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How do lenses form images, and why do objects have colour?

Lenses and visible light: how convex and concave lenses refract light, ray diagrams and magnification, and how colour depends on reflection, transmission and absorption (separate physics).

A focused answer to AQA GCSE Physics 4.6.2, covering how convex and concave lenses refract light to form images, drawing ray diagrams and using the magnification equation, and how the colour of objects depends on the reflection, transmission and absorption of light.

Generated by Claude Opus 4.89 min answer

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  1. What this dot point is asking
  2. Convex and concave lenses
  3. Magnification
  4. Colour of objects
  5. Try this

What this dot point is asking

AQA wants you to describe how convex and concave lenses refract light, draw ray diagrams to find the image, use the magnification equation, and explain how the colour of an object depends on which wavelengths it reflects, transmits or absorbs. This is part of topic 4.6.2 of the AQA GCSE Physics (8463) specification and is separate physics only.

Convex and concave lenses

The focal length is the distance from the centre of the lens to the principal focus, where the rays meet (or appear to come from). To find an image position by drawing a ray diagram for a convex lens, you draw two standard rays from the top of the object: one travelling parallel to the axis, which is refracted through the principal focus on the far side, and one passing straight through the centre of the lens undeviated. Where these rays meet locates the top of the image. If the rays actually cross, the image is real and can be projected onto a screen; if they only appear to come from a point behind the object (so you extend them backwards as dashed lines), the image is virtual. A real image is always inverted, while a virtual image is upright. The closer the object is brought towards the focal point, the larger and further away the real image becomes.

Magnification

A convex lens is described as converging because it brings parallel rays together; a concave lens is diverging because it spreads them apart. The power of a lens to bend light depends on its shape: a fatter convex lens has a shorter focal length and bends light more strongly. Convex lenses are used in magnifying glasses, cameras and the human eye, while concave lenses are used to correct short sight by spreading the light out before it reaches the eye.

Colour of objects

Visible light is the small band of the electromagnetic spectrum the eye can detect, and white light is a mixture of all its colours, from red (longest wavelength) to violet (shortest). This is why a prism can split white light into a spectrum: each colour refracts by a slightly different amount. Objects can be opaque (no light passes through), transparent (light passes through clearly) or translucent (light passes through but is scattered). For a transparent or translucent coloured object, the colour seen depends on which wavelengths it transmits as well as reflects, while for an opaque object it depends only on what it reflects.

Try this

Q1. State what type of image a concave lens always produces. [1 mark]

  • Cue. A virtual, upright, diminished image.

Q2. An object 3 cm3\,cm tall produces an image 12 cm12\,cm tall. Calculate the magnification. [2 marks]

  • Cue. magnification=123=4\text{magnification} = \dfrac{12}{3} = 4.

Exam-style practice questions

Practice questions written in the style of AQA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.

AQA 20204 marksA convex lens produces an image of an object. The object is 5.0 mm5.0\,\text{mm} tall and the image formed is 20 mm20\,\text{mm} tall. Calculate the magnification, and state whether the image is larger or smaller than the object and by what factor.
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Use the magnification equation magnification=image heightobject height=205.0=4\text{magnification} = \frac{\text{image height}}{\text{object height}} = \frac{20}{5.0} = 4 (2 marks; the heights are in the same units so they cancel and the answer has no units). The image is larger than the object, because the magnification is greater than 11, and specifically it is four times larger (2 marks). Markers reward the correct ratio, recognising that magnification has no units, and interpreting a value greater than 11 as an enlarged image. A common error is to invert the ratio (object over image).

AQA 20214 marksExplain why a red apple looks red when viewed in white light, and explain what colour the same apple would appear when viewed through a blue filter.
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White light contains all the colours of the visible spectrum (1 mark). When white light falls on the red apple, the surface reflects red light and absorbs the other colours, so only red light reaches the eye and the apple looks red (1 mark). A blue filter transmits only blue light and absorbs all other colours, so when the apple is viewed through it, only blue light can pass through the filter, but the apple reflects red and absorbs blue (1 mark). Since no red light can pass through the blue filter and the apple does not reflect blue, almost no light reaches the eye from the apple, so it appears black (or very dark) (1 mark). Markers reward the reflect-its-own-colour-absorb-the-rest idea and reasoning through the filter to conclude the apple looks black.

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