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Images

An image is a copy of an object that is caused by the reflection or refraction of light.

When you look at yourself in a mirror you are looking at an image of yourself.

There are two different types of images:

  • Real images can be projected onto a screen.

    A photocopier makes images of a document that are projected and copied onto a piece of paper.

    A projector in a cinema makes large images that appear on the screen in front of the audience.

  • Virtual images cannot be projected onto a screen.

    You cannot project the images you see in a mirror onto a screen.

    A magnifying glass also creates virtual images.

This magnified image cannot be formed on a screen. It is virtual.
This magnified image cannot be formed on a screen. It is virtual.

A mirror image is where the rays of light 'seem' to be coming from after they have been reflected by a mirror. Mirrors create images of objects by reflection.

The images created by flat mirrors are

  • the same distance behind the mirror as the object is in front of the mirror
  • left-right reversed
  • the same size as the object
  • virtual (they cannot be projected onto a screen)
The observer sees an image of the arrow in the mirror.
The observer sees an image of the arrow in the mirror.

To locate the position of a mirror image trace the reflected rays back from the observer over to the other side of the mirror until they meet.

The point at which the rays meet is the position of the image.

Converging lenses can focus rays of light from an object to form an image on the other side of the lens.

  • Images can be diminished (smaller than the object) or magnified (larger than the object).
  • Images can be upright (the same way up as the object) or inverted (the other way up to the object).
  • Images can be real (can be projected onto a screen) or virtual (cannot be projected onto a screen).

The properties of the image formed depend on the position of the object relative to the lens and the focal length, $$F$$, of the lens.

Position of object Image properties
Beyond $$ 2 \, F $$ real, inverted, diminished
Between $$ F $$ and $$ 2 \, F $$ real, inverted, magnified
Between the lens and $$ F $$ virtual, upright, magnified

Magnifying glasses create images that are magnified, upright and virtual.

An object is placed at A. The converging lens produces a real image of the object at A' which can be projected onto a screen.
An object is placed at A. The converging lens produces a real image of the object at A' which can be projected onto a screen.

Ray diagrams can be used to work out the appearance of images created by lenses and whether they are real or virtual.

1) Draw the lens and the $$\Tgreen{\text{principal foci}}$$ on both sides of the lens, and mark the position of the $$\Tviolet{\text{object}}$$.

2) Draw $$\Tred{\text{one ray}}$$ from the object straight through the centre of the lens without it changing direction.

3) Draw a $$\Tblue{\text{second ray}}$$ parallel to the axis heading towards the lens. At the centre of the lens, it is refracted so that it passes through the principal focus.

How to construct a ray diagram
How to construct a ray diagram

A real $$\Torange{\text{image}}$$ is formed where the two lines meet. If the image is on the same side of the axis as the object it is upright. If the image is further from the axis than the object it is magnified.

If the two lines do not meet, a virtual image is formed. To draw it, extend the rays coming from the lens to the side of the lens where the object is found. This is how a magnifying glass works.