Light: Mirrors and Lenses · Lesson 2 of 7
Images Formed by Spherical Mirrors
“Follow how mirror images change with distance and use those patterns to explain everyday applications.”
• Describe images using size, orientation, and lateral reversal. • Compare plane, concave, and convex mirror images as the object moves. • Identify mirror types from a sequence of image observations. • Connect dental, lighting, telescope, traffic, and surveillance uses to mirror properties. • Explain why a vehicle’s convex mirror requires careful distance judgement.
Describe an image before explaining it
Knowing whether a surface curves inward or outward is only the beginning. We also need a clear way to describe what appears in it. Use two separate questions: Is the image upright or upside down? Is it larger, smaller, or the same size as the actual object? Separating these questions prevents confusing a large image with an upright one.
An erect image has the same upright orientation as the object. An inverted image is upside down relative to the object.
An enlarged image is larger than the object. A diminished image is smaller than the object. An image can also be the same size as its object.
Lateral inversion refers to the familiar sideways reversal in a mirror, such as lettering that appears reversed. It is different from an image becoming upside down. The source draws attention to lateral inversion in all three mirror types; do not use sideways reversal alone to identify the type. Compare size and orientation as well.
Move an object in front of two mirrors
A single look at a mirror can be misleading because some image properties depend on distance. This investigation compares how the image changes while the mirror and object remain the same. Moving the object, rather than changing several things at once, makes the observations easier to interpret.
Support a concave and a convex mirror upright, side by side, and use a small toy. Start with the toy a few centimetres from each mirror; the source suggests about 3–4 cm as a starting point, not a universal boundary. Keep it at a similar height to the mirrors.
- Observe each image near the mirror. Record orientation, size compared with the toy, and any sideways reversal.
- Move the toy slowly farther away and record how the image changes.
- Repeat with each mirror individually if looking at both together is confusing.
- Compare the records and describe a pattern for each mirror. Check the same toy in a plane mirror as a reference.
| Mirror | Object close to mirror | As object is taken farther away |
|---|---|---|
| Plane | Erect; same size as the object | Remains erect and the same size |
| Concave | At a sufficiently small distance: erect and enlarged | At suitable farther distances: inverted; may be enlarged, same-size, or diminished |
| Convex | Erect and diminished | Remains erect and diminished; image becomes smaller as distance increases |
The concave mirror has the changing pattern. Close enough to it, the image is upright and enlarged. At suitable greater distances it becomes inverted. Among these inverted images, size can be larger than the object, equal to it, or smaller. The chapter describes this change qualitatively; there is no single distance in centimetres that applies to every concave mirror.
A convex mirror has a steadier pattern: for an object placed in front of it, the image remains erect and diminished. As the object moves away, the image generally becomes smaller. A plane mirror provides the contrasting reference: it gives an erect, same-size image rather than the distance-dependent size changes of spherical mirrors.
Do not memorise 3–4 cm as the boundary between erect and inverted images. Mirrors with different curvatures can give different results at the same distance. Use the observed pattern and move the object through a range of distances.
Problem
A toy’s image is upright and enlarged when close to a mirror. Farther away, its image becomes inverted. Which mirror is consistent with these observations?
- 1.A plane mirror keeps an erect, same-size image, so it does not fit.
- 2.A convex mirror keeps an erect, diminished image, so it does not fit either.
- 3.A concave mirror can give an erect enlarged image nearby and an inverted image farther away. It fits the sequence.
Problem
A visitor starts far from a large concave mirror and walks slowly toward it. Describe the broad change she may observe.
- 1.At a sufficiently large distance, she sees an inverted image.
- 2.As she approaches through the range of inverted images, the image grows larger.
- 3.Once she is sufficiently close, the image is erect and enlarged. The transition should not be described as a smoothly shrinking upright image.
Why dentists and lighting devices use concave mirrors
The usefulness of a mirror follows from what it does to light and images. A dentist needs to see a small part of a tooth clearly. Holding a concave mirror close enough to the tooth provides an upright enlarged view, making details easier to inspect. The same mirror would not give this useful upright enlarged view at every distance.
Concave-shaped reflectors are also found in torches and vehicle headlights. With a suitable position of the light source, the reflector helps direct light into a useful beam. This is a lighting use, rather than a need to view a magnified image of a bulb. A later lesson will examine how curved mirrors change the direction of light beams.
The source notes that many modern telescopes use a large concave primary mirror. It collects and directs light from distant objects. A telescope is therefore another application of a curved reflecting surface, even though its arrangement differs from a dentist’s mirror.
With a teacher or parent, visit a dentist’s or ENT clinic if such a visit is available. Ask which instruments use mirrors and what each mirror helps the doctor see. Identify the mirror used in each instrument rather than assuming every medical instrument contains the same mirror type.
Problem
Would a convex mirror be a suitable replacement when a dentist needs an enlarged view of a nearby tooth?
- 1.Start with the purpose: the image should make the tooth’s details look larger.
- 2.A convex mirror gives an erect but diminished image, so it does not meet that purpose.
- 3.A concave mirror held at a suitable close distance can give an erect enlarged image and is the appropriate choice.
Why a wide view matters on roads and in stores
A driver needs to see more than one small patch of road. A convex mirror provides a wider field of view: it shows more of the surrounding area than a similarly sized plane mirror in a comparable arrangement. Its images remain upright, which helps the driver recognise vehicles, but the images are smaller than the actual vehicles.
The extent of the surrounding scene that can be seen through an optical arrangement or in a mirror from a particular viewing position.
This combination explains convex side-view mirrors, mirrors at sharp road bends and intersections, and surveillance mirrors in shops. At a bend, a driver can see traffic approaching from a direction that would otherwise be hidden. In a shop, staff can monitor a larger area. The useful property is the broad view, not enlarged detail.
A smaller-looking vehicle can be judged as farther away than it actually is. That is why a convex vehicle mirror carries a warning that objects are closer than they appear. The image does not move the real vehicle; it can mislead our distance judgement. A driver must allow for this when assessing traffic.
A convex mirror helps you see a larger area, while each vehicle’s image is diminished. A concave dental mirror enlarges a nearby tooth, but that does not make it the correct mirror for monitoring a wide road scene.
Check your understanding
Identify the observation or purpose first, then choose the mirror. This is more reliable than associating one image size with every curved surface.
Quiz
Which mirror always gives an erect same-size image of an ordinary object?
A nearby object has an erect enlarged image, which becomes inverted when it is moved away. Which mirror fits?
Which image is expected in a convex mirror?
Why is a convex mirror useful at a sharp road bend?
For an enlarged upright view of a tooth, a concave mirror should be:
Which statement correctly distinguishes lateral inversion from an inverted image?
Practice Problems
- Make a comparison table of image size and orientation for plane, concave, and convex mirrors.
- Describe how to identify three unlabelled mirrors by observing a toy at several distances. Explain why one observation may be insufficient.
- Explain why a dentist uses a concave mirror close to a tooth, while a shop uses a convex surveillance mirror.
- Explain the vehicle-mirror warning using image size and distance judgement.
- A person walks toward a concave mirror from far away. Describe the broad image changes and compare them with approaching a plane mirror.
- List the concave-mirror applications discussed here and distinguish uses for viewing images from uses for directing or collecting light.
Key Takeaways
• Describe orientation and size separately: erect or inverted; enlarged, diminished, or same-size. • A plane mirror gives an erect, same-size image. • A concave mirror’s image depends on object distance; a suitably close object gives an erect enlarged image. • A convex mirror gives an erect diminished image and a useful wide field of view. • Dental mirrors, lighting reflectors, and reflecting telescopes use concave surfaces for different purposes. • Convex road and surveillance mirrors show a wide area, but their smaller images require careful distance judgement.