengths correspond to the primary colours- red, green and blue .

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