Lenses

What is a lens?

A transparent material with curved surfaces that bends light.

What is a convex lens?

A converging lens that brings light rays together.

What is a concave lens?

A diverging lens that spreads light rays apart.

Define principal axis.

The line through the optical centre, perpendicular to the plane of the lens.

Define optical centre.

The midpoint of the lens; rays through it are undeviated.

Define principal focus (F).

The point where rays parallel to the principal axis converge after refraction.

Define focal length (f).

The distance between the optical centre (C) and the principal focus (F).

What is the focal plane?

The vertical plane passing through the focus, perpendicular to the principal axis.

What is a real image?

An image formed when actual rays meet; can be captured on a screen.

What is a virtual image?

An image formed when rays appear to meet; cannot be captured on a screen.

What happens to a ray parallel to the principal axis?

It refracts through the principal focus.

What happens to a ray through the optical centre?

It passes undeviated.

What happens to a ray through the principal focus?

It refracts parallel to the principal axis.

Why does a converging lens form different images at different distances?

Because object distance (u) relative to focal length (f) determines where rays converge.

Where is the focus when parallel rays not along the axis enter the lens?

At a point on the focal plane.

What image is formed when u = ∞?

Real, inverted, diminished (uses: telescope objective).

What image is formed when u > 2f?

Real, inverted, diminished (uses: camera, eye).

What image is formed when u = 2f?

Real, inverted, same size (uses: photocopier).

What image is formed when f < u < 2f?

Real, inverted, magnified (uses: projector, enlarger).

What happens when u = f?

No image is formed (lens used as telescope eyepiece).

What image is formed when u < f?

Virtual, upright, magnified (uses: magnifying glass).

What does a concave lens do to light?

Diverges rays away from a focal point.

Where is the focal point of a concave lens?

The point where diverging rays appear to meet when extended backward.

What type of image does a concave lens always produce?

Virtual, upright, diminished.

Which lens is used in a magnifying glass?

Converging lens (u < f).

Which lens is used in telescopes?

Converging lens (objective: u = ∞, eyepiece: u = f).

Which lens is used in cameras?

Converging lens (u > 2f).

Which lens is used in spectacles for short-sightedness?

Diverging lens.

Which lens is used in spectacles for long-sightedness?

Converging lens.