Lenses and dispersion Cambridge IGCSE Physics revision
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In plain words
A converging lens (fatter in the middle) bends light inwards to a point called the principal focus. Hold one a long way from an object and you get an upside-down image you can catch on a screen, as a projector does. Hold it very close and it becomes a magnifying glass.
Three things to know
- A converging (convex) lens brings parallel rays to the principal focus. The distance from the lens to that point is the focal length.
- An object further than the focal length from a converging lens gives a real, inverted image. Closer than the focal length, it gives a virtual, upright, magnified image: a magnifying glass.
- White light is dispersed by a prism into red, orange, yellow, green, blue, indigo and violet. Red is refracted least, violet most.
Tips and tricks
- In a ray diagram, draw two rays from the top of the object: one parallel to the axis, which then passes through the focus, and one straight through the centre of the lens. The image is where they cross.
- A real image is where rays actually meet, and it can be shown on a screen. A virtual image can't.
It lands in your notebook with its questions as flashcards.
Lenses and dispersion: 6 questions and answers
These are the quiz’s questions. Do the quiz first, then come back here for the ones that got you.
What does a converging lens do to rays parallel to its axis?
They meet at the principal focus.
Which colour of light is refracted most by a glass prism?
Violet is slowed most by the glass, red least.
An image formed by a lens can be caught on a screen. What kind of image is it?
Only a real image, where rays actually meet, can be shown on a screen.
How is a converging lens used as a magnifying glass?
The image is then virtual, upright and enlarged.
What is the splitting of white light into colours called?
Each colour bends by a different amount.
Which lens is used to correct long sight?
It bends the rays more, so they focus on the retina rather than behind it.
Quiz
6 questions
Tap an answer and you’ll see straight away whether it’s right, and why.
Worksheet
4 questions, 8 marks. Write your answers on paper, then check them.
Lenses and dispersion
Cambridge IGCSE Physics 0625 · 8 marks · papermunch.org
Name ______________________________ Date ______________
State what is meant by the focal length of a converging lens.[2]
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The distance from the centre of the lens to the principal focus, where rays parallel to the axis meet.
An object is placed closer to a converging lens than its focal length. Describe the image.[2]
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Virtual, upright and magnified (on the same side of the lens as the object).
Describe what a prism does to a beam of white light.[2]
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It splits it into a spectrum of colours (dispersion), because each colour is refracted by a different amount.
State which type of lens corrects short sight, and why.[2]
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A diverging lens: it spreads the rays slightly so that they focus on the retina instead of in front of it.
Answers: Lenses and dispersion
- 1. The distance from the centre of the lens to the principal focus, where rays parallel to the axis meet.
- 2. Virtual, upright and magnified (on the same side of the lens as the object).
- 3. It splits it into a spectrum of colours (dispersion), because each colour is refracted by a different amount.
- 4. A diverging lens: it spreads the rays slightly so that they focus on the retina instead of in front of it.