Lenses

Aug 2, 2024

2-minute read

Table of Contents

A lens is a section of transparent material which is used to refract light. There are two types of lenses:

Converging/convex lens - bends light entering it parallel to the principal axis towards the focal point
Diverging/concave lens - bends light entering it parallel to the principal axis away from the focal point

Credits: Toppr

Parts of a lens

Below are the basic parts of a lens and their definitions.

Focal point/Focus the point through which parallel rays of light are directed when they pass through the lens. In the case of a diverging lens, it is the point from which parallel rays of light apparently originate
Focal length the distance from the center of the lens to the focal point
Principal/optical axis the line of symmetry of the lens. It passes through the focal points and the center of the lens
Center of curvature The center of the imaginary sphere that the lens would have been cut from
Focal plane the plane which is perpendicular to the principal axis and intersects it at the focus. Parallel rays which do not enter the lens parallel to the principal axis will still intersect on the focal plane

We can visualize how various rays of light will travel through a lens by using a ray diagram. It is useful to know that:

Rays passing through the lens parallel to the principal axis will be redirected through the focus on the other side of the lens for converging lens (the emergent side). For diverging lens they will be drawn as if coming out of the focus on the same side from which they enter (the incident side)
Rays passing through the center of the lens are unaffected
Rays passing through the focus will emerge parallel to the principal axis. For converging lens we draw a line through the focus on the incident side and for diverging lens we draw it through the focus on the emergent side

We can investigate these ray patterns using a simulation: https://ophysics.com/l12.html

Run the simulation
Try different object distances and heights, including object distances $O$ to $F$, $F$ to $2F$ and $>2F$
Pay attention to how the rays behave and the image characteristics
Change the lens to a diverging lens by dragging the focus to the other side of the lens
Repeat steps 2 and 3