Real images in a concave mirror

The idea

For a concave mirror, an object at infinity, beyond C, at C or between C and F gives a real, inverted image at F (point-sized), between F and C (diminished), at C (same size) or beyond C (enlarged) respectively.

A far away flame

A concave mirror faces a candle flame that is very far away. A small white screen is held in front of the mirror, in the right place to catch the image.

What will the picture of the flame on the screen look like, compared with the flame?

• Tiny and upside down
• The same size and upright
• Very big and upright

Four object positions

In every picture F is 20 cm and C is 40 cm from the mirror. The red ray runs parallel to the axis. The blue ray passes through F. Open every tab.

The image walks away

As the object came closer to the mirror, the image moved away from the mirror and grew larger. In all four pictures the image is real and inverted.

For a concave mirror, an object at infinity, beyond C, at C or between C and F gives a real, inverted image at F (point-sized), between F and C (diminished), at C (same size) or beyond C (enlarged) respectively.

• Object | Image | Size | Nature
• At infinity | At F | Highly diminished, point-sized | Real and inverted
• Beyond C | Between F and C | Diminished | Real and inverted
• At C | At C | Same size | Real and inverted
• Between C and F | Beyond C | Enlarged | Real and inverted

Notes

A concave mirror forms a real, inverted image for an object at infinity, beyond C, at C and between C and F. The image is at F, between F and C, at C and beyond C, getting larger as the object comes closer.

Check yourself

An object is placed beyond C of a concave mirror. Where is its image and how big is it?

The real image of a concave mirror is exactly the same size as the object. Where is the object?

An object starts far beyond C and moves towards C. What happens to its real image?

Which statement is true for all four object positions of this lesson?

• Between F and C, smaller than the object — correct. Yes!
• Beyond C, larger than the object. That is the image for an object between C and F. For an object beyond C the image is between F and C and is diminished.
• At F, point-sized. A point-sized image at F is formed only when the object is at infinity.
• At F. An object at F gives no image on a screen; its image would be at infinity.
• At C — correct. Yes!
• Beyond C. An object beyond C gives a diminished image, smaller than the object.
• It moves towards the mirror and grows smaller. It is the other way round. The image is at F for a far away object and moves out to C as the object reaches C, growing larger.
• It stays at F and keeps the same size. The image stays at F only for an object at infinity. As the object comes closer the image moves away from F.
• It moves away from the mirror and grows larger — correct. Yes!
• The image is real and inverted — correct. Yes!
• The image is virtual and erect. A virtual, erect image is formed only when the object is between P and F.
• The image is real and erect. A real image formed by a concave mirror is inverted, as the table shows.