Images in a convex mirror

The idea

A convex mirror always forms a virtual, erect, diminished image behind the mirror: point-sized at F for an object at infinity, and between P and F for an object at any finite distance.

The mirror at Agra Fort

A small convex mirror is fitted in a wall of Agra Fort, facing the Taj Mahal. A visitor can see the full image of the huge Taj Mahal in that small mirror.

How does the image of the Taj Mahal in the mirror compare with the building itself?

• Much smaller, so the whole building fits
• The same size, so only a part fits
• Bigger, so only a small part shows

Near and far

Slide the object far away and then close to the mirror. Notice where the image is and how big it is.

Always small and behind

For every distance the image stayed behind the mirror, upright and smaller than the object. It was always between P and F.

A convex mirror always forms a virtual, erect, diminished image behind the mirror: point-sized at F for an object at infinity, and between P and F for an object at any finite distance.

Notes

A convex mirror always gives a virtual, erect, diminished image behind the mirror: at F for an object at infinity and between P and F for any object at a finite distance.

Check yourself

A convex mirror forms an image of an object at a finite distance. What is the image like?

An object is at infinity in front of a convex mirror. Where is its image?

A friend says that a convex mirror gives an enlarged image if the object is very close to it. Is that right?

Why can a small convex mirror show the whole of a tall building?

• Real, inverted and diminished, in front of the mirror. A convex mirror does not form a real image of an object in front of it. Its image is behind the mirror.
• Virtual, erect and diminished, between P and F behind the mirror — correct. Yes!
• Virtual, erect and enlarged, behind the mirror. A convex mirror always gives a diminished image. An enlarged virtual image is formed by a concave mirror for an object between P and F.
• At the pole, with the same size as the object. The image of an object at infinity is point-sized and lies at F, behind the mirror.
• At C, in front of the mirror. A convex mirror forms its image behind the mirror. For an object at infinity it is at F.
• At F, behind the mirror, point-sized — correct. Yes!
• No, the image is diminished at every distance — correct. Yes!
• Yes, any object close to a mirror looks bigger. For a convex mirror the image is diminished for an object at any finite distance, even a close one.
• Yes, if the object is between P and F. The focus of a convex mirror is behind the mirror, so an object in front cannot be between P and F. The image is diminished anyway.
• Its image is enlarged and clear. A convex mirror does not enlarge. It gives a diminished image, so a large object fits in a small mirror.
• Its image of the building is diminished — correct. Yes!
• The building moves closer to the mirror. The distance of the building does not change. The image is diminished, which lets the whole building fit.