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thickest in middle & thinnest at edge causes rays parallel to principal axis to
converge through a single point (principal focus) after refraction
1. Converging Lenses
Optical Centre (O) – centre of the lens
Principal Axis – line through the optical centre
Principal Focus (F) – point on principal axis where light rays that are parallel to principal axis converge after refraction
Converging Lens Terms
Secondary Principal Focus (F’) - light can strike lens from either side- both sides of lens can focus parallel light- F’ is focus on same side of lens as incident
rays- F and F’ alternate sides depending on which
side the incident rays are coming from
rays are refracted twice, way in and way out
only incident ray entering and ray leaving are important, so we use a shortcut
Path of Light Through a Lens
3 rules:1. ray parallel to the principal axis is refracted
through the principal focus (F).2. ray through the secondary principal focus(F’) is
refracted parallel to the principal axis.3. ray through the optical centre (O) continues
straight through without being refracted.
Locating Images in Converging Lenses
thinnest in middle and thickest near edge causes parallel incident rays to diverge
(spread) after refraction
2. Diverging Lenses
same as Converging Lens: Optical Centre (O) Principal Axis Principal Focus (F)
Secondary Principal Focus (F’) – different
F is on same side of lens as incident rays F’ is on opposite side as incident rays
Diverging Lens Terms
Locating Images in Diverging Lenses 3 rules:1. ray parallel to the principal axis is refracted as if
it had come through the principal focus (F)2. ray that appears to pass through the secondary
principal focus (F’) is refracted parallel to the principal axis.
3. ray through the optical centre (O) continues straight through on its path.
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