Starter quiz
- Converging rays are rays that are coming together. ______ rays are rays that are spreading out.
- 'diverging' ✓
- The imaginary line through the centre of a lens, at right angles to the lens diameter, is called the ______.
- 'principal axis' ✓
- Any ray that passes through the centre of a lens …
- does not change direction. ✓
- changes direction to pass through the principal focus.
- changes direction to become parallel to the principal axis.
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- These terms can be used to describe images made by lenses. Match each term with its meaning.
- diminished⇔smaller than the object ✓
- inverted⇔upside-down (compared with the object) ✓
- magnified⇔larger than the object ✓
- upright⇔the same way up as the object ✓
- Which of these convex lens ray diagrams is correct?
- The power of a lens is greater if …
- it has greater curvature. ✓
- it is placed closer to the object.
- the focal point is further from the lens.
- the material it is made from refracts light through greater angles. ✓
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Exit quiz
- An image that light rays appear to come from, but do not actually come from, is called a ______ image.
- 'virtual' ✓
- What is the principal focus for a concave lens?
- the point at which an image appears
- the point to which all rays are focused
- the point to which rays parallel to the principal axis converge
- the point that rays parallel to the principal axis appear to have diverged from ✓
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- Which of these statements are true about short-sightedness?
- It can be corrected using convex lenses.
- The lens in the person’s eye does not have enough power.
- Images of distant objects appear in front of the retina (back of the eye). ✓
- A short-sighted person can focus on nearby objects but not distant objects. ✓
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This scale ray diagram shows an object (an arrow), a concave lens and an image. Which of these words describe the image?- diminished ✓
- inverted
- magnified
- real
- upright ✓
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- A convex lens makes a magnified image (Q) of an object (P). Which of these images correctly shows real rays (solid black lines) and virtual rays (dashed black lines)?
- Which of these statements are correct?
- A convex lens can make real images. ✓
- A convex lens can make virtual images. ✓
- A concave lens can make real images.
- A concave lens can make virtual images. ✓
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Worksheet
Presentation
Video
Lesson Details
Key learning points
- When rays of light are brought to a focus a real image forms. A real image can be projected onto a screen.
- Virtual images occur at points where rays appear to (but do not actually) originate from. They cannot form on a screen.
- The size and position of a virtual image is found by drawing virtual rays to find where rays appear to originate from.
- Upright virtual images are produced by concave lenses, and objects within a focal length of a convex lens.
- Convex lenses fix long sight by shortening the focus. Concave lenses fix short sight by lengthening the focus.
Common misconception
Real images always or only form on screens, or a screen is necessary to observe a real image.
Be clear that real images can form on screens, but can also be observed without a screen. Demonstrate looking through a convex lens (e.g. a magnifying glass) at a diminished, inverted real image of a distant object.
Keywords
Principal axis - The principal axis is a line through the centre of a lens, 90° to the optical axis of the lens.
Principal focus - The principal focus for a convex lens is the point to which rays parallel to the principal axis are focused. For a concave lens it is the point from which such rays appear to be diverged.
Real image - When rays of light are brought to a focus a real image forms. A real image can be projected onto a screen.
Virtual image - Virtual images occur at points where rays appear to (but do not actually) originate from. They cannot form on a screen.
Virtual ray - Virtual rays are dashed lines drawn to represent where rays of light appear to have come from.