CAPF AC 2021 Q4: Science & Technology – Physics

CAPF AC 2021, Paper I (General Ability and Intelligence) · Science & Technology

1. (a) 5 cm, converging
2. (b) 10 cm, diverging
3. (c) 20 cm, converging
4. (d) 20 cm, diverging

Answer: (a) 5 cm, converging

Explanation

Image of the same size by a lens

Given: the object is 10 cm in front of the lens.

The image is real, inverted and of the same size as the object.

How to think: first decide the type of lens, then find the focal length.

A diverging (concave) lens always gives a virtual, erect, smaller image.

So a real image means the lens is converging (convex).

That removes (b) and (d) at once.

Convex lens: where the object is and what the image looks like

Working:

Answer: (a) 5 cm, converging.

Why each option is right or wrong

• (a) CORRECT. Same size real image means the object is at 2F, so 2f = 10 cm and f = 5 cm, convex.
• (b) Incorrect. A diverging lens never forms a real image.
• (c) Incorrect. The lens type is right, but this doubles 10 cm instead of halving it.
• (d) Incorrect. A diverging lens never forms a real image, and 20 cm is also wrong.

Key point

Convex lens, object at 2F: image is real, inverted, same size and also at 2F. So object distance = 2f. Here 2f = 10 cm, f = 5 cm.

Remember

Same size, real, inverted image: object is at 2F.

Common mistakes

• Doubling the distance to get 20 cm instead of halving it to 5 cm.
• Taking the object distance itself, 10 cm, as the focal length.
• Forgetting that a concave lens can never give a real image.

Exam tip

Educated guess: a real image needs a converging lens, so (b) and (d) go. Between 5 cm and 20 cm, remember same size means object at 2F.

Sources

• NCERT Class X Science — Light: Reflection and Refraction (image formation by lenses)
• NCERT Class XII Physics — Ray Optics and Optical Instruments (thin lens formula)

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