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14.4 Lenses

GCSE Physics · AQA · Chapter 14: Light

A free GCSE Physics lesson for AQA (8463) from OBA School: convex and concave lenses, principal focus and focal length, real and virtual images, magnification. 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

  • Describe what convex (converging) and concave (diverging) lenses do to parallel rays of light
  • Define the principal focus and the focal length, and draw the symbols for each lens
  • Describe how the image formed by a convex lens changes as the object moves closer, and say whether it is real or virtual
  • Calculate magnification = image height ÷ object height and rearrange it

Key words

  • Convex lens: A converging lens, thicker in the middle: it makes parallel rays meet at a point.
  • Concave lens: A diverging lens, thinner in the middle: it makes parallel rays spread out.
  • Principal axis: The line through the centre of the lens at 90° to it.
  • Principal focus (F): Convex: the point where rays parallel to the axis meet. Concave: the point they appear to come from.
  • Focal length (f): The distance from the centre of the lens to the principal focus.
  • Real image: Formed where rays actually meet; it can be shown on a screen.
  • Virtual image: Formed where rays only appear to come from; it cannot be shown on a screen.
  • Magnification: How many times bigger the image is than the object (image height ÷ object height).

Two kinds of lens

A lens is a shaped piece of glass or clear plastic. Light refracts as it enters and again as it leaves the lens. Because the surfaces are curved, rays passing through different parts of the lens are bent by different amounts, so a lens can bring rays together or spread them out.

  • A convex lens is a converging lens. Its principal focus is where parallel rays actually meet, so it is a real focus. A ray along the principal axis goes straight through.
  • A concave lens is a diverging lens. Parallel rays spread out as though they came from the principal focus on the same side as the incoming light.
  • Every lens has a principal focus on each side, the same distance (f) from the centre. A more strongly curved lens bends light more and has a shorter focal length.

Worked example: finding the focal length of a convex lens

Hold the lens near a wall opposite a window on the far side of the room. Move it until a sharp, upside-down image of the window appears on the wall.
Light from a very distant object arrives almost parallel, so the image forms at the principal focus.
The distance from the centre of the lens to the wall is measured as 12.5 cm.

Answer: focal length ≈ 12.5 cm

Key fact: A concave lens always gives a virtual, upright, diminished image. It is used in glasses to correct short sight. Convex lenses are used in cameras, projectors and magnifying glasses.

Real and virtual images from a convex lens

You can investigate the images a convex lens makes with a lamp (the object), the lens and a white screen on a metre rule.

  • Object far away: a small, inverted, real image forms close to F.
  • Object moved closer (but still further than F): the screen must be moved further from the lens and the real image gets bigger.
  • Object closer than F: no image can be caught on the screen. Looking through the lens you see a magnified, upright, virtual image. The lens is acting as a magnifying glass.

Magnification

magnification = image height ÷ object height

Both heights must be in the same unit (mm or cm). Magnification is a ratio, so it has no unit.

Rearranged: image height = magnification × object height; object height = image height ÷ magnification

  • magnification greater than 1: the image is bigger than the object (magnified)
  • magnification less than 1: the image is smaller than the object (diminished)

Worked example: a magnifying glass

An ant 1.2 cm long is viewed through a magnifying glass. Its image is 54 mm long.
Same units: image height = 54 mm = 5.4 cm
magnification = 5.4 ÷ 1.2 = 4.5

Answer: magnification = 4.5 (no unit)

Worked example: a classroom projector (rearranging)

A projector lens gives a magnification of 30. The picture on the screen is 1.5 m tall.
object height = image height ÷ magnification = 150 cm ÷ 30

Answer: 5.0 cm

Exam tip: Convert first: if one height is in mm and the other in cm, change one of them before dividing. Never write a unit after a magnification.

Common mistakes

  • Mixing up convex and concave: convex lenses bulge outwards and converge light; concave lenses “cave in” and diverge light.
  • Calculating magnification as object height ÷ image height. It is always image ÷ object.
  • Dividing heights in different units (e.g. mm by cm), or giving magnification a unit.
  • Saying a virtual image can be caught on a screen. Only real images can.
  • Measuring the focal length from the edge of the lens instead of its centre.

Revision questions

Try these, then check your answers below each one.

Question 1. Which type of lens makes parallel rays of light converge to a point? (Choose from: convex, concave, plane, diverging.)

Answer: convex

Working: A convex (converging) lens brings parallel rays together at its principal focus.

Question 2. A lens forms an image 7.5 cm tall of an object 2.5 cm tall. Calculate the magnification.

Answer: 3

Working: magnification = 7.5 ÷ 2.5 = 3. It has no unit.

Question 3. A grain of rice 4.0 mm long is viewed through a magnifying glass. Its image is 2.0 cm long. Calculate the magnification.

Answer: 5

Working: 2.0 cm = 20 mm. magnification = 20 ÷ 4.0 = 5.

Related lessons

Continue in the full course

This lesson is section 14.4 (AGCPH1404) 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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