Telescopes
Lenses
Lenses use the process of refraction to change the direction of light at their two surfaces.
• Convex / Converging Lens – Focuses incident light.
• Concave / Diverging Lens – Spreads out incident light.
Definitions:
Principal Axis – A construction line that passes through the centre of the lens at 90º to its
surface.
Principal Focus (F):
o In a converging lens: the point where incident beams passing parallel to the
principal axis will converge.
o In a diverging lens: the point from which the light rays appear to come from.
This is the same distance either side of the lens.
Focal Length (f) – The distance between the centre of the lens (O), and the principal focus
(F). The shorter the focal length, the stronger the lens.
,Rules for drawing converging Lens
(d) Parallel rays incident not parallel to the principal axis converge on the focal plane.
, Real Image – Formed where rays cross after refraction by a lens.
o Real Images can be cast onto a screen.
Virtual Image – Formed on the same side of the lens.
o Light rays don’t cross, so a virtual image cannot be seen on a screen.
Lenses
Lenses use the process of refraction to change the direction of light at their two surfaces.
• Convex / Converging Lens – Focuses incident light.
• Concave / Diverging Lens – Spreads out incident light.
Definitions:
Principal Axis – A construction line that passes through the centre of the lens at 90º to its
surface.
Principal Focus (F):
o In a converging lens: the point where incident beams passing parallel to the
principal axis will converge.
o In a diverging lens: the point from which the light rays appear to come from.
This is the same distance either side of the lens.
Focal Length (f) – The distance between the centre of the lens (O), and the principal focus
(F). The shorter the focal length, the stronger the lens.
,Rules for drawing converging Lens
(d) Parallel rays incident not parallel to the principal axis converge on the focal plane.
, Real Image – Formed where rays cross after refraction by a lens.
o Real Images can be cast onto a screen.
Virtual Image – Formed on the same side of the lens.
o Light rays don’t cross, so a virtual image cannot be seen on a screen.
