How do converging and diverging lenses form images, and how does this relate to the eye?
Converging and diverging lenses, how they refract light to form images, the structure of the eye, and correcting short and long sight.
A CCEA GCSE Physics answer on converging and diverging lenses, how they refract light to form images, the basic structure of the eye, and how lenses correct short sight and long sight.
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What this dot point is asking
CCEA wants you to describe converging and diverging lenses, explain how they refract light to form images, outline the basic structure of the eye, and explain how lenses correct short and long sight. Lenses build directly on refraction.
The answer
Converging and diverging lenses
Lenses work by refraction: light bends as it enters and leaves the curved glass surfaces, just as it does in a glass block.
Forming images
A converging lens can form a real image (one that can be projected onto a screen) when the object is beyond the focal length, as in a camera or the eye. When the object is closer than the focal length it acts as a magnifying glass, giving an enlarged, upright (virtual) image.
The eye
Correcting sight defects
Worked example: choosing a lens
Examples in context
- Example 1. A camera
- A converging lens forms a real, inverted image on the sensor, just as the eye forms one on the retina. Moving the lens adjusts the focus for near or far objects.
- Example 2. A magnifying glass
- Held close to small print, a converging lens produces an enlarged, upright image, helping to read tiny text, the same lens type used to correct long sight.
- Example 3. Projecting a slide
- A projector uses a converging lens to form a large, real image of a small slide on a distant screen. The image is inverted, so slides are loaded upside down; this is the same kind of real image the eye forms on the retina.
Remember that lenses focus light entirely by refraction at their two curved surfaces. The more strongly curved a converging lens is, the more it bends the light and the shorter its focal length, so the more powerful it is. This is why strong reading glasses use fatter, more curved lenses than weak ones.
Try this
Q1. Which lens is thicker in the middle, converging or diverging? [1 mark]
- Cue. A converging (convex) lens.
Q2. Where does the eye form a sharp image? [1 mark]
- Cue. On the retina at the back of the eye.
Q3. State the lens used to correct short sight. [1 mark]
- Cue. A diverging (concave) lens.
Exam-style practice questions
Practice questions written in the style of CCEA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
CCEA style4 marksDescribe the difference between a converging and a diverging lens, and state the type of lens used to correct short sight (myopia).Show worked answer →
A converging (convex) lens is thicker in the middle and brings parallel rays of light together to a focus (the focal point).
A diverging (concave) lens is thinner in the middle and spreads parallel rays of light apart, so they appear to come from a focal point behind the lens.
Short sight (myopia) is corrected with a diverging (concave) lens.
Markers reward: converging brings rays together (thicker middle); diverging spreads rays out (thinner middle); and a diverging lens for short sight.
CCEA style3 marksExplain what happens in an eye with long sight (hyperopia) and how a lens corrects it.Show worked answer →
In long sight the eye cannot focus on near objects; the image of a near object would form behind the retina, so it looks blurred.
A converging (convex) lens is used. It brings the light rays together a little before they enter the eye, so the eye can then focus the image onto the retina.
Markers reward: image forms behind the retina for near objects; corrected with a converging (convex) lens that focuses the image onto the retina.
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Sources & how we know this
- CCEA GCSE Physics specification — CCEA (2017)