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Lenses and image formation — KCSE Physics

KCSE Physics · 133 practice questions · 11 syllabus objectives

43 easy45 medium45 hard

What You'll Learn

Key learning outcomes for this topic, aligned to the KNEC KCSE syllabus.

Identify parts and functions of optical devices: camera, simple and compound microscopes

Define focal point, focal length and power of a lens; distinguish between converging and diverging lenses

Apply the lens formula (1/f = 1/u + 1/v) and magnification formula (m = v/u) to solve lens problems

Construct ray diagrams for converging and diverging lenses and describe the nature and position of the image formed

Distinguish between converging and diverging lenses; identify principal focus and optical centre

Draw and complete ray diagrams for converging and diverging lenses to locate images

State characteristics of images formed by lenses and calculate magnification

Apply the thin lens formula 1/f = 1/u + 1/v to solve problems

Describe experiments to determine the focal length of a converging lens

Describe image formation in the human eye, accommodation, and vision defects with corrections

Lenses and image formation

Sample Questions

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1
easySHORT ANSWER4 marks

An object is placed 25 cm from a converging lens with a focal length of 10 cm. (a) Calculate the image distance using the lens formula. (2 marks) (b) State the characteristics of the image formed. (2 marks)

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Part (a) — 2 marks
Use the lens formula 1/f = 1/v - 1/u (1 mk)
Correct value of image distance v = 50 cm (1 mk)
Part (b) — 2 marks
Image is real (1 mk)
Image is inverted (1 mk)
2
easySHORT ANSWER4 marks

State the differences between a real image and a virtual image formed by a lens. (4 marks)

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Part (a) — 4 marks
A real image can be projected onto a screen, while a virtual image cannot. (1 mk)
Real images are inverted, whereas virtual images are upright. (1 mk)
Real images are formed on the opposite side of the lens from the object, while virtual images are formed on the same side. (1 mk)
Real images are usually larger or smaller depending on the object distance, while virtual images maintain the object's size. (1 mk)
3
easySHORT ANSWER3 marks

State the characteristics of the image formed when an object is placed at a distance greater than twice the focal length of a converging lens. (3 marks)

View Marking Scheme
Part (a) — 3 marks
The image is real (1 mk)
The image is inverted (1 mk)
The image is smaller than the object (1 mk)
4

An object is placed 30 cm from a converging lens with a focal length of 10 cm. (a) Calculate the image distance using the lens formula. (2 marks) (b) State the nature of the image formed. (1 mark)

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