Real & Virtual Images (Oxford AQA IGCSE Physics)

Images

• Images produced by lenses can be one of two types:

  • A real image

  • A virtual image

• The size of an image produced by either lens depends on the distance between the object and the lens

Real images

• A real image is defined as:

An image is formed when light rays from an object converge to meet each other and can be projected onto a screen

• A real image is one produced by the convergence of light towards a focus

• Real images are always inverted (upside down compared to the object)

• Real images can be projected onto pieces of paper or screens

  • An example of a real image is the image formed on a cinema screen

A real image formed by a projector

Virtual images

• A virtual image is defined as:

An image is formed when the light rays from an object do not meet but appear to meet behind the lens and cannot be projected onto a screen

• A virtual image is formed by the divergence of light away from a point

• Virtual images are always upright

• Virtual images cannot be projected onto a piece of paper or a screen

  • An example of a virtual image is a person's reflection in a mirror

A virtual image formed by a mirror

Ray Diagrams

• Ray diagrams can be used to show the images formed by lenses

• Whether an image is real or virtual is determined by:

  • The distance between the object and lens compared to the focal length

• The size of the image (magnified, diminished or same size) is also determined by:

  • The distance between the object and lens compared to the focal length

• Whether the image is upright or inverted is determined by:

  • Whether the image is real or virtual

  • Upright images are virtual and inverted images are real

Convex lenses and real images

Object placed between f and 2f

• The diagram below shows the image formed when the object is placed at a distance between one focal length (f) and two focal lengths (2f) from the lens

Real, enlarged and inverted image formed by a converging lens

• In this case, the image is:

  • Real

  • Enlarged

  • Inverted

Object placed further than 2f

• The following diagram shows what happens when the object is more distanced – further than twice the focal length (2f) from the lens

Real, diminished and inverted image formed by a converging lens

• In this case, the image is:

  • Real

  • Diminished (smaller)

  • Inverted

Object placed at exactly 2f

• If the object is placed at exactly twice the focal length (2f) from the lens

Real, same size and inverted image formed by a converging lens

• In this case, the image is:

  • Real

  • Same size as the object

  • Inverted

• To draw an accurate ray diagram of real images formed using convex lenses:

1. Start by drawing a ray going from the top of the object through the centre of the lens. This ray will continue to travel in a straight line

2. Next draw a ray going from the top of the object, travelling parallel to the axis of the lens. When this ray emerges from the lens it will travel directly towards the principal focus

3. The image is found at the point where the above two rays meet

Convex lenses and virtual images

Virtual, magnified and upright image formed by a converging lens

• In this case, the image is:

  • Virtual: the light rays appear to meet when produced backwards

  • Magnified: the image is larger than the object

  • Upright: the image is formed on the same side of the principal axis

• To draw an accurate ray diagram of virtual images formed using convex lenses:

1. Start by drawing a ray going from the top of the object through the centre of the lens. This ray will continue to travel in a straight line

2. Draw a dashed line continuing this ray upwards

3. Next draw a ray going from the top of the object, travelling parallel to the axis to the lens. When this ray emerges from the lens it will travel directly through the principal focus f

4. Also, draw a dashed line continuing this ray upwards

5. The image is the line drawn from the axis to the point where the two dashed lines meet

Concave lenses and virtual images

Virtual, diminished and upright image formed by a diverging lens

• In this case, the image is:

  • Virtual: the light rays appear to meet when produced backwards

  • Diminished: the image is smaller than the object

  • Upright: the image is formed on the same side of the principal axis

• Concave lenses can also be used to form virtual images

• If an object is placed further from the lens than the principal focus f then a diverging lens ray diagram will be drawn in the following way:

1. Start by drawing a ray going from the top of the object through the centre of the lens. This ray will continue to travel in a straight line

2. Next draw a ray going from the top of the object, travelling parallel to the axis to the lens. When this ray emerges from the lens it will travel directly upwards away from the axis

3. Draw a dashed line continuing this ray downwards to the principal focus, f

4. The image is the line drawn from the axis to the point where the above two rays meet

Comparing converging and diverging lenses

• The image produced by a converging lens can be either real or virtual

  • This means the image can be inverted (real) or upright (virtual)

• The image produced by a diverging lens is always virtual

  • This means the image will always be upright