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Year 11
OCR
Foundation

Scale diagrams for convex lenses

I can describe uses of convex lenses and draw scale ray diagrams to determine the position of an image.

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New
New
Year 11
OCR
Foundation

Scale diagrams for convex lenses

I can describe uses of convex lenses and draw scale ray diagrams to determine the position of an image.

Download all resources
Share activities with pupils

Lesson details

Key learning points

  1. In cameras and the human eye, the required image distance < the object distance. The image is diminished and inverted.
  2. In projectors, the required image distance > the object distance. The image is magnified and inverted.
  3. A ray incident on a convex lens parallel to the principal axis is refracted to pass through principal focus.
  4. A ray incident on the centre of a convex lens is not refracted.
  5. A ray incident on a convex lens from the principal focus is refracted so it becomes parallel to the principal axis.

Common misconception

Covering over half of a lens will block out rays of light from half the object and the image will appear cut in half.

Talk about many rays of light passing through the lens from each point of the object and show pupils that blocking half the lens keeps the same image, only less bright.

Keywords

  • Object distance - The object distance is the distance between an object and a lens.

  • Image distance - The image distance is the distance between a lens and an image formed by that lens.

  • Principal focus - The principal focus is the point to which parallel rays (from distant objects) are focused.

  • Focal length - The focal length is the distance between the centre of a lens and the principal focus.

  • Principal axis - The principal axis is a line through the centre of a lens, 90° to the optical axis of the lens.

Provide students with lots of practice with ray diagrams. It can be a good strategy to use mini-whiteboards initially to allow students some initial lower-stakes practice at drawing out the 'rough shape'. Pupils can quickly gain confidence and move onto the more precise to-scale versions on paper.
Teacher tip

Licence

This content is © Oak National Academy Limited (2024), licensed on Open Government Licence version 3.0 except where otherwise stated. See Oak's terms & conditions (Collection 2).

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Starter quiz

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6 Questions

Q1.
A lens that bends light rays towards each other so that they meet or cross is called a lens.
Correct Answer: convex
Q2.
A student wants to use a lens to create an image on a screen. At which position should the student place the screen to see a sharp image?
An image in a quiz
Position A
Position B
Correct answer: Position C
Position D
Q3.
The diagram shows light rays being focused by a lens. What are the correct names for the distance labelled A, the point labelled B and the line labelled C?
An image in a quiz
Correct Answer:A,focal length

focal length

Correct Answer:B,principal focus

principal focus

Correct Answer:C,principal axis

principal axis

Q4.
Which of these words can be used to describe images made on screens by lenses?
Correct answer: diminished
Correct answer: inverted
Correct answer: life-sized
Correct answer: magnified
upright
Q5.
The diagrams show parallel light rays being brought to a focus by three different lenses, labelled P, Q and R. Lenses P and Q have the same curvature. Which of these statements must be correct?
An image in a quiz
Lens P refracts rays through the largest angles.
Correct answer: Lens P and lens Q are made of different materials.
Correct answer: Lens R has the highest power.
Correct answer: Lens R has the shortest focal length.
Q6.
A student uses a lens to make a sharp (clear) image of an object on a screen. Which of these statements must be correct?
The light rays hitting the lens are parallel or very nearly parallel.
By moving the screen, the student can see sharp images of different sizes.
Correct answer: The lens brings all rays from one point on the object to one point on the image.
The distance between the lens and the image equals the focal length of the lens.

Exit quiz

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6 Questions

Q1.
Which of these statements are correct about rays of light that hit a lens?
Correct answer: Rays from a nearby object are diverging.
Rays from a nearby object are converging.
Rays from a nearby object are very nearly parallel.
Rays from a very distant object are converging.
Correct answer: Rays from a very distant object are very nearly parallel.
Q2.
This scale ray diagram shows an object (an arrow), a convex lens and an image. Which of these words describe the image?
An image in a quiz
diminished
Correct answer: inverted
life-sized
Correct answer: magnified
upright
Q3.
When a lens forms an image on a screen, which of these statements correctly describe the image distance?
It always equals the object distance.
Correct answer: It is the distance between the image and the lens.
It is the distance between the image and the object.
Correct answer: It is the distance needed between the screen and lens for the image to be sharp.
Q4.
The diagram shows rays of light from an object entering an eye through its lens. The rays are focused onto the back of the eye which acts as a screen. How does the eye focus on a more distant object?
An image in a quiz
No change is needed because the back of the eye is at the principal focus.
The lens becomes fatter so that it has higher power.
Correct answer: The lens becomes thinner so that it has lower power.
The eye becomes longer so that the screen is further from the lens.
The eye becomes shorter so that the screen is closer to the lens.
Q5.
In this ray diagram, the principal focus on each side of the convex lens is shown as a cross. Three rays from the top of the object (an arrow) are shown. Match each ray with its description.
An image in a quiz
Correct Answer:Ray P,parallel to principal axis, then bends to pass through principal focus

parallel to principal axis, then bends to pass through principal focus

Correct Answer:Ray Q,passes through centre of lens without being refracted

passes through centre of lens without being refracted

Correct Answer:Ray R,passes through principal focus, then bends parallel to principal axis

passes through principal focus, then bends parallel to principal axis

Q6.
A student tries to use a convex lens to make an image of a bright object. He varies the distance between the screen and lens but finds no image. Which of the following are possible explanations?
Correct answer: The object is too close to the lens.
The object is too far from the lens.
The rays are converging when they leave the lens.
Correct answer: The lens cannot refract the rays enough to make them converge.
Correct answer: The student has placed the screen between the lens and the object.