Light: Mirrors and Lenses · Lesson 5 of 7
What Is a Lens, and How Does It Change an Image?
“Explore curved transparent materials and discover why lenses change the appearance of objects.”
• Explain what a lens is and distinguish it from a mirror. • Identify convex and concave lenses using their relative thickness. • Investigate a water drop as a simple lens. • Describe image changes when an object moves relative to either lens type. • Explain the changed appearance of a pencil viewed through water qualitatively.
Look through a curved transparent surface
A mirror sends light back by reflection. Now imagine looking through a clear window, a water drop, or a magnifying glass. These materials allow light to pass through. A flat window usually lets you see objects with little obvious change in size, but a curved transparent surface can change how the light travels and hence how an object appears.
The difference is not simply that one object is shiny and another is smooth. A mirror is used for its reflecting surface. A lens is used mainly for the light transmitted through it. Both can change the appearance of an object, but they interact with light in different ways.
Make a water-drop lens
Before naming manufactured lenses, examine a small curved drop of water. Its rounded surface is easy to produce on a transparent strip. Printed text gives a useful reference because you can compare letters beneath the drop with neighbouring letters that are not beneath it.
Collect a flat clear plastic strip or glass strip, a dropper, water, a little oil or wax, and printed text. A clear ruler can serve as the strip. Use a thin oil or wax coating on a small area to help the water remain as a rounded drop rather than spread into a flat film.
- Place a small water drop on the coated area and observe whether its surface is flat or bulges outward.
- Position the strip above printed text so some letters lie beneath the drop.
- Look down through the drop and compare those letters with nearby uncovered letters.
- Record whether the letters appear larger and describe what is special about the drop’s shape.
With a suitable arrangement, the letters beneath the drop appear larger. The actual print has not grown; the curved transparent drop changes the paths of transmitted light so that its image looks enlarged. The water drop is acting as a simple lens. The oil or wax helps maintain the rounded shape, while the drop’s curved transparent surface produces the lens effect.
Problem
Letters under a water drop look bigger than letters nearby. Has the printed ink spread or become larger?
- 1.Compare the same letters after removing the drop: the printed text remains unchanged.
- 2.The drop changes the path of light reaching your eye from the letters.
- 3.The image appears enlarged; the actual letters have not grown. The difference is optical, not a change to the paper.
Convex and concave lenses
A lens is made from a transparent material, commonly glass or plastic, and has at least one curved surface. The shape affects transmitted light. The two types introduced here can be distinguished by comparing their thickness in the middle with their thickness near the edges. This thickness test is different from identifying the reflecting face of a mirror.
A piece of transparent material with one or more curved surfaces that changes the paths of light passing through it.
A lens that is thicker at the middle than near its edges.
A lens that is thinner at the middle than near its edges.
In a simple diagram, the lens outline communicates its shape; unlike a mirror drawing, it is not shaded to mark a non-reflecting back. A lens lets us look through it from one side toward an object on the other side. A flat clear glass piece is a useful comparison because it lacks the lens’s curved shape and strong converging or diverging effect.
For a mirror, inspect whether the reflecting face curves inward or outward. For the lenses studied here, compare centre and edge thickness. Do not assume that a convex lens must form the same kind of image as a convex mirror just because their names match.
Problem
A clear lens has thick edges and a narrow middle. Is it convex or concave?
- 1.Look at the thickness of the lens material, not just the frame around it.
- 2.The middle is thinner than the edges.
- 3.That is a concave lens. A convex lens has the opposite thickness pattern.
Move an object behind a lens
The image seen through a lens can depend on object distance, just as a concave mirror’s image does. To study the dependence, keep the lens upright and move the same small object. Place the object on the opposite side of the lens from your eye and keep it near the lens’s height so that it is easy to observe.
Use a convex lens, a concave lens, a holder, and a small toy. First place the convex lens upright and the toy close to it on the far side. Observe from the other side of the lens. Slowly move the toy farther away, record its apparent size and orientation, then repeat with the concave lens.
For a convex lens, an object at a suitable small distance appears erect and enlarged. This is the useful arrangement for a magnifying glass held near print. At suitable greater distances, the image becomes inverted. Among these inverted images, its size can be enlarged, the same as the object, or diminished as the object position changes.
For a concave lens, an ordinary object viewed through it appears erect and diminished. The apparent size changes as the object distance increases, but the image stays upright and smaller than the object. The source does not use focal-length calculations or image-distance equations; observations establish the broad comparison.
| Transparent object | At a suitable close object distance | At suitable greater object distances |
|---|---|---|
| Convex lens | Erect and enlarged | Inverted; can be enlarged, same-size, or diminished |
| Concave lens | Erect and diminished | Remains erect and diminished; apparent size changes |
| Flat clear glass piece | No obvious enlargement in this comparison | No convex- or concave-lens image pattern |
Problem
A student holds a lens near small print and sees the letters upright and enlarged. Which lens is suitable?
- 1.The required image is upright and larger than the print.
- 2.A concave lens makes ordinary objects appear diminished, so it does not fit.
- 3.A convex lens at a suitable small distance gives an erect enlarged view. A magnifying glass uses this arrangement.
Problem
Small print looks larger through a magnifying glass. The glass is then moved farther from the print. Must the view remain upright and enlarged?
- 1.The magnifying glass is a convex lens and its image depends on object-to-lens distance.
- 2.Its useful upright enlarged view occurs when the print is suitably close to the lens.
- 3.At suitable greater separations, an inverted image can be seen. Moving away does not preserve the original magnifying arrangement.
The toy or printed text remains in its original orientation. It is the appearance through the convex lens that changes. Keep object distance, image size, and image orientation separate when recording the investigation.
Why can water change a pencil’s appearance?
The chapter’s final pencil question extends the same idea to a clear tumbler containing water. Put a straight pencil behind the tumbler and look at it through the glass. After filling the tumbler halfway, the lower part is seen through water as well as the curved glass, while the upper part is seen through a different arrangement. Their apparent positions or sizes can differ.
Light can change direction when it passes from one transparent material to another, such as water to air. This change is called refraction. A curved water-and-glass arrangement can change the paths of different rays differently, so part of the pencil may look enlarged, displaced, or distorted. The actual pencil remains straight. This brief explanation supports the observation; detailed refraction laws are beyond the chapter’s treatment.
A change in the direction of light as it passes between transparent materials in suitable conditions. It helps explain why objects can appear changed when viewed through water or lenses.
Problem
The lower part of a pencil looks wider through a water-filled tumbler. A friend says the water has changed the pencil’s actual thickness. How would you check?
- 1.View the pencil directly, without looking through the water-and-glass arrangement.
- 2.If it is unchanged, the larger appearance came from the altered light paths, not a change to the pencil.
- 3.Explain that transmitted light can be redirected by transparent materials and curved surfaces. The viewed image is different from the object itself.
Check your understanding
Use the thickness pattern to identify a lens and the image pattern to confirm your explanation. Remember that the actual object does not change merely because its image looks different.
Quiz
Which statement describes a convex lens?
What is the main difference between using a lens and using a mirror in these observations?
What helps the water drop act as a simple lens?
An ordinary object viewed through a concave lens appears:
Which observation can occur with a convex lens at a suitable greater object distance?
Why can a straight pencil appear distorted through water in a curved glass?
Practice Problems
- Sketch simple side views of convex and concave lenses, labelling the centre and edges.
- Describe the water-drop investigation and explain why the printed letters do not actually grow.
- Compare the image of a toy through convex and concave lenses as its distance changes.
- Explain why “a convex mirror and convex lens always give identical images” is incorrect.
- Use a magnifying glass over print and describe the changes you observe as its distance from the print increases.
- View a pencil through an empty tumbler and then through a half-filled one. Describe the change and explain why it is an optical appearance rather than the pencil changing shape.
Key Takeaways
• A lens is a curved transparent piece of material used to change the paths of transmitted light. • A convex lens is thicker in the middle; a concave lens is thinner in the middle. • A rounded water drop can act as a simple lens and make suitably placed text look larger. • A convex lens can give an erect enlarged image nearby and inverted images at suitable greater distances. • A concave lens gives an erect diminished image of an ordinary object. • Changes seen through a lens or water-filled glass are changes of appearance, not changes to the actual object. • Refraction provides a qualitative explanation of altered light paths through transparent materials.