Light: Mirrors and Lenses · Lesson 6 of 7
How Lenses Converge and Diverge Light
“Compare transmitted light beams and connect lens behaviour to cameras, glasses, and the eye.”
• Compare the effects of flat glass, convex lenses, and concave lenses on parallel beams. • Use converging and diverging to describe the two lens types. • Explain heating by concentrated sunlight passing through a convex lens. • Distinguish beam direction through lenses from beam direction at mirrors. • Describe applications of lenses and the eye lens’s ability to change shape.
Follow the light that passes through
The previous lesson described how an object looks through a lens. Now we follow the transmitted light itself. This investigation parallels the mirror-beam investigation, but the light continues through the transparent device instead of being sent back from a reflecting surface. Observing the outgoing paths helps us connect lens shape with image behaviour and applications.
A convex lens does not act like a convex mirror. Although both names contain convex, one transmits light through curved surfaces while the other reflects it from an outward-bulging face. Compare what happens to a parallel incoming beam rather than trying to infer the effect from the word alone.
Compare flat glass and two lenses
A thin flat transparent glass plate provides the reference. In the source’s experiment, it leaves the outgoing beams approximately parallel, without the strong gathering or spreading effect of a lens. The convex and concave lenses change the separation between the outgoing beams in different ways.
Collect a thin transparent glass plate, convex and concave lenses, a torch, a comb, a paper clip, two similar books, and white sheets. Use the comb to obtain multiple narrow, approximately parallel beams. Support the plate or lens upright between the books, with paper on the books so the incident and transmitted paths can be seen. Keep the glass secure and handle it carefully.
- Send the parallel beams through the flat plate and observe their outgoing directions.
- Replace the plate with a convex lens and observe whether the transmitted beams approach one another.
- Repeat with a concave lens and observe whether the transmitted beams spread apart.
- Draw each pattern with arrows continuing through the transparent object. Record the difference between input and output.
The convex lens makes suitable parallel incident beams move toward one another after transmission. We therefore call it a converging lens. The concave lens makes the beams spread apart after transmission and is called a diverging lens. The names describe the outgoing paths, not a judgement about whether the device is useful or powerful.
A convex lens, named for its ability to bring suitable parallel incident rays closer together after they pass through it.
A concave lens, named for its ability to spread parallel incident rays apart after they pass through it.
| Device | Interaction used | Effect on initially parallel rays |
|---|---|---|
| Plane mirror | Reflection | Remain parallel after returning from the mirror |
| Concave mirror | Reflection | Converge after returning from the mirror |
| Convex mirror | Reflection | Diverge after returning from the mirror |
| Thin flat glass plate | Transmission | Outgoing beams remain approximately parallel |
| Convex lens | Transmission | Converge after passing through the lens |
| Concave lens | Transmission | Diverge after passing through the lens |
The table reveals a useful connection: a concave mirror and a convex lens can both bring a parallel beam together. A convex mirror and a concave lens can both spread it apart. The pairing concerns beam behaviour, while the paths themselves differ: mirrors return light toward the incident side, and lenses transmit it to the other side.
The source’s flat-glass comparison shows no convergence or divergence of the outgoing parallel beams. It does not mean that light can never change direction at a flat transparent boundary. Here the important result is that the beams do not acquire the gathering or spreading pattern of the curved lenses.
Problem
Three transparent devices receive parallel beams. One leaves them parallel, one gathers them, and one spreads them. Identify the devices.
- 1.The device with parallel outgoing beams is consistent with the thin flat plate in this investigation.
- 2.Gathering outgoing beams identify the convex, converging lens.
- 3.Spreading outgoing beams identify the concave, diverging lens. Check that the light passes through each device rather than reflecting back.
Problem
Which lens has the same broad gathering effect on a parallel beam as a concave mirror?
- 1.A concave mirror makes suitable reflected rays converge.
- 2.A convex lens makes suitable transmitted rays converge.
- 3.Their broad gathering effect matches, but their light paths differ because one reflects and the other transmits.
A convex lens can concentrate sunlight
Because a convex lens converges approximately parallel sunlight, it can produce a small bright spot on a suitably placed surface beyond it. The same energy-concentration reasoning used for a concave mirror applies: light collected over a larger area is directed into a smaller receiving region. An absorbing material in that region can heat strongly.
In the source demonstration, a convex lens replaces the concave mirror and is put in the path of sunlight. The paper is placed on the far side of the lens and its distance adjusted to obtain a small bright spot. Paper can smoke or ignite when enough sunlight is absorbed. A concave lens, which spreads the incident parallel beam, does not provide the same concentrating arrangement.
This demonstration requires teacher or adult supervision and a controlled setting. Never look directly at the Sun or through a lens toward it; a lens can concentrate sunlight and injure the eyes. Do not direct concentrated light toward people. The paper-heating setup also requires fire precautions; a supervised demonstration or video is sufficient for learning the principle.
Observe an adult-managed demonstration or a suitable video of sunlight directed onto paper through a convex lens. Record the side on which the bright spot forms, how changing the paper position changes the spot, and the evidence of heating. Compare these observations with the concave-mirror demonstration without attempting either alone.
Problem
A friend says a magnifying glass heats paper because its glass becomes hotter and transfers that heat to the paper. Is this the main explanation for the sunlight demonstration?
- 1.In the demonstration, the lens and paper are separated; the lens need not touch the paper.
- 2.The convex lens redirects transmitted sunlight into a small bright region on the paper.
- 3.Absorption of that concentrated light heats the paper. Direct heat conduction from a touching lens is not the mechanism being demonstrated.
Lenses in devices around us
Eyeglasses use lenses to help people see clearly. Some glasses have two viewing regions, commonly called bifocal glasses: an upper region is used for more distant viewing and a lower region for close work such as reading. The curved boundary visible in some designs separates these regions. This explains the opening observation without requiring a study of prescriptions or eye-defect calculations.
Camera lenses guide light to form an image on a recording surface or sensor. Smartphone cameras therefore also contain lenses, even though the individual lens parts may be small. Microscopes use lenses to help us examine small structures, and lens-based telescopes help us view distant objects. Some telescopes use mirrors as their main light-collecting element, so the presence of a telescope does not mean every telescope has an identical design.
Notice the different purposes: a camera records an image, a microscope helps view very small things, and a telescope helps view distant things. The shared feature is controlled use of light. Avoid assuming that every instrument consists of a single simple lens or that every image is upright and enlarged in every arrangement.
| Application | Why lenses matter |
|---|---|
| Eyeglasses | Help the wearer see clearly for the intended viewing task |
| Cameras and smartphone cameras | Guide light to form a recordable image |
| Microscopes | Help view small structures |
| Lens-based telescopes | Help view distant objects |
| Human eye | A convex lens participates in forming the view of objects at different distances |
The eye has a lens that changes shape
Your eye contains a transparent convex lens. Unlike a simple classroom glass lens with a fixed shape, the eye’s lens can change its shape. This changes how it redirects light, helping you switch between near tasks such as reading a book and viewing something farther away. The actual object does not move inside the eye; the optical arrangement adjusts to the object distance.
The source introduces this ability briefly, but it makes an important connection between optics and everyday experience. You can appreciate that the eye adjusts without learning additional calculations or detailed eye anatomy here. The key idea is that lens shape influences transmitted light, and the eye can adjust that shape.
Problem
What important difference should you mention when comparing a simple classroom glass lens with the eye lens?
- 1.Both are transparent and redirect light passing through them.
- 2.The classroom lens has a fixed shape in the investigation, so its image pattern changes when the object position changes.
- 3.The eye’s convex lens can change its shape, helping the eye adjust when viewing near and far objects.
Check your understanding
Use the words reflection and transmission to keep the devices distinct. Then connect convergence and divergence to each application.
Quiz
Which lens is also called a converging lens?
The outgoing parallel-beam pattern from a concave lens is:
Which pair can both gather initially parallel light?
Where is the concentrated sunlight region in a convex-lens demonstration?
What does the eye’s lens do when viewing tasks change between near and far objects?
Which statement about optical instruments is best supported by the chapter?
Practice Problems
- Draw parallel rays passing through a flat plate, a convex lens, and a concave lens. Use arrows and label the outgoing patterns.
- Explain why a convex lens and a convex mirror do not have the same beam behaviour.
- Compare the bright-region positions in concave-mirror and convex-lens sunlight demonstrations.
- Explain the heating mechanism in the convex-lens demonstration without saying that the lens creates energy.
- Give four lens applications and explain the purpose of each in a sentence.
- Explain how the eye lens differs from a fixed glass lens when viewing nearby and distant objects.
- Observe, without handling someone’s prescription unnecessarily, a picture of reading glasses with different viewing regions. Form a question about how the regions help different viewing tasks.
Key Takeaways
• A convex lens converges suitable parallel rays; a concave lens diverges them. • A thin flat glass plate leaves outgoing parallel beams approximately parallel in the source comparison. • A concave mirror and a convex lens both gather light, using reflection and transmission respectively. • A convex lens can concentrate sunlight into a small region beyond the lens and cause strong heating. • Never look at the Sun through a lens; heating demonstrations require adult supervision and fire precautions. • Lenses are used in eyeglasses, cameras, microscopes, and lens-based telescopes. • The eye’s convex lens can change shape to help with viewing near and distant objects.