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A convex lens is fat in the middle and brings light rays together, while a concave lens is thin in the middle and spreads them apart. Free notes and FAQs.
Convex and Concave Lenses is part of Electromagnetic waves in GCSE AQA Higher Physics.
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A convex lens is fat in the middle and brings light rays together, while a concave lens is thin in the middle and spreads them apart. Free notes and FAQs.
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Use this lesson to stay inside Electromagnetic waves while connecting the detail back to the wider Waves topic.
Read the core explanations from Convex and Concave Lenses before testing yourself from memory.
Hold a magnifying glass up to sunlight and you can focus the rays to a tiny, brilliant spot. That's a lens at work — bending light in a predictable way. But why does light bend, and how do different lens shapes create completely different effects?
A lens works by refraction — the bending of light as it passes from one material to another. When light travels from air into glass, it slows down and changes direction. The curved shape of the lens means different parts of a light ray hit the glass at different angles, causing the whole ray to bend in a controlled way.
There are two main types of lens, each with opposite effects. Convex lenses are fat in the middle and thin at the edges — they bring light rays together. Concave lenses are thin in the middle and thick at the edges — they spread light rays apart. This fundamental difference determines everything about how they form images.
Picture parallel rays of sunlight hitting a convex lens. The lens is thicker in the middle, so light passing through the centre travels through more glass than light at the edges. This means the central rays are slowed down and bent more than the edge rays.
The result is remarkable — all the parallel rays converge to meet at a single point called the principal focus. This happens because the curved shape ensures that every ray, no matter where it hits the lens, gets bent by exactly the right amount to reach the same focal point.
The distance from the lens to this focal point is the focal length. A powerful lens (like a magnifying glass) has a short focal length — it bends light sharply. A weak lens (like reading glasses) has a long focal length — it bends light gently.
In ray diagrams, we represent a convex lens with a vertical line that has outward-pointing arrows at the top and bottom. This symbol reminds us that the lens is thicker in the middle and converges light rays.
Now imagine the opposite lens shape — concave lenses are thin in the middle and thick at the edges. When parallel rays hit this lens, the edge rays travel through more glass than the central rays. This means the edge rays are slowed down and bent more.
The effect is the reverse of a convex lens — the rays spread out or diverge after passing through. If you trace these diverging rays backwards, they appear to come from a single point on the same side of the lens as the incoming light. This point is called the principal focus of the concave lens.
Concave lenses never actually bring light rays together to a real focus point. Instead, they create a virtual focus — a point where the rays would meet if you extended them backwards. This is why concave lenses always produce virtual images that appear smaller than the original object.
In ray diagrams, we represent a concave lens with a vertical line that has inward-pointing arrows at the top and bottom. This symbol reminds us that the lens is thinner in the middle and diverges light rays.
The key difference between convex and concave lenses lies in the type of images they produce. Understanding real versus virtual images is crucial for predicting what you'll see when looking through different lenses.
A real image forms when light rays actually converge at a point. You can project a real image onto a screen because the light rays physically meet there. Convex lenses can produce real images when the object is placed beyond the focal length. These images can be larger or smaller than the object, and they're always inverted (upside down).
A virtual image forms when light rays only appear to come from a point — they don't actually meet there. You can't project a virtual image onto a screen because no light rays physically converge. Virtual images always appear upright and are formed when you look through the lens towards the object.
Concave lenses always produce virtual images. No matter where you place the object, the diverging rays never actually meet — they only appear to come from a point when traced backwards. These virtual images are always upright, smaller than the object, and appear on the same side of the lens as the object.
Ray diagrams are the key to predicting where images will form and what they'll look like. There are three special rays that are straightforward to draw and always follow predictable paths through lenses.
For convex lenses: Ray 1 travels parallel to the axis and refracts through the principal focus. Ray 2 passes through the centre of the lens and continues straight (no deviation). Ray 3 passes through the principal focus and emerges parallel to the axis. Where any two of these rays intersect is where the image forms.
For concave lenses: Ray 1 travels parallel to the axis and refracts as if it came from the principal focus. Ray 2 passes through the centre and continues straight. Ray 3 heads towards the principal focus on the far side but refracts to emerge parallel to the axis. The image appears where the rays seem to come from when traced backwards.
Worked example: Drawing a Ray Diagram for a Convex Lens
An object is placed 30 cm from a convex lens with focal length 10 cm. Draw the ray diagram to find the image position.
Answer: Real, inverted image formed 15 cm from lens on opposite side
Lens questions often trip students up because they confuse the two lens types or mix up real and virtual images. The key is to remember that convex lenses can produce both types of image depending on object position, while concave lenses always produce virtual images.
When drawing ray diagrams, accuracy matters for marks. Use a ruler for straight lines and ensure rays actually pass through the focal points. Many students lose marks by drawing sloppy diagrams where the rays don't quite meet or don't pass through the correct points.
For describe questions about image properties, use the correct terminology: real or virtual, upright or inverted, magnified or diminished. Avoid vague terms like 'bigger' or 'smaller' — use 'magnified' or 'diminished' instead.
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Move to the related lessons or back to the topic page when you need the wider sequence before exam-style practice.
A convex lens is fat in the middle and thin at the edges and brings parallel light rays together at a principal focus. A concave lens is thin in the middle and thick at the edges and spreads light rays apart, so they only appear to come from a virtual focus.
Concave and convex lenses both bend light by refraction, but in opposite ways. A convex lens converges parallel rays to a principal focus, while a concave lens makes them diverge, so they appear to come from a virtual focus on the same side as the incoming light.
Focal length is the distance from the centre of a lens to its principal focus, the point where parallel rays converge after passing through a convex lens. A powerful lens, like a magnifying glass, has a short focal length and bends light sharply.
A convex lens is fat in the middle and thin at the edges and brings light rays together, while a concave lens is thin in the middle and thick at the edges and spreads light rays apart. Convex lenses can form real or virtual images, but concave lenses only form virtual images.
A real image forms where light rays actually converge, so it can be projected onto a screen; real images from a convex lens are always inverted. A virtual image forms where rays only appear to come from, so it cannot be projected, and it appears upright.
In a ray diagram, a convex lens is drawn as a vertical line with outward-pointing arrows at the top and bottom, which shows it is thicker in the middle and converges light rays. A concave lens has inward-pointing arrows.
A magnifying glass is a powerful lens with a short focal length, so it bends light sharply. A weak lens, like reading glasses, has a long focal length and bends light gently.
A concave lens always produces a virtual image that is upright, smaller than the object, and on the same side of the lens as the object. The diverging rays never actually meet; they only appear to come from a point when traced backwards.
A lens bends light by refraction: light slows down and changes direction as it passes from air into glass. The curved shape means different parts of the light hit the glass at different angles, so the rays bend in a controlled way.
The principal focus is the point where parallel rays of light converge after passing through a convex lens. The distance from the centre of the lens to the principal focus is the focal length.
A virtual focus is the point from which diverging rays appear to come when traced backwards through a concave lens. Concave lenses never bring light rays together to a real focus point.
A real image can be projected onto a screen because the light rays physically meet there. A virtual image cannot be projected, because the rays only appear to come from a point and never actually converge.
Real images formed by a convex lens are always inverted (upside down), and they can be larger or smaller than the object. Virtual images are always upright.
An object 30 cm from a convex lens with a focal length of 10 cm forms a real, inverted, smaller image 15 cm from the lens on the opposite side. The object is beyond the focal length, so the image is real.
A lens image is described as real or virtual, upright or inverted, and magnified or diminished. 'Magnified' or 'diminished' should be used instead of vague terms like 'bigger' or 'smaller'.