Lenses
Revise Lenses for GCSE Physics with this free worksheet and full mark scheme — Foundation and Higher exam-style questions with worked answers for AQA, Edexcel and OCR. A separate (triple) physics topic: convex lenses converge light and concave lenses diverge it, and ray diagrams are used to find the image that is formed.
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These worksheets and mark schemes are original, written for Virtus Academy and checked against the current AQA, Edexcel and OCR specifications. Every worksheet comes with a full mark scheme.
Topic overview
A lens forms an image by refracting light. This is a Triple-only topic.
A convex lens is thicker in the middle and causes parallel rays to converge to a point called the principal focus. A concave lens is thinner in the middle and causes parallel rays to diverge, appearing to come from a principal focus behind the lens.
The distance from the lens to the principal focus is the focal length. Magnification is calculated as image height divided by object height, and it has no units because it is a ratio of two lengths. A convex lens can form a real image, which can be projected onto a screen; a concave lens always forms a virtual image.
Revision notes
Convex and concave lenses
A convex lens is thicker in the middle and converges parallel rays to a principal focus.
A concave lens is thinner in the middle and diverges parallel rays, which appear to come from a principal focus on the same side as the object.
Focal length and images
The focal length is the distance from the lens to the principal focus.
A convex lens can form a real image, which can be projected onto a screen because light rays actually meet there. A concave lens always forms a virtual image, which cannot be projected.
Magnification
Magnification = image height ÷ object height.
It has no units because it is a ratio of two lengths. A magnification greater than 1 means the image is larger than the object; less than 1 means smaller.
Key points
- A convex lens is thicker in the middle and converges rays.
- A concave lens is thinner in the middle and diverges rays.
- Focal length is the distance to the principal focus.
- Magnification = image height ÷ object height.
- Magnification has no units.
- A real image can be projected onto a screen.
Worked examples
Example 1
An object 5 cm tall produces an image 15 cm tall. Calculate the magnification. [2 marks]
Model answer
Magnification = image height ÷ object height = 15 ÷ 5 (1 mark) = 3 (1 mark).
Example 2
State the difference between a convex and a concave lens. [2 marks]
Model answer
A convex lens is thicker in the middle and causes parallel rays of light to converge (1 mark). A concave lens is thinner in the middle and causes parallel rays to diverge (1 mark).
Example 3
Explain the difference between a real and a virtual image. [2 marks]
Model answer
A real image is formed where light rays actually meet, so it can be projected onto a screen (1 mark). A virtual image is formed where rays only appear to come from, so it cannot be projected (1 mark).
Common mistakes
Giving magnification a unit.
It is a ratio of two lengths, so it has no units.
Confusing convex and concave.
Convex is thicker in the middle and converges; concave is thinner and diverges.
Saying a concave lens forms a real image.
It always forms a virtual image.
Dividing object height by image height.
Magnification is image ÷ object.
Exam tips
- Remember magnification has no units.
- Learn convex as converging and concave as diverging.
- Distinguish real and virtual by whether the image can be projected.
- Remember this is a Triple-only topic.
Key terms
- Convex lens
- A lens thicker in the middle that converges light.
- Concave lens
- A lens thinner in the middle that diverges light.
- Principal focus
- The point where parallel rays converge or appear to diverge from.
- Magnification
- Image height divided by object height.
Related topics
Written and reviewed against the current AQA, Edexcel and OCR specifications. Spotted an error? Let us know.