A Lens Can Set Paper Alight With Nothing but Sunshine
Learn to tell a convex lens from a concave one by shape and by what it does to light, concentrate sunlight safely to a bright spot, compare the images each lens forms, and pick the right lens for a job.
How can a piece of glass set paper on fire?
By gathering sunlight that fell over the whole lens and concentrating it onto one tiny spot. All that energy arriving on a small area makes the spot hot enough to scorch and then burn the paper.
Only a convex lens can do it, because only a convex lens brings light together. This page covers everything in the CBSE Class 8 Science chapter's second part: types of lens, the burning-spot activity, the images lenses form, and their everyday uses.
Only a convex lens can do it, because only a convex lens brings light together. This page covers everything in the CBSE Class 8 Science chapter's second part: types of lens, the burning-spot activity, the images lenses form, and their everyday uses.
How do you tell a convex lens from a concave one?
A lens is a piece of transparent material, usually glass, with at least one curved surface. The shape and the effect on light go together.
- Convex lens — thicker in the middle and thinner at the edges. It bends parallel rays inward so they meet at a point, so it is a converging lens.
- Concave lens — thinner in the middle and thicker at the edges. It spreads parallel rays outward, so it is a diverging lens.
You can test an unknown lens in two ways:
- By touch and sight — feel whether it is thicker at the centre or at the rim.
- By looking through it at print — a convex lens held close makes the letters look larger; a concave lens makes them look smaller.
A drop of water resting on a flat surface, or a full glass tumbler of water held up to a page, acts as a convex lens and magnifies the text behind it.
The word to attach to each is the one that matters in later chapters: convex converges, concave diverges. Both start with the same letters as their action if you notice that a cave goes inward — the concave lens is hollowed out in the middle, and a hollow cannot bring anything together.
- Convex lens — thicker in the middle and thinner at the edges. It bends parallel rays inward so they meet at a point, so it is a converging lens.
- Concave lens — thinner in the middle and thicker at the edges. It spreads parallel rays outward, so it is a diverging lens.
You can test an unknown lens in two ways:
- By touch and sight — feel whether it is thicker at the centre or at the rim.
- By looking through it at print — a convex lens held close makes the letters look larger; a concave lens makes them look smaller.
A drop of water resting on a flat surface, or a full glass tumbler of water held up to a page, acts as a convex lens and magnifies the text behind it.
The word to attach to each is the one that matters in later chapters: convex converges, concave diverges. Both start with the same letters as their action if you notice that a cave goes inward — the concave lens is hollowed out in the middle, and a hollow cannot bring anything together.
How do you show that a convex lens concentrates sunlight, and what precautions apply?
The activity. Take a convex lens outdoors in bright sunshine and hold it above a sheet of paper lying on the ground.
1. Hold the lens parallel to the paper, a little above it.
2. Move the lens up and down slowly, watching the patch of light on the paper.
3. At one particular distance the patch becomes the smallest and brightest possible — a sharp bright spot.
4. Hold it steady there. After a short while the paper darkens, smokes and chars.
The explanation: the lens collects the sunlight falling on its whole surface and converges it onto that small spot, so a large amount of light energy arrives on a very small area and heats it strongly.
Repeat the activity with a concave lens and nothing happens. It spreads the light out instead, so no bright spot forms and the paper stays cool — which is the clearest proof of the difference between them.
Safety precautions, all of which are examinable:
- Never look at the Sun through a lens, or directly at all. The converged light can permanently damage the eye.
- Do the activity under a teacher's supervision, outdoors.
- Keep it away from anything inflammable, and never point it at a person, an animal or clothing.
- Keep water or sand nearby, and do not leave the paper unattended once it begins to smoke.
- Do not leave a lens lying in sunlight near paper or cloth, since it can start a fire unattended.
The first precaution is the one that matters most. The same concentration of energy that chars paper would be focused inside the eye, and eye damage from looking at the Sun is immediate and permanent — which is why this is a supervised activity rather than one to try alone.
1. Hold the lens parallel to the paper, a little above it.
2. Move the lens up and down slowly, watching the patch of light on the paper.
3. At one particular distance the patch becomes the smallest and brightest possible — a sharp bright spot.
4. Hold it steady there. After a short while the paper darkens, smokes and chars.
The explanation: the lens collects the sunlight falling on its whole surface and converges it onto that small spot, so a large amount of light energy arrives on a very small area and heats it strongly.
Repeat the activity with a concave lens and nothing happens. It spreads the light out instead, so no bright spot forms and the paper stays cool — which is the clearest proof of the difference between them.
Safety precautions, all of which are examinable:
- Never look at the Sun through a lens, or directly at all. The converged light can permanently damage the eye.
- Do the activity under a teacher's supervision, outdoors.
- Keep it away from anything inflammable, and never point it at a person, an animal or clothing.
- Keep water or sand nearby, and do not leave the paper unattended once it begins to smoke.
- Do not leave a lens lying in sunlight near paper or cloth, since it can start a fire unattended.
The first precaution is the one that matters most. The same concentration of energy that chars paper would be focused inside the eye, and eye damage from looking at the Sun is immediate and permanent — which is why this is a supervised activity rather than one to try alone.
What images do convex and concave lenses form?
A concave lens always behaves the same way, while a convex lens changes with distance.
Concave lens — whatever the object's position:
- Diminished, smaller than the object
- Erect
- Virtual, so it cannot be caught on a screen
Move the object nearer or further and the image stays small and upright; only its size changes a little.
Convex lens — the image depends on how far the object is:
- Object very close to the lens — image is enlarged, erect and virtual. This is the magnifying glass.
- Object further away — image is inverted and real, and can be caught on a screen.
- Object very far away — image is diminished, inverted and real, formed close to the lens.
You can watch the change with a single lens. Hold a convex lens close to print and the letters are large and upright. Slowly move it away and at some point the view flips upside down and shows the room behind.
A real image can be obtained on a screen because the rays genuinely meet; a virtual image cannot, because they only appear to come from it.
So the convex lens is the versatile one, and the concave lens the predictable one. That is exactly why a convex lens can serve as both a magnifier and a camera lens depending on distance, while a concave lens is chosen when a reliably smaller, upright image is wanted.
Concave lens — whatever the object's position:
- Diminished, smaller than the object
- Erect
- Virtual, so it cannot be caught on a screen
Move the object nearer or further and the image stays small and upright; only its size changes a little.
Convex lens — the image depends on how far the object is:
- Object very close to the lens — image is enlarged, erect and virtual. This is the magnifying glass.
- Object further away — image is inverted and real, and can be caught on a screen.
- Object very far away — image is diminished, inverted and real, formed close to the lens.
You can watch the change with a single lens. Hold a convex lens close to print and the letters are large and upright. Slowly move it away and at some point the view flips upside down and shows the room behind.
A real image can be obtained on a screen because the rays genuinely meet; a virtual image cannot, because they only appear to come from it.
So the convex lens is the versatile one, and the concave lens the predictable one. That is exactly why a convex lens can serve as both a magnifier and a camera lens depending on distance, while a concave lens is chosen when a reliably smaller, upright image is wanted.
Which lens is used in a magnifying glass, spectacles and a camera?
Match the lens to what the device needs to do.
Convex lenses are used in:
- Magnifying glass — held close to the object to give an enlarged, erect, virtual image, for reading small print, examining leaves or checking a coin
- Camera — forms a small, inverted, real image on the film or sensor
- Projector — forms an enlarged real image on a screen
- Spectacles for long-sightedness, where the eye cannot focus on near objects
- Microscope and telescope eyepieces and objectives
- The human eye itself contains a convex lens, which forms a real inverted image on the retina
Concave lenses are used in:
- Spectacles for short-sightedness, where the eye cannot focus on distant objects
- Peepholes in doors, giving a wide view of who is outside
- Combined with other lenses in some optical instruments
So choosing for a given use follows one question: is a bigger image wanted, or a smaller wider one? Enlargement needs a convex lens; reduction with a wide view needs a concave one.
Spectacles are the case worth reading carefully, because both types appear. A person who cannot see nearby things clearly needs a convex lens; one who cannot see distant things clearly needs a concave lens. An optician's prescription therefore names not just a strength but a type — and fitting the wrong one would make the sight worse rather than better.
Convex lenses are used in:
- Magnifying glass — held close to the object to give an enlarged, erect, virtual image, for reading small print, examining leaves or checking a coin
- Camera — forms a small, inverted, real image on the film or sensor
- Projector — forms an enlarged real image on a screen
- Spectacles for long-sightedness, where the eye cannot focus on near objects
- Microscope and telescope eyepieces and objectives
- The human eye itself contains a convex lens, which forms a real inverted image on the retina
Concave lenses are used in:
- Spectacles for short-sightedness, where the eye cannot focus on distant objects
- Peepholes in doors, giving a wide view of who is outside
- Combined with other lenses in some optical instruments
So choosing for a given use follows one question: is a bigger image wanted, or a smaller wider one? Enlargement needs a convex lens; reduction with a wide view needs a concave one.
Spectacles are the case worth reading carefully, because both types appear. A person who cannot see nearby things clearly needs a convex lens; one who cannot see distant things clearly needs a concave lens. An optician's prescription therefore names not just a strength but a type — and fitting the wrong one would make the sight worse rather than better.
Exam tip
Exam tip: describing shape and effect together
Lens questions are marked on both halves of the definition, so never give one alone.
Write it as a pair: a convex lens is thicker at the middle and converges light; a concave lens is thinner at the middle and diverges it.
For image questions, state all three features — size, erect or inverted, and real or virtual — and define real as can be obtained on a screen.
Remember that a concave lens always gives a diminished erect virtual image, while a convex lens depends on distance. That difference is a favourite question.
For the sunlight activity, include the step of moving the lens to get the smallest brightest spot, and give at least two safety precautions, starting with never look at the Sun through a lens.
And for uses, name the lens type with the device — convex for a magnifying glass and camera, concave for a peephole.
Write it as a pair: a convex lens is thicker at the middle and converges light; a concave lens is thinner at the middle and diverges it.
For image questions, state all three features — size, erect or inverted, and real or virtual — and define real as can be obtained on a screen.
Remember that a concave lens always gives a diminished erect virtual image, while a convex lens depends on distance. That difference is a favourite question.
For the sunlight activity, include the step of moving the lens to get the smallest brightest spot, and give at least two safety precautions, starting with never look at the Sun through a lens.
And for uses, name the lens type with the device — convex for a magnifying glass and camera, concave for a peephole.
Did you know
Why does a magnifying glass turn the view upside down if you move it back?
Because past a certain distance the converging rays actually cross before reaching your eye.
Held close to the page, the lens cannot bring the rays to a meeting point before they reach you, so your eye sees an enlarged upright image that only appears to be there — a virtual one.
Move the lens further away and the rays now converge and cross over in front of your eye. What was at the top of the object arrives from below, so the image flips and becomes a real, inverted one. The lens has not changed at all; only the distance has — which is the same reason a camera, sitting far from its subject, records an inverted image on its sensor.
Held close to the page, the lens cannot bring the rays to a meeting point before they reach you, so your eye sees an enlarged upright image that only appears to be there — a virtual one.
Move the lens further away and the rays now converge and cross over in front of your eye. What was at the top of the object arrives from below, so the image flips and becomes a real, inverted one. The lens has not changed at all; only the distance has — which is the same reason a camera, sitting far from its subject, records an inverted image on its sensor.
Key takeaways
Lenses: quick revision
- A convex lens is thicker in the middle and converges light; a concave lens is thinner in the middle and diverges it.
- Test an unknown lens by feel, or by looking at print — convex enlarges, concave diminishes.
- A convex lens held at the right distance concentrates sunlight to the smallest brightest spot, which chars paper; a concave lens cannot. Never look at the Sun through a lens.
- A concave lens always gives a diminished, erect, virtual image whatever the distance.
- A convex lens gives an enlarged erect virtual image when the object is very close, and an inverted real image when it is further away.
- Convex lenses are used in magnifying glasses, cameras, projectors and long-sight spectacles; concave lenses in short-sight spectacles and door peepholes.
You will remember all of this far better after answering five questions on it than after reading it twice.
- Test an unknown lens by feel, or by looking at print — convex enlarges, concave diminishes.
- A convex lens held at the right distance concentrates sunlight to the smallest brightest spot, which chars paper; a concave lens cannot. Never look at the Sun through a lens.
- A concave lens always gives a diminished, erect, virtual image whatever the distance.
- A convex lens gives an enlarged erect virtual image when the object is very close, and an inverted real image when it is further away.
- Convex lenses are used in magnifying glasses, cameras, projectors and long-sight spectacles; concave lenses in short-sight spectacles and door peepholes.
You will remember all of this far better after answering five questions on it than after reading it twice.