This MCQ module is based on: Concave and Convex Mirrors and Their Uses
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Concave and Convex Mirrors and Their Uses
🎓 Class 8
Science
CBSE
Theory
Ch 10 — Force and Pressure
⏱ ~26 min
🌐 Language: [gtranslate]
🧠 AI-Powered MCQ Assessment
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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"]
10.5 Drawing Ray Diagrams
To predict the image formed by any curved mirror, we trace just two out of four special rays. Wherever these two reflected rays meet (or appear to meet), the image forms.
Four Rules for Ray Tracing:
- A ray parallel to the principal axis reflects through F (concave) or appears to come from F (convex).
- A ray passing through F (concave) reflects parallel to the principal axis.
- A ray passing through C retraces its own path — it hits the mirror normally.
- A ray striking the pole reflects symmetrically about the principal axis.
10.6 Images in a Concave Mirror — Six Cases
(a) Object at Infinity
Rays from a very distant object (like the Sun) reach the mirror practically parallel, so they converge at the focus F. The image is a tiny, inverted, highly bright point. This is how solar cookers work!
(b) Object Beyond Centre of Curvature (C)
Image forms between F and C. It is real, inverted and diminished.
(c) Object at C
Image forms at C itself — real, inverted and the same size as the object.
(d) Object Between C and F
Image forms beyond C — real, inverted and enlarged. This is the principle of a movie projector.
(e) Object at F
Reflected rays leave parallel to each other. They never meet, so the image is formed at infinity — extremely large and inverted. This is how a searchlight or torch reflector throws out a parallel beam.
(f) Object Between P and F
Both reflected rays diverge, so they appear to come from a point behind the mirror. The image is virtual, erect and enlarged. This is what you see in a shaving or dentist's mirror.
| Object Position | Image Position | Nature | Size |
|---|---|---|---|
| At infinity | At F | Real, inverted | Point-sized |
| Beyond C | Between F and C | Real, inverted | Diminished |
| At C | At C | Real, inverted | Same size |
| Between C and F | Beyond C | Real, inverted | Enlarged |
| At F | At infinity | Real, inverted | Very large |
| Between P and F | Behind mirror | Virtual, erect | Enlarged |
10.7 Images in a Convex Mirror
A convex mirror has a simpler life — wherever the object is, the image is always:
- Virtual (behind the mirror),
- Erect (same way up),
- Diminished (smaller than the object).
10.8 Where These Mirrors Help Us
(a) Concave Mirror — the Converger
Shaving / Make-up Mirror
Held closer than F, it gives a large upright image of the face, so blemishes are easy to spot.
Dentist's Mirror
A small concave mirror at a handle — held close to a tooth, it enlarges the tooth for easy inspection.
Search Light & Torch
A bulb at the focus sends out a strong, almost parallel beam that carries far into the dark.
Solar Cooker / Solar Furnace
Large concave dish concentrates sunlight at its focus to melt metals or cook food.
Reflecting Telescope
A huge concave mirror gathers faint starlight and brings it to a focus for clear viewing.
Headlamps
Car headlamps use concave reflectors to throw a bright beam down the road.
(b) Convex Mirror — the Diverger
Rear-view & Side-view Mirrors
Provide a wide field of view so drivers can see vehicles behind them over a large angle.
Shop Surveillance
Corner-mounted convex mirrors let shopkeepers keep an eye on every aisle.
Hairpin Bends
Big convex mirrors fitted on mountain roads reveal traffic approaching from the other side.
ATMs & Lifts
Small convex mirrors show people what is happening behind them — safety without extra cameras.
⚠️ "Objects in mirror are closer than they appear." This warning is etched on every car's convex side mirror. Because a convex mirror forms a diminished image, our brain wrongly reads "small = far". In reality the object is nearer than it looks — always double-check before changing lanes!
🔆 Activity 10.2 — Catching Sunlight on a Paper
You will need: a small concave mirror (shaving mirror or spoon), a sunny window, a thick piece of dark paper.
- Take the mirror outside under the Sun (always supervise with an adult).
- Hold the concave side facing the Sun so sunlight falls on it.
- Place the dark paper on the other side and move it closer / farther until a bright spot becomes as small as possible.
- Measure the distance from the paper to the mirror.
🔍 Predict: What will you see on the paper when the spot is smallest? Why might the paper begin to smoke?
The tiny bright spot is the image of the Sun formed at the focus of the concave mirror. Its distance from the pole equals the focal length (f). Because all the Sun's energy striking the mirror is concentrated into a tiny area, the paper may darken or even start smoking — the same principle powers a solar cooker.
Caution: Never focus sunlight on skin, eyes or flammable items.
🎯 Competency-Based Questions
A village science club is designing a small solar cooker using a large dish-shaped concave mirror and a blackened cooking vessel. At the same time, one of the members is designing a safety mirror to be fixed near a sharp turn on the hill road outside the village.
Q1. L1 Remember At which point should the cooking vessel be placed on the solar cooker's concave dish?
Answer: At the focus (F) of the concave mirror, because parallel rays from the Sun converge there and heat is maximum.
Q2. L2 Understand For the hill-road safety mirror, which mirror is chosen and why?
Answer: A large convex mirror. It always gives a small, erect image with a very wide field of view, so drivers can see vehicles approaching from around the bend in one quick glance.
Q3. L3 Apply An object is placed 10 cm in front of a concave mirror of focal length 15 cm. Will the image be real or virtual? Why?
Answer: The object distance (10 cm) is less than the focal length (15 cm), so the object is between P and F. The image will be virtual, erect and enlarged — formed behind the mirror.
Q4. L4 Analyse Why is a convex mirror preferred over a plane mirror of the same size on a car, even though a convex mirror makes vehicles appear smaller?
Answer: A convex mirror fits a wider slice of the road behind into the same glass area. Reducing image size is a fair price for seeing a much larger region. A plane mirror of the same area shows only a narrow strip and can leave large blind spots.
Q5. L5 Evaluate Ravi claims: "A dentist should use a convex mirror because it shows a large field of view of the mouth." Evaluate.
Answer: Ravi is wrong. A convex mirror gives a diminished image, so a tooth would appear even smaller — useless for close inspection. A dentist needs a concave mirror, used close to the tooth (within its focal length), which gives an enlarged and erect image of the tooth.
🔗 Assertion–Reason Questions
Assertion (A): A concave mirror can be used as a solar cooker.
Reason (R): A concave mirror converges parallel rays of sunlight to a single focus, producing intense heat.
Answer: A. Concentrated heat at the focus is exactly what lets a solar cooker cook food.
Assertion (A): "Objects in mirror are closer than they appear" is written on vehicle side mirrors.
Reason (R): Side mirrors are concave and always produce enlarged images.
Answer: C. A is true, but R is false — side mirrors are convex and always produce diminished images. The warning is needed because the small image tricks drivers into thinking the vehicle is far away.
Assertion (A): A concave mirror with the object at its focus sends out a parallel beam of light.
Reason (R): A ray coming from F reflects parallel to the principal axis.
Answer: A. This is the principle used in torches, car headlamps and search lights.
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