Convex Lenses

AQA-style Exam Question

Draw a ray diagram to show how a convex lens forms an image of an object. Include:

• two light rays

• the principal focus (F)

• the principal axis

[3 marks]

• How does our eye see an object?

  1. Light rays from a light source reflect from the object to the LENS in our eye.

  2. Our LENS (convex in shape) refracts the light so it converges and forms the image back on the RETINA

• For simplicity, how do we model the lens?

  We assume the lens is:

  • Long line with arrow heads pointing outwards (if convex)

  • with set Principal Focus points (F) on either side

  • Note: the distance between the lens and the principal focus point is called focal length.

  The location of the focus points are a feature of the lens. They appears on either side, since the lens is symmetrical in shape.

• How do we model the object?

  • as sitting on a horizontal line called principal axis

• What other point matters?

  Optical centre: where the lens and the axis meet.

• How do we draw the first ray?

  From the top of the object:

  • Draw a ray parallel to the principal axis until it reaches the lens.

  • It then refracts and passes through the principal focus (F).

• How do we draw the second ray?

  Again, from the top of the object:

  • a diagonal line to cross the Optical Centre and extends beyond

  • Where both rays meet, the top of the image is mapped

Challenge:

• How do we describe the image in 3 words?

  • Diminished (smaller)

  • Inverted (upside down)

  • Real (the rays actually met here)

Continue Revising Paper 2

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