This MCQ module is based on: Lenses and Eye Defects
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Lenses and Eye Defects
🎓 Class 8
Science
CBSE
Theory
Ch 10 — Force and Pressure
⏱ ~28 min
🌐 Language: [gtranslate]
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Myaischool Lt Science Assessment
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[myaischool_lt_science_assessment grade_level="class_8" science_domain="physics" difficulty="basic"]
10.9 From Mirrors to Lenses
A mirror bends light by reflection. A lens is a transparent piece of glass or plastic that bends light by refraction — as the light passes through it. Every lens has two faces, at least one of which is curved.
Two Basic Shapes
- Convex (converging) lens — thicker in the middle, thinner at the edges. Brings parallel rays together at a focus.
- Concave (diverging) lens — thinner in the middle, thicker at the edges. Spreads parallel rays outwards, as if from a focus behind the lens.
10.10 Images Formed by Lenses
A convex lens behaves very much like a concave mirror — the nature of its image depends on where the object is placed relative to the focus. A concave lens behaves like a convex mirror — it always gives a small, virtual, erect image.
| Lens | Object Position | Image |
|---|---|---|
| Convex | Very far (at infinity) | At F, real, inverted, point-sized |
| Convex | Beyond 2F | Between F and 2F, real, inverted, smaller |
| Convex | Between F and 2F | Beyond 2F, real, inverted, enlarged |
| Convex | Between lens and F | Same side, virtual, erect, enlarged (magnifying glass!) |
| Concave | Anywhere | Same side, virtual, erect, diminished |
10.11 Where Lenses Help Us
Convex (Converging) Lens — Busiest Helper
Magnifying Glass
Object within focal length → enlarged virtual image. Used by watch-makers, jewellers and stamp collectors.
Camera Lens
Forms a small, real, inverted image of the scene on the sensor or film.
Microscope
Two convex lenses together magnify tiny objects like cells and microbes.
Telescope (refracting)
A large convex lens (objective) gathers light; a second convex lens (eyepiece) magnifies the image.
Projector
A convex lens creates a large, inverted real image on the screen. The slide is placed upside-down inside!
Human Eye
The eye itself uses a natural convex lens to focus light on the retina.
Concave (Diverging) Lens
Spectacles for Myopia
Corrects short-sightedness by diverging rays slightly before they reach the eye.
Galilean Telescope Eyepiece
Older-style opera glasses use a concave eyepiece so the image appears upright.
Peephole (door viewer)
A combination including concave lenses gives a wide-angle view of the corridor.
10.12 The Human Eye — a Living Camera
The human eye is the finest natural optical instrument. It behaves like a tiny camera: a convex lens focuses light on to a light-sensitive screen (the retina).
What Each Part Does
- Cornea — the clear front window of the eye. It does most of the bending of light (about two-thirds).
- Iris — the coloured ring that controls how wide the pupil opens.
- Pupil — the black opening in the centre. It shrinks in bright light and widens in dim light.
- Eye lens — a flexible convex lens. Tiny muscles squeeze or relax it to finish focusing the image sharply on the retina. This skill is called accommodation.
- Retina — the screen at the back packed with light-sensitive cells (rods and cones).
- Optic nerve — the cable that carries the signal from the retina to the brain, where the image is finally "seen".
🌗 Persistence of vision: An image stays on the retina for about 1/16 of a second after the object is removed. That is why fast-changing pictures (24 frames a second in a film!) appear as smooth motion.
🔍 Activity 10.3 — Your Pocket Magnifier
You will need: a small convex lens (a reading glass or a clear marble works), a newspaper, a white wall, a window that faces the outdoors.
- Hold the lens close (within a few cm) over a word on the newspaper. Move it slightly up and down. Notice what happens.
- Take the same lens to the window. Hold a sheet of paper a few cm behind the lens, facing the window.
- Move the paper slowly until a clear picture appears on it. Look at the picture — what's interesting about it?
🔍 Predict: Is the word on the newspaper larger or smaller than normal? Will the picture of the outside scene on the paper be the right way up or upside-down?
Step 1: Within focal length, the word appears larger — you have a virtual, erect, enlarged image. That is a magnifying glass.
Step 2: Beyond focal length, an inverted, smaller, real image of the outside scene forms on the paper. The same lens, behaving exactly like a camera! The image is upside-down because real images through a convex lens are always inverted.
10.13 Common Eye Defects and Their Correction
Myopia (Short-Sightedness)
A child with myopia can see nearby letters clearly but faraway objects like a blackboard or signboard look blurry. The eyeball is too long (or the lens is too strong), so light from far-away objects is focused in front of the retina. The cure is to make the rays diverge a little before they enter the eye — so a concave lens is used in spectacles.
Hypermetropia (Long-Sightedness)
The opposite problem — the person cannot read a book held at normal distance; letters are blurry up close, but distant things are fine. Here the eyeball is too short (or the lens too weak), so light from near objects would focus behind the retina. Remedy: use a convex lens in spectacles to converge rays a little extra before they enter the eye.
Presbyopia and Astigmatism
- Presbyopia — the lens stiffens with age (generally after 40 years), so reading small text becomes hard. Older people often need bifocals (two powers in one pair of glasses).
- Astigmatism — the cornea is not perfectly curved, so horizontal and vertical lines cannot be focused together. Corrected by specially shaped cylindrical lenses.
👁️ Eye-care tips: Read in good light, hold books about 25 cm away, give eyes a break from screens every 20 minutes (look at something 6 m away for 20 seconds), eat food rich in vitamin A (carrots, spinach, papaya, eggs) and see an eye doctor if writing on the blackboard looks fuzzy.
🎯 Competency-Based Questions
A school eye-screening camp tested 120 children. Of them, 14 could not read the last line on the chart from 6 m away, and 4 could not read the near-vision card held at 25 cm. The team prescribed spectacles for each group.
Q1. L1 Remember Name the light-sensitive screen at the back of the eye on which an image is formed.
Answer: The retina. It contains rod and cone cells that convert light into nerve signals sent to the brain via the optic nerve.
Q2. L2 Understand Which type of lens will the 14 children (who cannot see faraway letters) receive, and why?
Answer: A concave (diverging) lens. They suffer from myopia — the far image is forming in front of the retina. A concave lens diverges the incoming rays slightly so that the eye now focuses them exactly on the retina.
Q3. L3 Apply A watch-maker examining a tiny screw holds a convex lens 2 cm above it. Describe the image he sees and state its nature.
Answer: The screw is inside the focal length of the lens, so the image is virtual, erect and enlarged — the lens is acting as a magnifying glass. This lets him see tiny teeth of the screw clearly.
Q4. L4 Analyse Compare the human eye with a camera. List two features that are similar and one that is different.
Answer: Similar: Both have a convex lens that forms a real inverted image; both have a light-sensitive screen (retina / sensor). Different: The eye lens changes its own thickness (accommodation) to focus objects at different distances, while a camera moves the lens backwards and forwards.
Q5. L5 Evaluate A friend says: "Since both cataract and myopia affect vision, they must be corrected by the same kind of lens." Evaluate.
Answer: The friend is wrong. Myopia is a refraction problem (image forming before the retina) and is corrected by a concave lens. A cataract is a clouding of the eye lens itself — no spectacle lens can clear the fog; the cloudy lens has to be surgically removed and replaced by an artificial intra-ocular lens. The two defects are entirely different.
🔗 Assertion–Reason Questions
Assertion (A): A convex lens can act as a magnifying glass.
Reason (R): When the object is placed between the optical centre and the focus of a convex lens, the image is virtual, erect and enlarged.
Answer: A. Correct — that is precisely the magnifying-glass position.
Assertion (A): Myopia is corrected by using spectacles with convex lenses.
Reason (R): A convex lens diverges light rays before they enter the eye.
Answer: D. Both statements are false as written: myopia is corrected by a concave lens, and a convex lens actually converges rays. Only the fact that a concave lens diverges rays would fit here.
Assertion (A): The retina is the part of the eye on which the image is formed.
Reason (R): The retina contains light-sensitive cells that convert light into nerve signals.
Answer: B. Both statements are true. But R explains how the retina sends information, not why the image forms there. The image forms on the retina because it is at the focal plane of the eye lens.
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