Images — Combined Science 0653 - Physics Revision Notes & Topical Questions

Section 1 of 7Describing an image

Size

• Size Compared with the object, an image can be enlarged (bigger), the same size or diminished (smaller).
• Always compare the image with the object when you describe it.

Orientation

• Orientation An image is upright if it points the same way as the object and inverted if it is upside down.
• The image on the retina of the eye is inverted and diminished, and the brain interprets it so that we see the world the right way up.

Nature: real or virtual

• Nature: real or virtual A real image is formed where light rays actually cross and it can be caught on a screen.
• A virtual image is formed where rays only appear to come from; it cannot be projected on to a screen.
  • A magnifying glass gives an enlarged, upright, virtual image.

Section 2 of 7Three rules for drawing rays

1

A ray travelling through the optical centre, which is the middle of the lens, carries straight on without being bent.

2

A ray travelling parallel to the principal axis is refracted by the lens so that it passes through the principal focus on the far side.

3

A ray passing through the principal focus on the object side emerges from the lens parallel to the principal axis.

4

Draw any two of these rays from the top of the object; the image of that point is formed where the two rays meet.

Section 3 of 7Images from a converging lens

Object position	Image position	Size	Orientation	Nature
Beyond 2f	Between f and 2f	Diminished	Inverted	Real
At 2f	At 2f	Same size	Inverted	Real
Between f and 2f	Beyond 2f	Enlarged	Inverted	Real
Closer than f	Same side as the object	Enlarged	Upright	Virtual

Section 4 of 7Worked example: a diminished image

1

Draw a horizontal principal axis and a vertical line, or a thin ellipse, for the lens.

2

Mark the principal focus F on both sides of the lens at equal distances, and mark 2F as well.

3

Draw the object as an arrow on the left, standing on the axis and placed beyond 2F, since this is what gives a diminished image.

4

Draw a ray from the top of the object through the optical centre, going straight on.

5

Draw a second ray from the top of the object parallel to the axis, which bends at the lens and passes through F on the right.

6

Mark where the two rays cross and draw the image arrow down to the axis from there, then label object, image, F and the lens.

Section 5 of 7Everyday optical devices

Lens images in use

Magnifying glass

• Magnifying glass The object is held closer than f, so the image is enlarged, upright and virtual.

Camera

• Camera The object is far away, beyond 2f, so a real, inverted, diminished image is focused on the sensor.

Eye

• Eye The lens focuses a real, inverted and diminished image on the retina.

Projector

• Projector The slide sits between f and 2f so a real, enlarged, inverted image falls on the screen;
  • the slide is loaded upside down to compensate.

Section 6 of 7Key terms

Term	Meaning
Principal axis	The straight line through the centre of the lens at right angles to it.
Principal focus (f)	The point where rays parallel to the axis meet after passing through a converging lens.
Focal length	The distance from the optical centre of the lens to the principal focus.
Real image	An image formed by rays that actually meet, which can be shown on a screen.
Virtual image	An image formed where rays only appear to come from; it cannot be shown on a screen.
Diminished	Smaller than the object.

Section 7 of 7Exam technique

1

Drawing the rays so that they bend at the object rather than at the lens, or using the edge of the lens instead of the centre line.

2

Forgetting to label the principal foci, the object and the image on a ray diagram, which loses marks.

3

Describing an image only as 'big' or 'small' without comparing it with the object using enlarged, same size or diminished.

4

Saying a real image is upright. Real images from a single converging lens are inverted; only the virtual image is upright.

5

Drawing rays that are not straight, or that miss the focal point they are meant to pass through.