الأسبوع 6 · المحور الأول، الوحدة الأولى، المفهوم 1.3 «الضوء وحاسة البصر» (أ): كيف نرى وكيف ترى الحيوانات في الظلام · 3 دروس
3
Eyeshine: the mirror behind the eye
Discovery Education Science P4: Term 1, Unit 1, pp. 58–69
GoalExplain eyeshine: the shiny layer behind some animals’ eyes that bounces light back.
New words
eyeshine · the glow seen in some animals’ eyes when light hits them at nightshiny layer · a mirror-like layer behind the eye that bounces light backbounce · to hit something and come back
Explanation
At night, the eyes of a cat or a dog can seem to glow in the headlights of a car. This is eyeshine. It happens because many animals have a shiny layer at the back of the eye, like a tiny mirror. Light enters the eye, passes the part that senses light, and hits the shiny layer. The layer bounces the light back, so the eye gets a second chance to sense it. This helps the animal see in dim light. Some of the bounced light also leaves the eye, and that is the glow we see. Cats, dogs, horses and many other animals have this layer. People do not, so our eyes do not shine, and that is one reason we see worse than cats in the dark. Never shine a torch into an animal’s eyes or use a camera flash on it.
light enters the eye→hits the shiny layer→bounces back→second chance to sense it
Worked examples
A parked car’s headlights shine down a dark road. Two green dots shine at the roadside. They are the eyes of a cat. The shiny layer bounces the light back.
Foil bounces more light than plain paper. Hamza sticks foil on one card and nothing on another. In a dim room, under a torch from the side, the foil card looks brighter. The foil is a model of the shiny layer.
Shiny layer?
person
no
horse
yes
cat
yes
A person, a horse and a cat are compared. Only the horse and the cat have the shiny layer, so only their eyes shine at night.
Class activity
Mirror-layer model. A grown-up cuts two cards the same size. Glue kitchen foil on one and leave the other plain. In a dim room, hold the torch at the side, about one metre away, and shine it on each card in turn without looking into it. Which card looks brighter? Foil bounces more light, like the shiny layer in a cat’s eye. Keep the same distance for both cards.
Practice
1
Why do a cat’s eyes seem to glow in headlights?
A shiny layer behind the eye bounces light back.
2
How does the shiny layer help the cat see in dim light?
The light gets a second chance to be sensed.
3
Do people have a shiny layer behind the eye? Does it help or not?
No. So our eyes do not shine, and we see less well in the dark.
