Concave Lenses and Magnification

Draw a ray diagram for an image through a concave lens

• Pick a point on the top of the object. Draw a ray going from the object to the lens parallel to the axis of the lens.
• Draw another ray from the top of the object going right through the middle of the lens.
• The incident ray that’s parallel to the axis is refracted so it appears to have come from the principal focus. Draw a ray from the principal focus. Make it dotted before it reaches the lens
• The ray passing through the middle of the lens doesn’t bend.
• Mark where the refracted rays meet. That’s the top of the image.
• Repeat the process for a point on the bottom of the object. When the bottom of the object is on the axis, the bottom of the image is also on the axis.

Concave lenses always create virtual images

Unlike a convex lens, a concave lens always produces a virtual image. The image is the right way up, smaller than the object and on the same side of the lens as the object-no matter where the object is.

Magnifying glasses use convex lenses

Magnifying glasses work by creating a magnified virtual image.

• The object being magnified must be closer to the lens than the focal length.

• Since the image produced is a virtual image, the light rays don’t actually come from the place where the image appears to be.

• Remember’you can’t project a virtual image onto a screen’-that’s a useful phrase to use in the exam if they ask you about virtual images.

• You can use the magnifying formula to work out the magnification produced by a lens at a given distance:

  Magnification = image height / object height