Rods and cones: threats, stars and roses

The Weekend Australian - Travel - - General Appointments -

What are they? Light-sen­si­tive cells in the retina of our eye. They are called pho­tore­cep­tors, which means they re­spond to light that hits our retina, send­ing sig­nals down nerves to our brain, en­abling us to see. There are about 100 mil­lion rods and 5 mil­lion cones in the retina. Why do we have two types of pho­tore­cep­tors? Be­cause they do very dif­fer­ent jobs. Cones are sen­si­tive to colour and don’t work well in low light. Rods don’t re­spond to colour but come into their own in very dim light. We see very lit­tle colour in a dark room be­cause it’s pre­dom­i­nantly the rods that are work­ing then. How can rods pro­tect us from lurk­ing dan­ger? Rods and cones also dif­fer in their po­si­tion on the retina: the cones are con­cen­trated in the cen­tre; and the rods, which are also very sen­si­tive to ob­jects mov­ing in our field of vi­sion, pre­dom­i­nate in the pe­riph­ery. So the pho­tore­cep­tors that dom­i­nate in our pe­riph­eral vi­sion are the most light- and move­ment-sen­si­tive, help­ing us spot threats in the cor­ner of our eye. And help us see stars? If you want to see a dim star more clearly, don’t look straight at it. Use your ul­tra­sen­si­tive rods to get a bet­ter view by look­ing slightly away from it. How can rods and cones make red roses change colour? Rods aren’t very sen­si­tive to red light. So in bright day­light, when the colour-sen­si­tive cones pre­dom­i­nate, red rose petals ap­pear bright com­pared to the green leaves. But when the light fades, and the rods be­come more ef­fec­tive, the red petals look dull in com­par­i­son to the leaves.

Il­lus­tra­tion: Nathalie Gar­cia

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