New
New
Year 11
OCR
Higher

Virtual images from convex and concave lenses

I can describe the formation of virtual images by convex and concave lenses and draw ray diagrams to find the position and magnification of the image.

New
New
Year 11
OCR
Higher

Virtual images from convex and concave lenses

I can describe the formation of virtual images by convex and concave lenses and draw ray diagrams to find the position and magnification of the image.

Lesson details

Key learning points

  1. When rays of light are brought to a focus a real image forms. A real image can be projected onto a screen.
  2. Virtual images occur at points where rays appear to (but do not actually) originate from. They cannot form on a screen.
  3. The size and position of a virtual image is found by drawing virtual rays to find where rays appear to originate from.
  4. Upright virtual images are produced by concave lenses, and objects within a focal length of a convex lens.
  5. 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.

Support pupil's learning about lenses and images by linking simple observations you can make using lenses in the classroom to their physics representations. Online animations such as 'Geometric optics' from PhET can be incredibly useful here.
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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6 Questions

Q1.
Converging rays are rays that are coming together. rays are rays that are spreading out.
Correct Answer: diverging
Q2.
The imaginary line through the centre of a lens, at right angles to the lens diameter, is called the .
Correct Answer: principal axis
Q3.
Any ray that passes through the centre of a lens …
Correct answer: does not change direction.
changes direction to pass through the principal focus.
changes direction to become parallel to the principal axis.
Q4.
These terms can be used to describe images made by lenses. Match each term with its meaning.
Correct Answer:diminished,smaller than the object

smaller than the object

Correct Answer:inverted,upside-down (compared with the object)

upside-down (compared with the object)

Correct Answer:magnified,larger than the object

larger than the object

Correct Answer:upright,the same way up as the object

the same way up as the object

Q5.
Which of these convex lens ray diagrams is correct?
An image in a quiz
Correct Answer: An image in a quiz
An image in a quiz
An image in a quiz
An image in a quiz
Q6.
The power of a lens is greater if …
Correct answer: it has greater curvature.
it is placed closer to the object.
the focal point is further from the lens.
Correct answer: the material it is made from refracts light through greater angles.

6 Questions

Q1.
An image that light rays appear to come from, but do not actually come from, is called a image.
Correct Answer: virtual
Q2.
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
Correct answer: the point that rays parallel to the principal axis appear to have diverged from
Q3.
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.
Correct answer: Images of distant objects appear in front of the retina (back of the eye).
Correct answer: A short-sighted person can focus on nearby objects but not distant objects.
Q4.
This scale ray diagram shows an object (an arrow), a concave lens and an image. Which of these words describe the image?
An image in a quiz
Correct answer: diminished
inverted
magnified
real
Correct answer: upright
Q5.
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)?
An image in a quiz
Correct Answer: An image in a quiz
An image in a quiz
An image in a quiz
An image in a quiz
Q6.
Which of these statements are correct?
Correct answer: A convex lens can make real images.
Correct answer: A convex lens can make virtual images.
A concave lens can make real images.
Correct answer: A concave lens can make virtual images.