Starter quiz
- A lens that bends light rays towards each other so that they meet or cross is called a ______ lens.
- 'convex' ✓
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?- Position A
- Position B
- Position C ✓
- Position D
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- A student uses a lens to create an image of a car. The real car is 400 cm long. The image of the car is 8 cm long. What is the magnification of the image?
- -50
- 0.02 ✓
- 50
- 392
- 3200
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- Which of these words can be used to describe images made on screens by lenses?
- diminished ✓
- inverted ✓
- life-sized ✓
- magnified ✓
- upright
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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 true?- Lens P refracts rays through the largest angles.
- Lens P and lens Q are made of different materials. ✓
- Lens R has the highest power. ✓
- Lens R has the shortest focal length. ✓
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- 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.
- 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.
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Exit quiz
- Which of these statements are correct about rays of light that hit a lens?
- 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.
- Rays from a very distant object are very nearly parallel. ✓
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This scale ray diagram shows an object (an arrow), a convex lens and an image. Which of these words describe the image?- diminished
- inverted ✓
- life-sized
- magnified ✓
- upright
-
This scale ray diagram shows the creation of an image by a convex lens. What is the magnification?- 0.2
- 0.5
- 2 ✓
- 10
- 20
-
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?- 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.
- 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.
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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.- Ray P⇔parallel to principal axis, then bends to pass through principal focus ✓
- Ray Q⇔passes through centre of lens without being refracted ✓
- Ray R⇔passes through principal focus, then bends parallel to principal axis ✓
- 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?
- 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.
- The lens cannot refract the rays enough to make them converge. ✓
- The student has placed the screen between the lens and the object. ✓
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Worksheet
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Presentation
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Video
Lesson Details
Key learning points
- In cameras and the human eye, the required image distance < the object distance so the image is diminished and inverted.
- In projectors, the required image distance > the object distance so the image is magnified and inverted.
- A ray incident on a convex lens parallel to the principal axis is refracted to pass through principal focus.
- A ray incident on the centre of a convex lens is not refracted.
- 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.
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