The shape of the lesson
By the end of the lesson, learners can
- Receptors and the eye: Click each part of the cut-away eye (or its chip) to read what it does.
- The pupil reflex: Drag the light slider from dim to bright and watch the iris.
- Focusing near and far: accommodation: Extended.
- Cornea refracts; lens focuses on to the retina; iris controls light entering the pupil.
- Bright light: pupil smaller; (Extended) circular muscles contract, radial relax.
- Near object (Extended): ciliary muscles contract, ligaments slack, lens fat, more refraction.
- Rods (Extended): dim light, not at the fovea. Cones: three kinds, colour, packed in the fovea.
One picture every idea returns to
Sense organs are groups of receptor cells responding to specific stimuli: light, sound, touch, temperature and chemicals. In the eye the cornea refracts light, the iris controls how much light enters the pupil, the lens focuses light on to the retina, the retina contains light receptors (some sensitive to different colours) and the optic nerve carries impulses to the brain. The pupil reflex changes pupil diameter as light intensity changes.
Forty-five minutes, phase by phase
| Time | Phase | What happens in the room | Grouping |
|---|---|---|---|
| 0 to 5 min | Hook: predict | Open with the question in the title, Light in, impulses out.. Sense organs are groups of receptor cells, each tuned to one kind of stimulus. Put the figure on the board: it is the one picture every idea in the lesson returns to. | Whole class, then pairs |
| 5 to 13 min | Teach and check: Receptors and the eye | Click each part of the cut-away eye (or its chip) to read what it does. Project tab 4 of the lab (Parts of the eye) and let a student drive it while the class predicts. | Whole class, then pairs |
| 13 to 21 min | Teach and check: The pupil reflex | Drag the light slider from dim to bright and watch the iris. Project tab 5 of the lab (The pupil reflex) and let a student drive it while the class predicts. | Pairs |
| 21 to 30 min | Teach and check: Focusing near and far: accommodation | Extended. Project tab 6 of the lab (Accommodation for near and distant objects) and let a student drive it while the class predicts. | Cooperative Concept Mapping |
| 30 to 36 min | Check | The four Stage 1 questions from the student lesson, answered on mini whiteboards or by show of fingers. Re-teach the one most get wrong. | Individual, then whole class |
| 36 to 43 min | Plenary | The exam-style questions from Stage 2, one at a time: thirty seconds alone, then a random call. The feedback lines in the key give the examiner's wording. | Individual, random call |
| 43 to 45 min | Exit and homework | Set the worksheet below (6 questions, 23 marks). Learners self-assess against the objectives before they leave. | Individual |
Protect the parts that carry the learning
The sequence carries about fifty minutes of material, so the lab is driven by one student while the class predicts rather than explored freely. Times are a guide.
Protected: the check at 30 minutes. It is the only individual read on whether the key idea landed, and it is not cut.
Cut first if the build overruns: the last check in Focusing near and far: accommodation, never the check or the plenary.
In a 60 minute block: run worksheet questions 1 to 3 in class as a Rally Coach set and model both the hardest worked example and its check live.
Cooperative Concept Mapping
Teams receive the key terms on cards and arrange them on a large sheet, drawing and labelling the links. Compare maps between teams before showing the model map.
The full structure, with its variations and the other twelve, is on the cooperative-learning page.
What to head off, and how
| Trap learners fall into | Teaching move that pre-empts it |
|---|---|
| The blind spot and the fovea are different places. | The fovea has the most receptors (cones); the blind spot has none at all, because the optic nerve leaves there. |
| Circular muscles contracting make the pupil smaller, like pulling a drawstring. | Students often swap the two sets: radial muscles contract to make the pupil wider. |
| When the ciliary muscles contract, the ligaments go slack, not tight. | The ciliary ring gets smaller, so it stops pulling on the lens, and the elastic lens springs fatter. |
Sense organs and the eye: worksheet
6 questions, 23 marks. Written in the style of the papers; no past paper question is reproduced.
Describe what is meant by a sense organ and name the sense organ that responds to each of: (a) light, (b) chemicals in food.
State the function of each of these parts of the eye: cornea, iris, lens, optic nerve.
A bright torch is shone into a person's eye. Describe and explain the change in the pupil. (Extended candidates should refer to the iris muscles.)
Extended. Explain how the eye changes so that a person can focus on a page held 25 cm from the eye after looking at a distant mountain.
Extended. The table shows the number of receptors in small areas of the human retina at different distances from the fovea.
| Distance from fovea / degrees | Cones / thousands per mm2 | Rods / thousands per mm2 |
|---|---|---|
| 0 | 150 | 0 |
| 20 | 8 | 150 |
| 60 | 5 | 80 |
(a) Describe the distribution of rods and cones. (b) Use the data to explain why a dim star is easier to see when you look slightly to one side of it.
Extended. Explain why we can see colour clearly in daylight but not in very dim light.
Answer key and marking notes
- A sense organ is a group of receptor cells that responds to a specific stimulus.
- (a) The eye.
- (b) The tongue (also accept the nose for chemicals in the air).
- Cornea: refracts light (as it enters the eye).
- Iris: controls how much light enters the pupil.
- Lens: focuses light on to the retina.
- Optic nerve: carries impulses from the retina to the brain.
- The pupil becomes smaller (its diameter decreases).
- This reduces the amount of light entering the eye, which protects the retina from damage.
- It is a reflex: automatic and rapid, triggered by light receptors in the retina.
- Extended: the circular muscles of the iris contract and the radial muscles relax; they act antagonistically.
- Light from the near page is diverging, so it needs more refraction to focus on the retina.
- The ciliary muscles contract.
- This reduces the tension in the suspensory ligaments, which go slack.
- The elastic lens becomes fatter (more convex).
- The fatter lens refracts light more, so the image is focused on the retina.
- (a) Cones are concentrated at the fovea and are few elsewhere.
- Rods are absent from the fovea, are most numerous a short distance from it and become fewer towards the edge of the retina.
- (b) Rods are more sensitive to dim light than cones (they give night vision).
- Looking slightly to one side puts the star's image on a region with many rods, instead of the fovea, which has no rods.
- Colour vision needs cones; there are three kinds, each absorbing light of a different colour.
- Cones only work in bright light.
- In dim light only the rods respond, and they do not distinguish colours, so we see in shades of grey.
8 multiple-choice questions on this lesson
From the topic question bank. Assign them in the app for a marked score in your gradebook, or run the topic as a class game.
Which is the best description of a sense organ?
- AA group of receptor cells responding to a specific stimulus
- BA single neurone that carries impulses to the brain
- CAn organ that secretes hormones into the blood
- DA muscle or gland that carries out a response
Which part of the eye controls how much light enters the pupil?
- AThe iris
- BThe cornea
- CThe lens
- DThe retina
A disease damages the optic nerve of one eye. What is the most likely effect on that eye?
- AImpulses from the retina cannot reach the brain
- BLight can no longer be refracted on to the retina
- CThe pupil can no longer change size in bright light
- DThe lens can no longer change shape for near objects
The graph shows the pupil diameter of a person before and after a bright light is switched on. What is the decrease in pupil diameter?
- A4 mm
- B2 mm
- C6 mm
- D8 mm
Why does the pupil become smaller in bright light?
- ATo reduce the light entering and protect the retina from damage
- BTo let more light reach the retina so the image is brighter
- CTo change the shape of the lens for focusing
- DTo stop light reaching the blind spot
Extended: in dim light, which change takes place in the iris?
- ARadial muscles contract and circular muscles relax
- BCircular muscles contract and radial muscles relax
- CBoth radial and circular muscles contract
- DCiliary muscles contract and the lens becomes fatter
Extended: as people age the lens becomes less elastic. Which task becomes hardest?
- AReading print held close to the eyes
- BSeeing a distant mountain clearly
- CSeeing in very dim light at night
- DTelling red from green objects
Extended: which statement about the fovea is correct?
- AIt is packed with cones and has no rods
- BIt is packed with rods and has no cones
- CIt has no receptors because the optic nerve leaves there
- DIt contains equal numbers of rods and cones
Quiz key
A sense organ is a group of receptor cells that respond to a specific stimulus such as light, sound, touch, temperature or chemicals. A single neurone only transmits impulses; it does not detect a stimulus as an organ. Secreting hormones describes an endocrine gland, and a muscle or gland that responds is an effector.
The iris contains muscles that change the size of the pupil, controlling how much light enters. The cornea refracts light, the lens focuses light on to the retina, and the retina contains the light receptors; none of them changes the size of the pupil.
The optic nerve carries impulses from the retina to the brain, so damage stops the image being seen even though light is still focused. Refraction is done by the cornea and lens, not the nerve. Changing pupil size is done by the iris, and changing lens shape by the ciliary muscles; neither is the job of the optic nerve.
The diameter falls from 6 mm before the light to a steady 2 mm afterwards, a decrease of 6 − 2 = 4 mm. 2 mm is the final diameter, not the change. 6 mm is the starting diameter. 8 mm is the top of the axis and was never reached.
In bright light the pupil reflex reduces the pupil diameter so less light enters, preventing damage to the retina. Letting in more light is the dim-light response. Lens shape is changed by the ciliary muscles in accommodation, not by the pupil. The blind spot has no receptors, so protecting it is not the purpose.
In dim light the radial muscles contract and the circular muscles relax, widening the pupil to let in more light; the two sets are antagonistic. Circular muscles contracting is the bright-light response. Both sets cannot contract at once in this reflex because they pull in opposite directions. Ciliary muscles and lens shape belong to accommodation, not the pupil reflex.
For near objects the ciliary muscles contract, the suspensory ligaments slacken and an elastic lens springs into a fatter shape to refract light more. A less elastic lens stays thin, so near objects blur. Distant vision needs a thin lens, which is still achieved. Night vision depends on rods and colour vision on cones, not on the lens.
The fovea is a small area of the retina packed with cones and with no rods, giving sharp colour vision. Rods are spread across the rest of the retina, not concentrated at the fovea. The place with no receptors where the optic nerve leaves is the blind spot. Rods and cones are not equal anywhere: cones dominate the fovea and rods the rest of the retina.
What to project
The student lesson embeds these tabs. Open them full screen on the projector; every slider has a drag handle on the stage.
Slides for this lesson
A projectable deck for every pack is on the way. Until then, present the student lesson full screen: its sections, figure and lab tabs follow the same order as the plan above.