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14.5 Using Lenses

GCSE Physics · AQA · Chapter 14: Light

A free GCSE Physics lesson for AQA (8463) from OBA School: drawing ray diagrams for lenses, how the image depends on the object position, the camera, the magnifying glass and the concave lens. Watch the video, read the explanation and worked examples, then try the revision questions. You can work through the full lesson, with every video, practice quizzes and printable worksheets with mark schemes, in our free online course.

What you will learn

  • Draw ray diagrams for a convex lens using the three construction rays
  • Describe the image (real or virtual, upright or inverted, magnified or diminished) for different object positions
  • Explain how a convex lens is used in a camera and as a magnifying glass
  • Draw a ray diagram for a concave lens and describe its image
  • Use a scale ray diagram to find the position and size of an image and the magnification

Key words

  • Principal axis: The straight line through the centre of the lens, at 90° to the lens.
  • Construction rays: The standard rays used to locate an image in a ray diagram.
  • Inverted: Upside down compared with the object. The opposite is upright.
  • Magnified: Bigger than the object. The opposite is diminished (smaller).
  • 2F: A point on the principal axis at twice the focal length from the lens.
  • Scale diagram: A diagram drawn so that 1 cm on paper represents a fixed real distance.

The three construction rays

Rays leave every point of an object in all directions. To find where the top of the image forms, we follow rays from the top of the object whose paths we can predict. Two rays are enough; a third checks your drawing.

Ray 1 travels parallel to the principal axis, then is refracted through F on the far side.

Ray 2 passes straight through the centre of the lens without changing direction.

Ray 3 passes through F on the near side, then leaves the lens parallel to the principal axis.

The image of the top of the object is where the rays cross. The bottom of the image is on the axis.

Exam tip: Use a ruler and a sharp pencil, put arrows on rays, draw the lens as a straight line with the correct arrowheads, and draw any virtual rays and images with dashed lines.

How the image depends on where the object is

For a convex lens, the type of image depends on how far the object is from the lens compared with the focal length.

  • very far away; Image position at F; Image real, inverted, very small; Use telescope objective, camera (distant scene)
  • beyond 2F; Image position between F and 2F; Image real, inverted, diminished; Use camera, eye
  • at 2F; Image position at 2F; Image real, inverted, same size; Use photocopier (same-size copies)
  • between F and 2F; Image position beyond 2F; Image real, inverted, magnified; Use projector
  • at F; Image position no image (rays leave parallel); Image –; Use spotlight / searchlight beam
  • between F and the lens; Image position same side as object; Image virtual, upright, magnified; Use magnifying glass

Worked example: a scale ray diagram

A convex lens has a focal length of 4.0 cm. An object 1.0 cm tall stands 6.0 cm from the lens. Draw to full scale (1 cm on paper = 1 cm).
Rays 1 and 2 from the top of the object cross 12.0 cm from the lens on the other side, 2.0 cm below the axis.
Image: real, inverted, magnified. magnification = 2.0 ÷ 1.0 = 2.0

Answer: image 12 cm from the lens, 2.0 cm tall; magnification 2.0

The magnifying glass

When the object is closer to the lens than F, the rays leaving the lens are still spreading out. They never meet, so no real image forms. Your eye traces them back to where they appear to come from: a virtual, upright, magnified image on the same side of the lens as the object. It can only be seen by looking through the lens.

The concave lens

A concave lens spreads light out wherever the object is. Ray 1 leaves the lens as if it came from F on the object side; ray 2 goes straight through the centre. Traced back, they meet between the object and the lens.

This is why a concave lens is no use as a magnifying glass or for projecting an image onto a screen.

The camera

A camera uses a convex lens to form a real, inverted, diminished image on a light-sensitive sensor (or film). The aperture controls how much light gets in.

  • For a very distant object, the image forms at F, so the lens–sensor distance equals the focal length.
  • The closer the object, the further from the lens the image forms, so the lens must be moved further from the sensor to focus it.

Common mistakes

  • Forgetting arrows on rays, or drawing rays that bend in the middle of nowhere instead of at the lens line.
  • Sending ray 1 through the F on the same side as the object (that is ray 3’s F).
  • Drawing a virtual image or the backward extensions of rays as solid lines.
  • Saying a magnifying glass forms a real image. It is virtual, upright and magnified.
  • Describing an image with only one word. Always say real/virtual, upright/inverted and magnified/diminished/same size.

Revision questions

Try these, then check your answers below each one.

Question 1. A convex lens is used as a magnifying glass. What is the image like? (Choose from: virtual, upright, magnified, real, inverted, magnified, virtual, inverted, diminished, real, upright, diminished.)

Answer: virtual, upright, magnified

Working: The rays leaving the lens diverge, so they only appear to come from an image behind the object: virtual, upright and magnified.

Question 2. What kind of image does a camera lens form on the sensor? (Choose from: real, inverted, diminished, virtual, upright, diminished, real, upright, magnified, virtual, inverted, magnified.)

Answer: real, inverted, diminished

Working: Objects are beyond 2F, so the image is real (on the sensor), inverted and diminished.

Question 3. An object is placed at 2F in front of a convex lens. Where is the image? (Choose from: at 2F on the other side, same size and inverted, at F on the other side, very small, beyond 2F, magnified, on the same side as the object, virtual.)

Answer: at 2F on the other side, same size and inverted

Working: At 2F the image is real, inverted, the same size and at 2F on the other side of the lens.

Related lessons

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This lesson is section 14.5 (AGCPH1405) of our free GCSE Physics course for AQA. In the full course you get every video for this section, a self-marking practice quiz, a printable worksheet with a mark scheme, chapter tests, and your progress saved. Go to the full GCSE Physics course.

 

 

 

 

 

 

 

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