Edexcel A Level Physics:复习笔记5.21 Real & Virtual Images

Real Images

• Are formed when light rays from a point on an object pass through another point in space
  • The light rays are really there
• Can be formed on a screen
• Are seen in ray diagrams at the point where rays cross
• Examples include:
  • Pictures projected onto a wall or screen
  • The image formed on the retina

A real image can be projected onto a screen

Virtual Images

• Virtual images are formed when light rays from a point on an object appear to have come from another point in
  space
• The light rays are not really where the image appears to be
• The image cannot be formed on a screen
• Examples include:
  • Images seen through a magnifying glass
  • All images formed by a diverging (concave) lens
  • Reflections in a mirror

A reflection in a mirror is an example of a virtual image

Ray diagrams

• Ray diagrams can be used to show whether an image will be real or virtual
• If the rays from the object naturally cross the image will be real
• If the rays have to be extended backwards to make them cross, the image is virtual
  • To signal virtual rays they are drawn as dashed lines rather than solid ones

Ray diagram for a real image in a converging lens

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

Diagram showing the formation of a real image by a lens

• The above diagram shows the image that is formed when the object is placed at a distance between one focal length (f) and two focal lengths (2f) from the lens
• In this case, the image is:
  • Real
  • Enlarged
  • Inverted

   

Ray diagram for a virtual image in a converging lens

• If the object is placed closer to the lens than the focal length, the emerging rays diverge and a real image is no longer formed
• When viewed from the right-hand side of the lens, the emerging rays appear to come from a point on the left. This point can be found by extending the rays backwards (creating virtual rays)
• A virtual image will be seen at the point where these virtual rays cross

A virtual image is formed by the divergence of rays from a point

• In this case the image is:
  • Virtual
  • Enlarged
  • Upright

   

• Using a lens in this way allows it to be used as a magnifying glass
• When using a magnifying glass, the lens should always be held close to the object

Ray diagram for a virtual image in a diverging lens

• The image formed by a diverging lens is always virtual
• To draw this diagram draw two rays from the top of the object
  • One ray passes through the centre of the lens with no refraction
  • The second is drawn parallel to the principal axis until it meets the centre of the lens
  • The ray refracts through the principal focus
  • To make the rays cross the line will need to be extended, forming a virtual meeting point

Concave lenses only produce virtual images

• In this case the image is:
  • Virtual
  • Diminished
  • Upright

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